Bacteria | Cell, Evolution, & Classification

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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/bacteria">bacteria</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/bacteria">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref39333"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/bacteria#ref39333">The bacterial cell</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref272356"><a class="w-100 link-gray-900" href="/science/bacteria#ref272356">Bacteria as prokaryotes</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref39334"><a class="w-100 link-gray-900" href="/science/bacteria/Diversity-of-structure-of-bacteria">Diversity of structure of bacteria</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref39335"><a class="w-100 link-gray-900" href="/science/bacteria/Diversity-of-structure-of-bacteria#ref39335">Morphological features of bacteria</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref39336"><a class="w-100 link-gray-900" href="/science/bacteria/Diversity-of-structure-of-bacteria#ref39336">The Gram stain</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39337"><a class="w-100 link-gray-900" href="/science/bacteria/The-cell-envelope">The cell envelope</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39338"><a class="w-100 link-gray-900" href="/science/bacteria/Capsules-and-slime-layers">Capsules and slime layers</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39339"><a class="w-100 link-gray-900" href="/science/bacteria/Capsules-and-slime-layers#ref39339">Flagella, fimbriae, and pili</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39340" class="has-children"><a class="w-100 link-gray-900" href="/science/bacteria/Capsules-and-slime-layers#ref39340">The cytoplasm</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref39341"><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content">Genetic content</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref39342"><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content#ref39342">Cytoplasmic structures</a></li></ul></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref272357"><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content#ref272357">Biotypes of bacteria</a></li></ul></div></li><li data-target="#ref272358"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content#ref272358">Bacterial reproduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref272359"><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content#ref272359">Reproductive processes</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref272360"><a class="w-100 link-gray-900" href="/science/bacteria/Genetic-content#ref272360">Binary fission</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref272361"><a class="w-100 link-gray-900" href="/science/bacteria/Budding">Budding</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref272362"><a class="w-100 link-gray-900" href="/science/bacteria/Budding#ref272362">Sporulation</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref272363"><a class="w-100 link-gray-900" href="/science/bacteria/Exchange-of-genetic-information">Exchange of genetic information</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref272364"><a class="w-100 link-gray-900" href="/science/bacteria/Growth-of-bacterial-populations">Growth of bacterial populations</a></li></ul></div></li><li data-target="#ref272365"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/bacteria/Growth-of-bacterial-populations#ref272365">Ecology of bacteria</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref272366"><a class="w-100 link-gray-900" href="/science/bacteria/Growth-of-bacterial-populations#ref272366">Distribution in nature</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref272367"><a class="w-100 link-gray-900" href="/science/bacteria/The-importance-of-bacteria-to-humans">The importance of bacteria to humans</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref272368"><a class="w-100 link-gray-900" href="/science/bacteria/The-importance-of-bacteria-to-humans#ref272368">Bacteria in food</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref272369"><a class="w-100 link-gray-900" href="/science/bacteria/Bacteria-in-industry">Bacteria in industry</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref272370"><a class="w-100 link-gray-900" href="/science/bacteria/Bacteria-in-medicine">Bacteria in medicine</a></li></ul></li></ul></div></li><li data-target="#ref272371"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/bacteria/Evolution-of-bacteria">Evolution of bacteria</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref39345"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/bacteria/Evolution-of-bacteria#ref39345">Biosynthesis, nutrition, and growth of bacteria</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref39346"><a class="w-100 link-gray-900" href="/science/bacteria/Evolution-of-bacteria#ref39346">Factors affecting bacterial growth</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref39347"><a class="w-100 link-gray-900" href="/science/bacteria/Evolution-of-bacteria#ref39347">Nutritional requirements</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39348" class="has-children"><a class="w-100 link-gray-900" href="/science/bacteria/Physical-requirements">Physical requirements</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref39349"><a class="w-100 link-gray-900" href="/science/bacteria/Physical-requirements#ref39349">Oxygen</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref39350"><a class="w-100 link-gray-900" href="/science/bacteria/Physical-requirements#ref39350">Temperature</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref39351"><a class="w-100 link-gray-900" href="/science/bacteria/Physical-requirements#ref39351">pH</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref39352"><a class="w-100 link-gray-900" href="/science/bacteria/Physical-requirements#ref39352">Salt and water</a></li></ul></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref39353"><a class="w-100 link-gray-900" href="/science/bacteria/Bacterial-metabolism">Bacterial metabolism</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref39354"><a class="w-100 link-gray-900" href="/science/bacteria/Bacterial-metabolism#ref39354">Heterotrophic metabolism</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39355"><a class="w-100 link-gray-900" href="/science/bacteria/Autotrophic-metabolism">Autotrophic metabolism</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref39356"><a class="w-100 link-gray-900" href="/science/bacteria/Autotrophic-metabolism#ref39356">Phototrophic metabolism</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref39357"><a class="w-100 link-gray-900" href="/science/bacteria/Biosynthetic-pathways-of-bacteria">Biosynthetic pathways of bacteria</a></li></ul></div></li><li data-target="#ref58987"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/bacteria/Biosynthetic-pathways-of-bacteria#ref58987">Classification of bacteria</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref272372"><a class="w-100 link-gray-900" href="/science/bacteria/Biosynthetic-pathways-of-bacteria#ref272372">Taxonomic rankings</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref328850"><a class="w-100 link-gray-900" href="/science/bacteria/Biosynthetic-pathways-of-bacteria#ref328850">The bacterial species problem</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref328851"><a class="w-100 link-gray-900" href="/science/bacteria/Biosynthetic-pathways-of-bacteria#ref328851">Genetic approaches</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref328852"><a class="w-100 link-gray-900" href="/science/bacteria/DNA-based-methods">DNA-based methods</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref328853"><a class="w-100 link-gray-900" href="/science/bacteria/DNA-based-methods#ref328853">16S rRNA analysis</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref39371"><a class="w-100 link-gray-900" href="/science/bacteria/Classification-by-morphology-biochemistry-and-other-features">Classification by morphology, biochemistry, and other features</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/bacteria/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/bacteria">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/bacteria/images-videos">Images & 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Kadner</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of Microbiology, School of Medicine, University of Virginia, Charlottesville. Coauthor of <i>Essentials of Medical Microbiology.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Robert J. Kadner</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/editor/Kara-Rogers/6713" > <div class="editor-title font-16 font-weight-bold">Kara Rogers</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Kara Rogers is the senior editor of biomedical sciences at Encyclopædia Britannica, where she oversees a range of content from medicine and genetics to microorganisms. She joined Britannica in 2006 and...</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Kara Rogers</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/science/bacteria/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2025-02-13T00:00:00CST" >Feb 13, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/bacteria/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn btn-sm border-2 btn-outline-red-400 border-red-400 module-spacing js-inline-ai-ask-button p-10 ml-5"> Ask the Chatbot </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Singular: </dt> <dd>bacterium</dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Joshua-Lederberg" topicid="334486">Joshua Lederberg</a></dd> <dd><a href="/biography/George-P-Smith" topicid="2131923">George P. Smith</a></dd> <dd><a href="/biography/Robert-Koch" topicid="320834">Robert Koch</a></dd> <dd><a href="/biography/Antonie-van-Leeuwenhoek" topicid="334699">Antonie van Leeuwenhoek</a></dd> <dd><a href="/biography/Ferdinand-Cohn" topicid="124611">Ferdinand Cohn</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/rickettsia-microorganism-group" topicid="502946">rickettsia</a></dd> <dd><a href="/science/campylobacter" topicid="1241383">campylobacter</a></dd> <dd><a href="/science/nitrogen-fixing-bacteria" topicid="967311">nitrogen-fixing bacteria</a></dd> <dd><a href="/science/sulfur-bacterium" topicid="572728">sulfur bacterium</a></dd> <dd><a href="/science/eubacterium" topicid="194742">eubacterium</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/bacteria"> See all related content </a> </div> </div> </div> </div><div class="ai-top-questions-module module-spacing ai-tq collapsed font-sans-serif"> <div class="font-weight-bold font-14"> Top Questions </div> <ul class="mt-5"> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="0"> <div>What are bacteria?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="1"> <div>Where can bacteria be found?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="2"> <div>How are bacteria important to our environment?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="3"> <div>What are the basic shapes of bacteria?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="4"> <div>How do bacteria reproduce?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="5"> <div>What role do bacteria play in the human body?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="6"> <div>How can bacteria cause disease?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="7"> <div>What is antibiotic resistance in bacteria?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="8"> <div>How are bacteria used in industrial applications?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="9"> <div>What are some ways scientists study and classify bacteria?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> </ul> <button class="show-more btn btn-unstyled font-14 p-5 pr-10 mb-10 rounded-sm d-none"><em class="material-icons " data-icon="expand_more"></em> Show more</button> <button class="show-less btn btn-unstyled font-14 p-5 pr-10 mb-10 rounded-sm"><em class="material-icons " data-icon="expand_less"></em> Show less</button> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://www.telegraph.co.uk/news/2025/02/12/secret-sex-bacteria-could-help-solve-crimes/" rel="nofollow">Secret ‘sex bacteria’ may help to solve crimes</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 10, 2025, 9:58 PM ET (The Telegraph) <button class="btn btn-link d-inline p-0 font-12 js-toggle-recent-news"> <span class="text-gray-500">...</span><span>(Show more)</span> </button> </span> </div> </div> <div class="rest-of-recent-news-items"> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.news-medical.net/news/20250123/New-discovery-sheds-light-on-how-bacteria-use-plasmids-to-build-resistance.aspx" rel="nofollow">New discovery sheds light on how bacteria use plasmids to build resistance</a> <span class="font-14 text-gray-600"> <span>•</span> Jan. 23, 2025, 11:41 PM ET (News-Medical) </span> </div> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref475162"></span>bacteria</strong>, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="environment" href="https://www.merriam-webster.com/dictionary/environment" data-type="MW">environment</a> on <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>, from <a href="https://www.britannica.com/science/deep-sea-vent" class="md-crosslink " data-show-preview="true">deep-sea vents</a> to deep below Earth’s surface to the digestive tracts of humans.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Bacteria lack a membrane-bound <a href="https://www.britannica.com/science/nucleus-biology" class="md-crosslink " data-show-preview="true">nucleus</a> and other internal structures and are therefore ranked among the unicellular life-forms called <a href="https://www.britannica.com/science/prokaryote" class="md-crosslink " data-show-preview="true">prokaryotes</a>. Prokaryotes are the dominant living creatures on Earth, having been present for perhaps three-quarters of Earth history and having adapted to almost all available ecological habitats. As a group, they display exceedingly <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diverse" href="https://www.merriam-webster.com/dictionary/diverse" data-type="MW">diverse</a> metabolic capabilities and can use almost any <a href="https://www.britannica.com/science/organic-compound" class="md-crosslink " data-show-preview="true">organic compound</a>, and some <a href="https://www.britannica.com/science/inorganic-compound" class="md-crosslink " data-show-preview="true">inorganic compounds</a>, as a food source. Some bacteria can cause diseases in humans, animals, or plants, but most are harmless and are <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="beneficial" href="https://www.merriam-webster.com/dictionary/beneficial" data-type="MW">beneficial</a> ecological agents whose metabolic activities sustain higher life-forms. Other bacteria are <a href="https://www.britannica.com/science/symbiosis" class="md-crosslink " data-show-preview="true">symbionts</a> of plants and invertebrates, where they carry out important functions for the host, such as <a href="https://www.britannica.com/science/nitrogen-fixation" class="md-crosslink " data-show-preview="true">nitrogen fixation</a> and <a href="https://www.britannica.com/science/cellulose" class="md-crosslink " data-show-preview="true">cellulose</a> <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="degradation" href="https://www.merriam-webster.com/dictionary/degradation" data-type="MW">degradation</a>. Without prokaryotes, soil would not be fertile, and dead organic material would decay much more slowly. Some bacteria are widely used in the preparation of foods, chemicals, and <a href="https://www.britannica.com/science/antibiotic" class="md-crosslink " data-show-preview="true">antibiotics</a>. Studies of the relationships between different groups of bacteria continue to yield new insights into the origin of life on Earth and mechanisms of <a href="https://www.britannica.com/science/evolution-scientific-theory" class="md-crosslink " data-show-preview="true">evolution</a>.</p><span class="marker MOD2 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref39333"> <h2 class="h1">The <span id="ref463521"></span><a href="https://www.britannica.com/science/prokaryote" class="md-crosslink " data-show-preview="true">bacterial</a> cell</h2> <section data-level="2" id="ref272356"> <h2 class="h2">Bacteria as <span id="ref463518"></span><a href="https://www.britannica.com/science/prokaryote" class="md-crosslink " data-show-preview="true">prokaryotes</a></h2> <span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="102103" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/48203/102103"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg"><img src="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg?w=300" alt="Bacterial, animal, and plant cells compared" data-width="1600" data-height="1600" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg" data-href="/media/1/48203/102103">Bacterial, animal, and plant cells compared</a><span>Bacterial cells differ from animal cells and plant cells in several ways. One fundamental difference is that bacterial cells lack intracellular organelles, such as mitochondria, chloroplasts, and a nucleus, which are present in both animal cells and plant cells.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">All living organisms on Earth are made up of one of two basic types of cells: <a href="https://www.britannica.com/science/eukaryote" class="md-crosslink " data-show-preview="true">eukaryotic</a> cells, in which the genetic material is enclosed within a nuclear membrane, or prokaryotic cells, in which the genetic material is not separated from the rest of the <a href="https://www.britannica.com/science/cell-biology" class="md-crosslink autoxref " data-show-preview="true">cell</a>. Traditionally, all prokaryotic cells were called bacteria and were classified in the prokaryotic kingdom <span id="ref475161"></span><a href="https://www.britannica.com/science/moneran" class="md-crosslink " data-show-preview="true">Monera</a>. However, their <a href="https://www.britannica.com/science/classification-biology" class="md-crosslink autoxref " data-show-preview="true">classification</a> as Monera, equivalent in <a href="https://www.britannica.com/science/taxonomy" class="md-crosslink " data-show-preview="true">taxonomy</a> to the other kingdoms—Plantae, <a href="https://www.britannica.com/animal/animal" class="md-crosslink autoxref " data-show-preview="true">Animalia</a>, <a href="https://www.britannica.com/science/fungus" class="md-crosslink autoxref " data-show-preview="true">Fungi</a>, and Protista—understated the remarkable genetic and metabolic <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diversity" href="https://www.merriam-webster.com/dictionary/diversity" data-type="MW">diversity</a> exhibited by prokaryotic cells relative to eukaryotic cells. In the late 1970s American microbiologist <a href="https://www.britannica.com/biography/Carl-Woese" class="md-crosslink " data-show-preview="true">Carl Woese</a> pioneered a major change in classification by placing all organisms into three domains—Eukarya, Bacteria (originally called Eubacteria), and <a href="https://www.britannica.com/science/archaea" class="md-crosslink " data-show-preview="true">Archaea</a> (originally called Archaebacteria)—to reflect the three ancient lines of <a href="https://www.britannica.com/science/evolution-scientific-theory" class="md-crosslink " data-show-preview="true">evolution</a>. The prokaryotic organisms that were formerly known as bacteria were then divided into two of these domains, Bacteria and Archaea. Bacteria and Archaea are superficially similar; for example, they do not have intracellular organelles, and they have circular <a href="https://www.britannica.com/science/DNA" class="md-crosslink " data-show-preview="true">DNA</a>. However, they are fundamentally distinct, and their separation is based on the genetic evidence for their ancient and separate evolutionary lineages, as well as fundamental differences in their chemistry and physiology. Members of these two prokaryotic domains are as different from one another as they are from eukaryotic cells.</p><span class="marker MOD3 mod-inline"></span> <span class="marker PREMOD4 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="707" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/66/6166-050-27423137/drawing-structure-cell-bacillus-type.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/48203/707"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/66/6166-050-27423137/drawing-structure-cell-bacillus-type.jpg"><img src="https://cdn.britannica.com/66/6166-050-27423137/drawing-structure-cell-bacillus-type.jpg?w=300" alt="Bacillus-type bacterial cell" data-width="1599" data-height="1102" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/66/6166-050-27423137/drawing-structure-cell-bacillus-type.jpg" data-href="/media/1/48203/707">Bacillus-type bacterial cell</a><span>Schematic drawing of the structure of a typical bacterial cell of the bacillus type.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">Prokaryotic cells (i.e., Bacteria and Archaea) are fundamentally different from the <span id="ref463519"></span><a href="https://www.britannica.com/science/eukaryote" class="md-crosslink " data-show-preview="true">eukaryotic</a> cells that <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constitute" href="https://www.merriam-webster.com/dictionary/constitute" data-type="MW">constitute</a> other forms of life. Prokaryotic cells are defined by a much simpler design than is found in eukaryotic cells. The most-apparent simplification is the lack of intracellular <span id="ref463520"></span><a href="https://www.britannica.com/science/organelle" class="md-crosslink " data-show-preview="true">organelles</a>, which are features characteristic of eukaryotic cells. Organelles are discrete membrane-enclosed structures that are contained in the <a href="https://www.britannica.com/science/cytoplasm" class="md-crosslink " data-show-preview="true">cytoplasm</a> and include the nucleus, where genetic information is retained, copied, and expressed; the <a href="https://www.britannica.com/science/mitochondrion" class="md-crosslink " data-show-preview="true">mitochondria</a> and <a href="https://www.britannica.com/science/chloroplast" class="md-crosslink " data-show-preview="true">chloroplasts</a>, where chemical or light energy is converted into metabolic energy; the <a href="https://www.britannica.com/science/lysosome" class="md-crosslink " data-show-preview="true">lysosome</a>, where ingested proteins are digested and other nutrients are made available; and the <a href="https://www.britannica.com/science/endoplasmic-reticulum" class="md-crosslink " data-show-preview="true">endoplasmic reticulum</a> and the <a href="https://www.britannica.com/science/Golgi-apparatus" class="md-crosslink " data-show-preview="true">Golgi apparatus</a>, where the <a href="https://www.britannica.com/science/protein" class="md-crosslink " data-show-preview="true">proteins</a> that are synthesized by and released from the cell are assembled, modified, and exported. All of the activities performed by organelles also take place in bacteria, but they are not carried out by specialized structures. In addition, prokaryotic cells are usually much smaller than eukaryotic cells. The small size, simple design, and broad metabolic capabilities of bacteria allow them to grow and divide very rapidly and to inhabit and flourish in almost any environment.</p><div class="one-good-fact-module"> </div><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="302395" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="253598"><a data-id="253598" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/What-is-the-difference-between-bacteria-and-viruses/-302395"><img src="https://cdn.britannica.com/98/253598-138-7DAEFFA9/What-is-the-difference-between-bacteria-and-viruses.jpg?w=800&h=450&c=crop" alt="The difference between bacteria and viruses" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/p6jedgj4.m3u8" } ], "image": "https://cdn.britannica.com/98/253598-138-7DAEFFA9/What-is-the-difference-between-bacteria-and-viruses.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/udt8IPmp.vtt", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/WYrpXBZx.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="253598">The difference between bacteria and viruses</span><span>Knowing the difference isn't just a technical distinction—it can save lives.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/bacteria/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">Prokaryotic and eukaryotic cells differ in many other ways, including <a href="https://www.britannica.com/science/lipid" class="md-crosslink autoxref " data-show-preview="true">lipid</a> <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="composition" href="https://www.merriam-webster.com/dictionary/composition" data-type="MW">composition</a>, structure of key metabolic <a href="https://www.britannica.com/science/enzyme" class="md-crosslink " data-show-preview="true">enzymes</a>, responses to antibiotics and <a href="https://www.britannica.com/science/toxin" class="md-crosslink " data-show-preview="true">toxins</a>, and the mechanism of expression of genetic information. Eukaryotic organisms contain multiple linear chromosomes with genes that are much larger than they need to be to encode the synthesis of proteins. Substantial portions of the ribonucleic acid (<a href="https://www.britannica.com/science/RNA" class="md-crosslink " data-show-preview="true">RNA</a>) copy of the genetic information (deoxyribonucleic acid, or DNA) are discarded, and the remaining <a href="https://www.britannica.com/science/messenger-RNA" class="md-crosslink " data-show-preview="true">messenger RNA</a> (mRNA) is substantially modified before it is translated into <a href="https://www.britannica.com/science/protein" class="md-crosslink autoxref " data-show-preview="true">protein</a>. In contrast, bacteria have one circular <a href="https://www.britannica.com/science/chromosome" class="md-crosslink autoxref " data-show-preview="true">chromosome</a> that contains all of their genetic information, and their mRNAs are exact copies of their <a href="https://www.britannica.com/science/gene" class="md-crosslink " data-show-preview="true">genes</a> and are not modified.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/viruses-bacteria-and-diseases" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/11/167111-131-607BA64C/woman-Caucasian-hand-pain-face-lockjaw-toothache.jpg" alt="Adult Caucasian woman with hand on her face as if in pain. lockjaw, toothache, healthcare and medicine, human jaw bone, female" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Viruses, Bacteria, and Diseases</div> </div> </a><span class="marker MOD5 mod-inline"></span> </section> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-simplify-root"></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":18,"pageId":48203,"pageLength":763,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.36"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-page.js?v=3.133.36' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":true} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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