Bacteria are microorganisms that have circular double-stranded DNA and (except for mycoplasmas) cell walls. Most bacteria live extracellularly. Some bacteria (eg, Salmonella typhi; Neisseria gonorrhoeae; Legionella, Mycobacterium, Chlamydia, and Chlamydophila spp) preferentially reside and replicate intracellularly. Some bacteria such as chlamydiae and rickettsiae are obligate intracellular pathogens (ie, able to grow, reproduce, and cause disease only within the cells of the host). Others (eg, Salmonella typhi, Brucella sp, Francisella tularensis, N. gonorrhoeae, N. meningitidis, Legionella and Listeria spp, Mycobacterium tuberculosis) are facultative intracellular pathogens.

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

Bacteria are classified by the following criteria (see Table 1: Bacteria and Antibacterial Drugs: Classification of Common Pathogenic Bacteria).

Morphology

Bacteria may be cylindric (bacilli), spherical (cocci), or spiral (spirochetes). A few coccal, many bacillary, and most spirochetal species are motile.

Staining

The most common stain for general bacterial identification is Gram stain. Gram-positive bacteria retain crystal violet dye (appearing dark blue) after iodine fixation and alcohol decolorization; gram-negative bacteria do not. 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 Biology of Infectious Disease: Factors Facilitating Microbial Invasion.)

Ziehl-Neelsen stain (acid-fast stain) is used to identify mainly mycobacteria, particularly M. tuberculosis. It also can identify Nocardia sp. Carbolfuchsin is applied with heat, followed by decolorization with hydrochloric acid and ethanol and counterstaining with methylene blue.

Encapsulation

Some bacteria are enclosed in capsules; for some encapsulated bacteria (eg, Streptococcus pneumoniae, Haemophilus influenzae), the capsule helps protect them from ingestion by phagocytes. Encapsulation increases bacterial virulence.

Oxygen requirements

Aerobic bacteria (obligate aerobes) require O₂ to produce energy and to grow in culture. They produce energy using aerobic cellular respiration.

Anaerobic bacteria (obligate anaerobes) do not require O₂ and do not grow in culture if air is present. They produce energy using fermentation or anaerobic respiration. Anaerobic bacteria are common in the GI tract, vagina, dental crevices, and wounds when blood supply is impaired.

Facultative bacteria can grow with or without O₂. They produce energy by fermentation or anaerobic respiration when O₂ is absent and by aerobic cellular respiration when O₂ is present. Microaerophilic bacteria prefer a reduced O₂ tension (eg, 2 to 10%). Chlamydiae are obligate intracellular parasites that acquire energy from the host cell and do not produce it themselves.

Table 1
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Classification of Common Pathogenic Bacteria Type Bacteria Obligate aerobic Gram-negative cocci Moraxella catarrhalis, Neisseria gonorrhoeae, N. meningitidis Gram-positive bacilli Corynebacterium jeikeium Acid-fast bacilli Mycobacterium avium complex, M. kansasii, M. leprae, M. tuberculosis, Nocardia sp Nonfermentative, non-Enterobacteriaceae Acinetobacter calcoaceticus, Flavobacterium meningosepticum, Pseudomonas aeruginosa, P. alcaligenes, other Pseudomonas sp, Stenotrophomonas maltophilia Fastidious gram-negative coccobacilli and bacilli Brucella, Bordetella, Francisella, and Legionella spp Treponemataceae (spiral bacteria) Leptospira sp Obligate anaerobic Gram-negative bacilli Bacteroides fragilis, other Bacteroides sp, Fusobacterium sp, Prevotella sp Gram-negative cocci Veillonella sp Gram-positive cocci Peptococcus niger, Peptostreptococcus sp Non–spore-forming gram-positive bacilli Actinomyces, Bifidobacterium, Eubacterium, and Propionibacterium spp Endospore-forming gram-positive bacilli Clostridium botulinum, C. perfringens, C. tetani, other Clostridium sp Facultative anaerobic Gram-positive cocci, catalase-positive Staphylococcus aureus (coagulase-positive), S. epidermidis (coagulase-negative), other coagulase-negative staphylococci Gram-positive cocci, catalase-negative Enterococcus faecalis, E. faecium, Streptococcus agalactiae (group B streptococcus), S. bovis, S. pneumoniae, S. pyogenes (group A streptococcus), viridans group streptococci (S. mutans, S. mitis, S. salivarius, S. sanguis), S. anginosus group (S. anginosus, S. milleri, S. constellatus), Gemella morbillorum Gram-positive bacilli Bacillus anthracis, Erysipelothrix rhusiopathiae, Gardnerella vaginalis (gram-variable) Gram-negative bacilli Enterobacteriaceae (Citrobacter sp, Enterobacter aerogenes, Escherichia coli, Klebsiella sp, Morganella morganii, Proteus sp, Providencia rettgeri, Salmonella typhi, other Salmonella sp, Serratia marcescens, Shigella sp, Yersinia enterocolitica, Y. pestis) Fermentative, non-Enterobacteriaceae Aeromonas hydrophila, Chromobacterium violaceum, Pasturella multocida, Plesiomonas shigelloides Fastidious gram-negative coccobacilli and bacilli Actinobacillus actinomycetemcomitans, Bartonella bacilliformis, B. henselae, B. quintana, Eikenella corrodens, Haemophilus influenzae, other Haemophilus sp Mycoplasma Mycoplasma pneumoniae Treponemataceae (spiral bacteria) Borrelia burgdorferi, Treponema pallidum Microaerophilic Curved bacilli Campylobacter jejuni, Helicobacter pylori, Vibrio cholerae, V. vulnificus Obligate intracellular parasitic Chlamydiaceae Chlamydia trachomatis, Chlamydophila pneumoniae, C. psittaci Coxiellaceae Coxiella burnetii Rickettsiales Rickettsia prowazekii, R. rickettsii, R. typhi, R. tsutsugamushi, Ehrlichia chaffeensis, Anaplasma phagocytophilum Classification of Common Pathogenic Bacteria Type Bacteria Obligate aerobic Gram-negative cocci Moraxella catarrhalis, Neisseria gonorrhoeae, N. meningitidis Gram-positive bacilli Corynebacterium jeikeium Acid-fast bacilli Mycobacterium avium complex, M. kansasii, M. leprae, M. tuberculosis, Nocardia sp Nonfermentative, non-Enterobacteriaceae Acinetobacter calcoaceticus, Flavobacterium meningosepticum, Pseudomonas aeruginosa, P. alcaligenes, other Pseudomonas sp, Stenotrophomonas maltophilia Fastidious gram-negative coccobacilli and bacilli Brucella, Bordetella, Francisella, and Legionella spp Treponemataceae (spiral bacteria) Leptospira sp Obligate anaerobic Gram-negative bacilli Bacteroides fragilis, other Bacteroides sp, Fusobacterium sp, Prevotella sp Gram-negative cocci Veillonella sp Gram-positive cocci Peptococcus niger, Peptostreptococcus sp Non–spore-forming gram-positive bacilli Actinomyces, Bifidobacterium, Eubacterium, and Propionibacterium spp Endospore-forming gram-positive bacilli Clostridium botulinum, C. perfringens, C. tetani, other Clostridium sp Facultative anaerobic Gram-positive cocci, catalase-positive Staphylococcus aureus (coagulase-positive), S. epidermidis (coagulase-negative), other coagulase-negative staphylococci Gram-positive cocci, catalase-negative Enterococcus faecalis, E. faecium, Streptococcus agalactiae (group B streptococcus), S. bovis, S. pneumoniae, S. pyogenes (group A streptococcus), viridans group streptococci (S. mutans, S. mitis, S. salivarius, S. sanguis), S. anginosus group (S. anginosus, S. milleri, S. constellatus), Gemella morbillorum Gram-positive bacilli Bacillus anthracis, Erysipelothrix rhusiopathiae, Gardnerella vaginalis (gram-variable) Gram-negative bacilli Enterobacteriaceae (Citrobacter sp, Enterobacter aerogenes, Escherichia coli, Klebsiella sp, Morganella morganii, Proteus sp, Providencia rettgeri, Salmonella typhi, other Salmonella sp, Serratia marcescens, Shigella sp, Yersinia enterocolitica, Y. pestis) Fermentative, non-Enterobacteriaceae Aeromonas hydrophila, Chromobacterium violaceum, Pasturella multocida, Plesiomonas shigelloides Fastidious gram-negative coccobacilli and bacilli Actinobacillus actinomycetemcomitans, Bartonella bacilliformis, B. henselae, B. quintana, Eikenella corrodens, Haemophilus influenzae, other Haemophilus sp Mycoplasma Mycoplasma pneumoniae Treponemataceae (spiral bacteria) Borrelia burgdorferi, Treponema pallidum Microaerophilic Curved bacilli Campylobacter jejuni, Helicobacter pylori, Vibrio cholerae, V. vulnificus Obligate intracellular parasitic Chlamydiaceae Chlamydia trachomatis, Chlamydophila pneumoniae, C. psittaci Coxiellaceae Coxiella burnetii Rickettsiales Rickettsia prowazekii, R. rickettsii, R. typhi, R. tsutsugamushi, Ehrlichia chaffeensis, Anaplasma phagocytophilum

Last full review/revision July 2009 by Matthew E. Levison, MD