Within a very short period immunity

The adaptive side is sometimes called specific immunity because it has the ability to develop new responses that are highly specific to molecular components of infectious agents, called antigens. These encounters trigger the development of new cellular responses and production of circulating antibodies, which have a component of memory if the invader returns. Artificially creating this memory is, of course, the goal of vaccines.

Innate immunity includes the role of physical, cellular, and chemical systems that are in place and that respond to all aspects of “foreignness.” These include mucosal barriers, phagocytic cells, and the action of circulating glycoproteins such as complement.

The immune response to infection is presented as two major components—innate immunity and adaptive immunity. The primary effectors of both are cells that are members of the white blood cell series derived from hematopoietic stem cells in the bone marrow

knowledge of the immune response to infection is integral to understanding the pathogenesis of infectious diseases. It turns out that one of the main attributes of a successful pathogen is evading or confounding the immune system.

Some factors that increase emergence or reemergence of infectious pathogens include: ++ Human and animal demographics and population movement with intrusion into new habitats (particularly tropical forests) Irrigation, especially primitive irrigation systems, which fail to control arthropods and enteric organisms Uncontrolled urbanization, with vector populations breeding in stagnant water Increased international commerce and travel with contact or transport of vectors and pathogens (globalization) Breakdown in public health measures, including sanitation, vector control, immunization programs related to social unrest, civil wars, and major natural disasters Ecological changes, including global climate change and deforestation, with farmers and their animals exposed to new arthropods, floods, and drought Microbial evolution whether related to indiscriminate use of anti-infective ... Your Access profile is currently affiliated with '[InstitutionA]' and is in the process of switching affiliations to '[InstitutionB]'. Please click ‘Continue’ to continue the affiliation switch, otherwise click ‘Cancel’ to cancel signing in. Get Free Access Through Your Institution Learn how to see if your library subscribes to McGraw Hill Medical products. Subscribe: Institutional or Individual Sign In Username Error: Please enter User Name Password Error: Please enter Password Forgot Password? Forgot Username? Sign in via OpenAthens Sign in via Shibboleth Pop-up div Successfully Displayed This div only appears when the trigger link is hovered over. Otherwise it is hidden from view. Please Wait AccessMedicine Books Library Updates Quick Reference Quick Medical Diagnosis & Treatment 2 Minute Medicine® DDx Diagnostic Tests Goodman & Gilman’s : FDA Approvals Huppert's Notes Guidelines — Primary Care Infographics Drugs Multimedia Auscultation Classroom Big Picture Physiology Videos Diagnostic and Imaging Studies Harrison's Pathophysiology Animations Human Anatomy Modules Lectures Patient Interview Patient Safety Modules Pharmacology Physical Exam POCUS Pearls Podcasts Procedural Videos Cases Case Files® - Preview Clinical Neuroanatomy Cases Critical Concept Mastery Series Family Medicine Board Review Fluid/Electrolyte and Acid-Base Cases G&G Pharm Cases Harrison’s Visual Case Challenge Internal Medicine Cases Medical Microbiology Cases Pathophysiology Cases Principles of Rehabilitation Medicine Case-Based Board Review Vanderbilt IM/Peds Curriculum Study Tools Review Questions Clerkship Topics Flashcards Patient Ed About AccessMedicine Advisory Board Hospital Corner For Instructors McGraw Hill Medical Sites AccessAnesthesiology AccessAPN Accessartmed AccessBiomedical Science AccessCardiology AccessDermatologyDxRx AccessEmergency Medicine AccessHemOnc AccessMedicina AccessMedicine AccessNeurology AccessObGyn AccessPediatrics AccessPharmacy AccessPhysiotherapy AccessSurgery Case Files Collection Clinical Sports Medicine Collection F.A. Davis AT Collection F.A. Davis PT Collection JAMAevidence Murtagh Collection OMMBID Pharmacotherapy Principles & Practice Support Training & Support Contact Us Submit Feedback Subscribe Institutional Subscriptions Individual Subscriptions McGraw Hill Apple store Google Play See Terms Copyright © McGraw HillAll rights reserved.Your IP address is 105.161.30.222 Terms of Use   •  Privacy Policy   •  Notice   •  Accessibility   •  Browser Support Silverchair Back To Top .index .dynamic-widget-module.flex-container .text-wrap { padding: 0; } .index .dynamic-widget-module.flex-container > .dynamic-widget-inner-wrap { padding: 1rem; } .index .dynamic-widget-module.flex-container>.dynamic-widget-inner-wrap h2 { position: relative; top: 0; } .index .dynamic-widget-module.flex-container>.dynamic-widget-inner-wrap .item { padding: 0; } .index .dynamic-widget-module.flex-container>.dynamic-widget-inner-wrap [class*='ssHomeWatch'] h2 { position: absolute; top: -2rem; } .index .dynamic-widget-module.flex-container>.dynamic-widget-inner-wrap [class*='ssHomeLearn'] h2 { position: absolute; top: -2rem; } .index .dynamic-widget-module.flex-container>.dynamic-widget-inner-wrap [class*='ssExtrabox'] h2 { position: absolute; top: -2rem; } /*MHGEN-8550*/ .cmpl_iframe_div.cadmore-player-wrap { padding-top: 130%; } .pg_umbrella-main .carousel-item { width: auto; } .pg_umbrella-main .carousel-item img { max-height: 350px; } .pg_umbrella-main .widget-instance-ssUmbrellaAccessCards img { max-height: 300px; } .widget-SitesDropdown .site-name { margin-top: .5rem !important; } .widget-SitesDropdown .site-name a, .user-services a { color: #333 !important; font-weight: bold; text-transform: uppercase; } #micrositeHeader_liAccessMedicina a { line-height: 52px; } //Track Audio Only for JE $('.dropdown.audio a').click(function (e) { //DO Tracking DoClientTrackingForMGH($(this), 'https://books.mcgraw-hill.com/podcast/jamaevidence/'); }); $(function () { $("#microsite-footer li.users-guides-to-the-medical-literature a").each(function () { var getContent = $(this).text(); var newString = getContent.replace('to the Medical Literature', '<br />to the Medical Literature'); $(this).html(newString); }); $('.for-librarians').find('a').removeClass().attr('target', '_blank'); }); //<![CDATA[ var Page_Validators = new Array(document.getElementById("micrositeHeader_RequiredFieldValidator1")); //]]> //<![CDATA[ var micrositeHeader_RequiredFieldValidator1 = document.all ? document.all["micrositeHeader_RequiredFieldValidator1"] : document.getElementById("micrositeHeader_RequiredFieldValidator1"); micrositeHeader_RequiredFieldValidator1.controltovalidate = "txtSearchTerm"; micrositeHeader_RequiredFieldValidator1.errormessage = "Please Enter a Search Term"; micrositeHeader_RequiredFieldValidator1.validationGroup = "headersearch"; micrositeHeader_RequiredFieldValidator1.evaluationfunction = "RequiredFieldValidatorEvaluateIsValid"; micrositeHeader_RequiredFieldValidator1.initialvalue = ""; //]]> //<![CDATA[ var Page_ValidationActive = false; if (typeof(ValidatorOnLoad) == "function") { ValidatorOnLoad(); } function ValidatorOnSubmit() { if (Page_ValidationActive) { return ValidatorCommonOnSubmit(); } else { return true; } } document.getElementById('micrositeHeader_RequiredFieldValidator1').dispose = function() { Array.remove(Page_Validators, document.getElementById('micrositeHeader_RequiredFieldValidator1')); } //]]> (function(){function b(){!1===c&&(c=!0,Munchkin.init("303-FKF-702"))}var c=!1,a=document.createElement("script");a.type="text/javascript";a.async=!0;a.src="//munchkin.marketo.net/munchkin.js";a.onreadystatechange=function(){"complete"!=this.readyState&&"loaded"!=this.readyState||b()};a.onload=b;document.getElementsByTagName("head")[0].appendChild(a)})(); !function(b,e,f,g,a,c,d){b.fbq||(a=b.fbq=function(){a.callMethod?a.callMethod.apply(a,arguments):a.queue.push(arguments)},b._fbq||(b._fbq=a),a.push=a,a.loaded=!0,a.version="2.0",a.queue=[],c=e.createElement(f),c.async=!0,c.src=g,d=e.getElementsByTagName(f)[0],d.parentNode.insertBefore(c,d))}(window,document,"script","https://connect.facebook.net/en_US/fbevents.js");fbq("init","472393460782729");fbq("track","PageView"); <img height="1" width="1" style="display:none" src="https://www.facebook.com/tr?id=472393460782729&amp;ev=PageView&amp;noscript=1"> !function(b,e,f,g,a,c,d){b.fbq||(a=b.fbq=function(){a.callMethod?a.callMethod.apply(a,arguments):a.queue.push(arguments)},b._fbq||(b._fbq=a),a.push=a,a.loaded=!0,a.version="2.0",a.queue=[],c=e.createElement(f),c.async=!0,c.src=g,d=e.getElementsByTagName(f)[0],d.parentNode.insertBefore(c,d))}(window,document,"script","https://connect.facebook.net/en_US/fbevents.js");fbq("init","491404515608644");fbq("track","PageView"); <img height="1" width="1" style="display:none" src="https://www.facebook.com/tr?id=491404515608644&amp;ev=PageView&amp;noscript=1"> !function(b,e,f,g,a,c,d){b.saq||(a=b.saq=function(){a.callMethod?a.callMethod.apply(a,arguments):a.queue.push(arguments)},b._saq||(b._saq=a),a.push=a,a.loaded=!0,a.version="1.0",a.queue=[],c=e.createElement(f),c.async=!0,c.src=g,d=e.getElementsByTagName(f)[0],d.parentNode.insertBefore(c,d))}(window,document,"script","https://tags.srv.stackadapt.com/events.js");saq("ts","gzdqW7I-rc7jNTBJ8xilNw"); Display.Common.DefaultUpdateTarget("ucDefaultUpdateTarget", asdfasdf) $(function () { // hide right widget column for front matter if (0 == 1) { $('#toolbox-widgets').hide(); } if ($('.emptyBookmarked').length) { $('.view-favorites').css('display', 'none'); $('.no-filtered-favorites').css('display', 'inline'); } else { $('.view-favorites').css('display', 'inline'); $('.no-filtered-favorites').css('display', 'none'); } // expand main content column for guideline book if (0 == 1) { if (String("RyanMedMic8").valueOf() == "guid") { $('.row').addClass("row-fluid").removeClass("row"); $('article').removeClass("span6").addClass("span12").show(); $('#content-navbar').removeClass("span6").addClass("span12").show(); } else { // others, just expand to the right column but leave the section nav to the left $('article').removeClass("span6").addClass("span9").show(); $('#content-navbar').removeClass("span6").addClass("span9").show(); } } if ("False" == "True") { $('.searchTextBook').hide(); } $('article').show(); $('#content-navbar').show(); scrollToHash(); getFavoriteStar(); window.addEventListener("SCM.AddFavorites.Complete", function (e) { $('.widget-AddFavorites').each(function () { $(this).addClass('rs_skip_always'); $(this).addClass('rs_preserve') }); }); $(document).ajaxComplete(function () { var cmeClinicalQuestionElement = $('.widget-CmeClinicalQuestions'); if (cmeClinicalQuestionElement.length) { AssigningToCliniclaQUeries(); } }); }); var sectionID = utility.getUrlParams('sectionid'); addRecentlyViewed(sectionID, "section", false); <div class="loader"> <img src="Images/spinnerLarge.gif" /> </div> {{if d.glossaryTerm }} <div class="glossaryTerm">{{html d.glossaryTerm}}</div> <div class="glossaryDefinition">{{html d.glossaryDefinition}}</div> <div class="bordered-bottom"> <a href="/SearchResults.aspx?q={{html d.glossarySearchTerm}}">Run a search on this term</a> </div> <div>{{html d.glossarySource}}</div> {{else}} <div class="msg_error">No glossary found!</div> {{/if}} { "showHighlights": false } This site uses cookies to provide, maintain and improve your experience. MH Privacy

. New methods of detection (eg, molecular) and surveillance (eg, global) have greatly improved our ability to detect and characterize emerging and reemerging infectious diseases.

Emerging infectious diseases reflect the arrival of a new pathogen (newly emerging) or an old pathogen that is increasing in incidence, clinical or laboratory characteristics, or geographic range (re-emerging or resurging).

An emerging disease is an infectious disease whose incidence has increased in the past two decades and/or that threatens to increase soon

Knowledge of the principles and practice of epidemiology is essential for clinicians (those treating individual patients) and public health practitioners (those focused on the health of the community) alike. Care of patients with suspected infections requires consideration of the likelihood of possible exposures in the community (acquisition) and to the community (spread to others).

Epidemiologic studies have informed public health measures and thereby have been critical to the control of epidemics, such as those due to cholera, plague, smallpox, yellow fever, and typhus.

Epidemiology is the study of the distribution and determinants of disease, both infectious and noninfectious, and other perturbations in health.

Ideally, a test would have both excellent sensitivity and specificity

Sensitivity is capacity of test to rule OUT a diagnosis ❋ Specificity is ability of test to rule IN or confirm diagnosis

specificity (the test’s ability to rule in [spin] or confirm an etiology because there are few false-positive results)

These characteristics are sensitivity (the test’s ability to rule out [snout] a disease because there are few false-negative results and thus fewer cases missed)

The best results are obtained when communication between the clinician and laboratory is optimal.

A combination of science and art on the part of both the clinician and laboratory worker is required:

This clinical diagnosis suggests a number of possible etiologic agents based on knowledge of infectious syndromes and their courses. T

The diagnosis of a microbial infection begins with an assessment of the clinical and epidemiologic features and formulation of a diagnostic hypothesis. Anatomic localization of the infection depends on physical and radiologic findings (eg, right lower lobe pneumonia, subphrenic abscess).

Antiseptics are disinfecting agents that can be used on body surfaces, such as the skin or vaginal tract, to reduce the numbers of pathogenic agents in the local microbiota. They have lower toxicity than disinfectants used environmentally, but are usually less active in killing vegetative organisms. ...

It implies the destruction of pathogenic microorganisms by processes that fail to meet the criteria for sterilization.

Pasteurization is the use of heat at a temperature sufficient to inactivate important pathogenic organisms in liquids such as water or milk, but at a temperature lower than that needed to ensure sterilization.

t can be accomplished by incineration, nondestructive heat treatment, certain gases, exposure to ionizing radiation, some liquid chemicals, and filtration.

Sterilization is an absolute term. It means complete killing, or removal, of all living organisms from a particular location or material

Absence of growth does not necessarily indicate sterility

A period of exposure to visible light may then activate an enzyme that breaks the dimers and restores viability by a process known as photoreactivation. In addition, mechanisms exist for repair of the damage without light. Such considerations are of great significance in the preparation of safe vaccines from inactivated virulent organisms.

ultraviolet (UV) irradiation of bacteria can result in the formation of thymine dimers in the DNA with loss of ability to replicate. A period of exposure to visible light

methods work has become of increasing importance in an environment that includes immunocompromised patients, transplantation, indwelling devices, and Covid-19.

killing microbes before they reach patients has been a major strategy for preventing infection. In fact, Ignaz Semmelweis successfully applied disinfection principles decades before bacteria were first isolated.

Antibiotic therapy, particularly with broad-spectrum agents, may so alter the microbiota of the gastrointestinal tract

Sterile animals have little immunity Low exposure correlates with asthma

Intestinal lactobacilli may protect against diarrheal agents

Flora that reach sterile sites may cause disease Virulence factors increase opportunity for invasion

BV is associated with a shift in vaginal microbiota

ormonal changes affect the vaginal flora Use of epithelial glycogen by lactobacilli produces low pH

Lower tract is protected by mucociliary action

S aureus is carried in anterior nares

Colonic flora predominantly anaerobic C difficile causes colitis

The colon carries the most abundant and diverse microbiota in the body. In the adult, feces are 25% or more bacteria by weight (about 1010 organisms per gram). More than 90% are anaerobes, predominantly members of the genera Bacteroides, Fusobacterium, Eubacterium, and Clostridium.

Small intestinal flora is scanty but increases toward lower ileum

zymes on bacteria. One species, H pylori, long thought to be a common resident, is now known to be the primary cause of ulcers. The small intestine has a scanty resident flora, except in the lower ileum, where it begins to resemble that of the colon. ++ H pylor

The total number of organisms in the oral cavity is very high, and it varies from site to site. Saliva usually contains a mixed flora of about 108 organisms per milliliter, derived mostly from the various epithelial colonization sites. The genera include Actinomyces, Bacteroides, Prevotella, Streptococcus, and others.

Oropharynx has streptococci and anaerobes

The mouth and pharynx contain large numbers of facultative and anaerobic bacteria.

Propionibacteria, staphylococci dominant bacteria Skin flora is not easily removed

The flora is more abundant on moist skin areas (axillae, perineum, and between toes).

The skin surface provides a dry, slightly acidic, aerobic environment. It plays host to an abundant flora that varies according to the presence of its appendages (hair, nails) and the activity of sebaceous and sweat glands.

Such transient bacteremia may be the source of infection when structures such as damaged heart valves and foreign bodies (prostheses) are in the bloodstream.

In health, the blood, body fluids, and tissues are sterile. Occasional organisms may be displaced across epithelial barriers as a result of trauma or during childbirth; they may be briefly recoverable from the bloodstream before they are filtered out in the pulmonary capillaries or removed by cells of the reticuloendothelial system

Flora may stay for short or extended periods If pathogens involved, the relationship is called the carrier state

Physiologic conditions influence colonization Adherence counteracts mechanical flushing Must compete for nutrients

Initial flora acquired during and immediately after birth

role of the microbiota because of its significance both as a defense mechanism against infection and as a source of potentially pathogenic organisms. In addition, it is important for physicians to know the typical composition of the microbiota at various sites to avoid confusion when interpreting laboratory culture results.

The term carrier state is used when organisms known to be potentially pathogenic are involved, although its implication of risk is not always justified. For example, Streptococcus pneumoniae, a cause of pneumonia

Transients are acquired from the environment and establish themselves briefly, but they tend to be excluded by competition from residents or by the host’s innate or immune defense mechanisms

Residents are strains that have an established niche at one of the many body sites, which they occupy indefinitely.

Residents are strains that have an established niche at one of the many body sites, which they occupy indefinitely.

Organisms of the microbiota may have a symbiotic relationship that benefits the host or may simply live as commensals with a neutral relationship to the host

we harbor 10 times more microbial cells than human cells. This population, formerly called the normal flora, is now referred to as our microbiota or microbiome. These microorganisms, which are overwhelmingly bacteria, are frequently found colonizing various body sites in healthy individuals.

Most parasites are free living, but some depend on combinations of animal, arthropod, or crustacean hosts for their survival.

Parasites are the most diverse of all microorganisms. They range from unicellular amoebas of 10 to 12 μm to multicellular tapeworms 1 m long. The individual cell plan is eukaryotic, but organisms such as worms are highly differentiated and have their own organ systems

Most fungi are free living and widely distributed in nature. Generally, fungi grow more slowly than bacteria,

Fungi are eukaryotic, and both yeasts and molds have a rigid external cell wall composed of their own unique polymers, called glucan, mannan, and chitin. Their genome may exist in a diploid or haploid state and replicate by meiosis or simple mitosis.

multiply by budding

Fungi exist in either yeast or mold forms

The Archaea are similar to bacteria but evolutionarily distinct. They are prokaryotic, but they differ in the chemical structure of their cell walls and other features. The Archaea (archebacteria) can live in environments humans consider hostile (eg, hot springs, high salt areas) but are not associated with disease.

they divide by binary fission and can be grown in artificial culture, producing progeny sometimes in a matter of hours

Bacteria are the smallest (0.1-0 μm) independently living agents known. They have a cytoplasmic membrane surrounded by a cell wall; a unique interwoven polymer called peptidoglycan makes the wall rigid. The simple prokaryotic cell plan includes no mitochondria, lysosomes, endoplasmic reticulum, or other organelles (

The close association of the virus with the cell sometimes results in the integration of viral nucleic acid into the functional nucleic acid of the cell, producing a latent infection that can be transmitted intact to the progeny of the cell.

Infection of other cells by the newly formed viruses occurs either by seeding from or lysis of the infected cells.

Because viruses lack the protein-synthesizing enzymes and structural apparatus necessary for their own replication, they bear essentially no resemblance to a true eukaryotic or prokaryotic cell.

Viruses are strict intracellular parasites of other living cells, not only of mammalian and plant cells but also of simple unicellular organisms, including bacteria (the bacteriophages).

Viruses are not cells at all. They have a genome and some structural elements, but must take over the machinery of another living cell (eukaryotic or prokaryotic) to replicate.

Bacteria also have a cell wall, but with a cell plan called “prokaryotic” that lacks the organelles

Fungi are also eukaryotic, but they have a rigid external wall that makes them seem more like plants than animals.

Parasites exist as single or multicellular structures with the same compartmentalized eukaryotic cell plan of our own cells including a nucleus and cytoplasmic organelles like mitochondria

The major classes of microorganisms in terms of ascending size and complexity are viruses, bacteria, fungi, and parasites.

Some microbial species have adapted to a symbiotic relationship with higher forms of life. For example, bacteria that can fix atmospheric nitrogen colonize root systems of legumes and of a few trees, such as alders, and provide the plants with their nitrogen requirements.

the great advances in public health that initiated the decline in disease and death.

Microorganisms are responsible for much of the breakdown and natural recycling of organic material in the environment.

definition of microbiology as the study of microscopic living forms still holds if one can accept that some organisms can reproduce only within other cells (eg, all viruses and some bacteria) and that others include macroscopic forms in their life cycle (eg, fungal molds, parasitic worms).

Microbiology is a science defined by smallness. Its creation was made possible by the invention of the microscope

Almost as quickly, virtually all categories of infectious agents developed resistance to all categories of antimicrobial agents to counter these chemotherapeutic agents. +

In the first half of the 20th century, scientists studied the structure, physiology, and genetics of microbes in detail and began to answer questions relating to the links between specific microbial properties and disease

The science of medical microbiology dates back to the pioneering studies of Pasteur and Koch, who isolated specific agents and proved that they could cause disease by introducing the experimental method. The methods they developed lead to the first golden age of microbiology (1875-1910), when many bacterial diseases and the organisms responsible for them were defined. These efforts, combined with epidemiologic work begun by Semmelweis and Lister, which showed how these diseases spread

For students of medicine, understanding the fundamental basis of infectious diseases has more relevance than ever.

A new uneasiness that is part evolutionary, part discovery, and part diabolic has taken hold. Infectious agents once conquered have shown resistance to established therapy, such as multiresistant Mycobacterium tuberculosis, and diseases, such as acquired immunodeficiency syndrome (AIDS), have emerged. The spectrum of infection has widened, with discoveries that organisms earlier thought to be harmless can cause disease under certain circumstances.

The terror was due to the fact that, although some of the causes of infection were being discovered, little could be done to prevent or alter the course of disease. In the 20th century, advances in public sanitation and the development of vaccines and antimicrobial agents changed this (Figure 1–1), but only for the nations that can afford these interventions.

When Sir William Osler, the great physician/humanist, wrote these words, fever (infection) was indeed the scourge of the world. Tuberculosis and other forms of pulmonary infection were the leading causes of premature death among the well-to-do and the less fortunate

It has been estimated that we currently have the capacity to identify a surprisingly small number of the pathogens responsible for causing human disease.

The phase-contrast microscope was developed to improve contrast differences between cells and the surrounding medium, making it possible to see living cells without staining them

With this microscope, specimens are rendered visible because of the differences in contrast between them and the surrounding medium. Many bacteria are difficult to see well because of their lack of contrast with the surrounding medium. Dyes (stains) can be used to stain cells or their organelles and increase their contrast so that they can be more easily seen in the bright-field microscope. +++

These microscopes generally employ a 100-power objective lens with a 10-power ocular lens, thus magnifying the specimen 1000 times. Particles 0.2 µm in diameter are therefore magnified to about 0.2 mm and so become clearly visible. Further magnification would give no greater resolution of detail and would reduce the visible area (field).

The bright-field microscope is the most commonly used in microbiology courses and consists of two series of lenses (objective and ocular lens), which function together to resolve the image.

The useful magnification of a microscope is the magnification that makes visible the smallest resolvable particles.

The resolving power of the light microscope under ideal conditions is about half the wavelength of the light being used. (Resolving power is the distance that must separate two point sources of light if they are to be seen as two distinct images.)

the microscope first revealed the presence of bacteria and later the secrets of cell structure. Today, it remains a powerful tool in cell biology.

This chapter discusses the basic structure and function of the components that make up eukaryotic and prokaryotic cells

If the exchange operates primarily to the benefit of one party, the association is described as

. In biology, mutualism is called symbiosis, a continuing association of different organisms

Prediction, the practical outgrowth of science, is a product created by a blend of technique and theory. Biochemistry, molecular biology, and genetics provide the tools required for analysis of microorganisms. Microbiology, in turn, extends the horizons of these scientific disciplines

Nowhere is biologic diversity demonstrated more dramatically than by microorganisms, cells, or viruses that are not directly visible to the unaided eye. In form and function, be it biochemical property or genetic mechanism, analysis of microorganisms takes us to the limits of biologic understanding

8% of the DNA is derived from remnants of viral genomes.

Humans also have an intimate relationship with microorganisms; 50–60% of the cells in our bodies are microbes (see Chapter 10). The bacteria present in the average human gut weigh about 1 kg, and a human adult will excrete his or her own weight in fecal bacteria each year

They are responsible for cycling the chemical elements essential for life, including carbon, nitrogen, sulfur, hydrogen, and oxygen; more photosynthesis is carried out by microorganisms than by green plants

Microbiology is the study of microorganisms, a large and diverse group of microscopic organisms that exist as single cells or cell clusters; it also includes viruses, which are microscopic but not cellular