Merck Manual

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Overview of Bacteria


Brian J. Werth

, PharmD, University of Washington School of Pharmacy

Reviewed/Revised May 2022
Topic Resources

Bacteria are microorganisms that have circular double-stranded DNA and (except for mycoplasmas) cell walls. Most bacteria live extracellularly, but some preferentially reside and replicate intracellularly. Obligate intracellular pathogens are able to grow, reproduce, and cause disease only within the cells of the host. Examples of these pathogens include Chlamydiae, Chlamydophila species, and rickettsiae. Facultative intracellular pathogens are able to live and reproduce either inside or outside of host cells. Examples of these pathogens include Salmonella typhi Overview of Salmonella Infections The genus Salmonella is divided into 2 species, S. enterica and S. bongori, which include > 2500 known serotypes. Some of these serotypes are named. In such cases, common... read more , Brucella Brucellosis Brucellosis is caused by Brucella species, which are gram-negative bacteria. Symptoms begin as an acute febrile illness with few or no localized signs and may progress to a chronic stage... read more species, Francisella tularensis Tularemia Tularemia is a febrile disease caused by the gram-negative bacterium Francisella tularensis; it may resemble typhoid fever. Symptoms are a primary local ulcerative lesion, regional lymphadenopathy... read more Tularemia , Neisseria gonorrhoeae Gonorrhea Gonorrhea is caused by the bacterium Neisseria gonorrhoeae. It typically infects epithelia of the urethra, cervix, rectum, pharynx, or conjunctivae, causing irritation or pain and purulent... read more Gonorrhea , N. meningitidis Meningococcal Diseases Meningococci (Neisseria meningitidis) are gram-negative diplococci that cause meningitis and meningococcemia. Symptoms, usually severe, include headache, nausea, vomiting, photophobia... read more Meningococcal Diseases , Legionella Legionella Infections Legionella pneumophila is a gram-negative bacillus that most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic or urine antigen... read more <i >Legionella</i> Infections and Listeria Listeriosis Listeriosis is bacteremia, meningitis, cerebritis, dermatitis, an oculoglandular syndrome, intrauterine and neonatal infections, or rarely endocarditis caused by Listeria species. Symptoms... read more species, and Mycobacterium tuberculosis Tuberculosis (TB) Tuberculosis is a chronic, progressive mycobacterial infection, often with an asymptomatic latent period following initial infection. Tuberculosis most commonly affects the lungs. Symptoms include... read more Tuberculosis (TB) .

Many bacteria are present in humans as normal microbiota, often in large numbers and in many areas (eg, in the gastrointestinal tract and skin). Only a few bacterial species are human pathogens.

Bacteria are classified by the following criteria (see also table Classification of Common Pathogenic Bacteria Classification of Common Pathogenic Bacteria Classification of Common Pathogenic Bacteria ):

  • Morphology

  • Staining

  • Encapsulation

  • Oxygen requirements


Bacteria may be

  • Cylindric (bacilli)

  • Spherical (cocci)

  • Spiral (spirochetes)

A few coccal, many bacillary, and most spirochetal species are motile.


Gram stain is the most common stain for general bacterial identification. Gram-positive bacteria retain crystal violet dye (appearing dark blue) after iodine fixation, alcohol decolorization, and counterstaining with safranin; gram-negative bacteria, which do not retain crystal violet, appear red. Gram-negative bacteria have an additional outer membrane containing lipopolysaccharide (endotoxin), increasing the virulence of these bacteria. (For other factors that enhance bacterial pathogenicity, see Factors Facilitating Microbial Invasion Factors Facilitating Microbial Invasion Microbial invasion can be facilitated by the following: Virulence factors Microbial adherence Antimicrobial resistance Defects in host defense mechanisms read more .)

Fluorochrome stains (eg, auramine-rhodamine) also identify acid-fast organisms, but a special fluorescent microscope is required.


Oxygen requirements

Aerobic bacteria (obligate aerobes) require oxygen to produce energy and to grow in culture. They produce energy using aerobic cellular respiration.

Anaerobic bacteria (obligate anaerobes Overview of Anaerobic Bacteria Bacteria can be classified by their need and tolerance for oxygen: Facultative: Grow aerobically or anaerobically in the presence or absence of oxygen Microaerophilic: Require a low oxygen concentration... read more ) do not require oxygen and do not grow in culture if air is present. They produce energy using fermentation or anaerobic respiration. Anaerobic bacteria are common in the gastrointestinal tract, vagina, dental crevices, and chronic wounds when blood supply is impaired.

Facultative bacteria can grow with or without oxygen. They produce energy by fermentation or anaerobic respiration when oxygen is absent and by aerobic cellular respiration when oxygen is present. Microaerophilic bacteria prefer a reduced oxygen tension (eg, 2 to 10%).

Chlamydiae are obligate intracellular parasites that acquire energy from the host cell and do not produce it themselves.


Drugs Mentioned In This Article

Drug Name Select Trade
Ablysinol, Nozin
PROVAYBLUE, Urolene Blue
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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