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, Mycobacteria, Rickettsia, Chlamydia, and Chlamydophila species) preferentially reside and replicate intracellularly. Some bacteria such as chlamydiae, Chlamydophila species, 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 species, Francisella tularensis, N. gonorrhoeae, Neisseria meningitidis, Legionella and Listeria species, 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 gastrointestinal tract). Only a few bacterial species are human pathogens.
Bacteria are classified by the following criteria (see also table Classification of Common Pathogenic Bacteria):
Morphology
Staining
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.)
Ziehl-Neelsen and Kinyoun stains are acid-fast stains used to identify mainly mycobacteria, particularly M. tuberculosis. They also can identify the gram-positive bacilli Nocardia and the protozoans Cryptosporidia, Cyclospora, and Cystoisospora. Carbolfuchsin is applied, followed by decolorization with hydrochloric acid and ethanol and then counterstaining with methylene blue.
Fluorochrome stains (eg, auramine-rhodamine) also identify acid-fast organisms, but a special fluorescent microscope is required.
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 oxygen to produce energy and to grow in culture. They produce energy using aerobic cellular respiration.
Anaerobic bacteria (obligate anaerobes) 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.
Classification of Common Pathogenic Bacteria
Type |
Bacteria |
Obligate aerobic |
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Gram-negative cocci |
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Gram-positive bacilli |
Corynebacterium jeikeium |
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Acid-fast bacilli |
Mycobacterium avium complex, M. kansasii, M. leprae, M. tuberculosis, Nocardia species |
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Nonfermentative, non-Enterobacteriaceae |
Acinetobacter calcoaceticus, Elizabethkingia meningoseptica (previously Flavobacterium meningosepticum), Pseudomonas aeruginosa, P. alcaligenes, other Pseudomonas species, Stenotrophomonas maltophilia |
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Fastidious gram-negative coccobacilli and bacilli |
Brucella, Bordetella, Francisella, and Legionella species |
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Leptospiraceae (spiral bacteria) |
Leptospira species |
Obligate anaerobic |
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Bacteroides fragilis, other Bacteroides species, Fusobacterium species, Prevotella species |
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Gram-negative cocci |
Veillonella species |
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Gram-positive cocci |
Peptococcus niger, Peptostreptococcus species |
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Non–spore-forming gram-positive bacilli |
Actinomyces, Bifidobacterium, Eubacterium, and Cutibacterium (previously Propionibacterium) species |
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Endospore-forming gram-positive bacilli |
Clostridium botulinum, C. perfringens, C. tetani, other Clostridium species |
Facultative anaerobic |
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Gram-positive cocci, catalase-positive |
Staphylococcus aureus (coagulase-positive), S. epidermidis (coagulase-negative), other coagulase-negative staphylococci |
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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 |
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Gram-positive bacilli |
Bacillus anthracis, Erysipelothrix rhusiopathiae, Gardnerella vaginalis (gram-variable) |
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Enterobacteriaceae (Citrobacter species, Enterobacter species, Escherichia coli, Klebsiella species, Morganella morganii, Proteus species, Plesiomonas shigelloides, Providencia rettgeri, Salmonella typhi, other Salmonella species, Serratia marcescens, Shigella species, Yersinia enterocolitica, Y. pestis) |
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Fermentative, non-Enterobacteriaceae |
Aeromonas hydrophila, Chromobacterium violaceum, Pasteurella multocida |
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Fastidious gram-negative coccobacilli and bacilli |
Actinobacillus actinomycetemcomitans, Bartonella bacilliformis, B. henselae, B. quintana, Eikenella corrodens, Haemophilus influenzae, other Haemophilus species |
Microaerophilic |
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Curved bacilli |
Campylobacter jejuni, Helicobacter pylori, Vibrio cholerae, V. vulnificus |
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Spirochaetaceae (spiral bacteria) |
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Obligate intracellular parasitic |
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Chlamydiaceae |
Chlamydia trachomatis, Chlamydophila pneumoniae, C. psittaci |
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Coxiellaceae |
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Rickettsia prowazekii, R. rickettsii, R. typhi, Orientia tsutsugamushi, Ehrlichia chaffeensis, Anaplasma phagocytophilum |
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