Bacteria are microorganisms that have circular double-stranded DNA (except for Streptomyces species) and cell walls (except for mycoplasma species). 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 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, Brucella species, Francisella tularensis, Neisseria gonorrhoeae, N. meningitidis, Legionella and Listeria species, and Mycobacterium tuberculosis.
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):
Morphology
Staining
Encapsulation
Oxygen requirements
Morphology
Bacteria may be
Cylindric (bacilli)
Spherical (cocci)
Spiral (spirochetes)
A few coccal, many bacillary, and most spirochetal species are motile.
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 and antibiotic resistance of these bacteria. (For other factors that enhance bacterial pathogenicity, see Factors Facilitating Microbial Invasion.)
Ziehl-Neelsen stain and Kinyoun stain 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
Fluorochrome stains (eg, auramine-rhodamine) also identify acid-fast organisms, but a special fluorescent microscope is required.
(See also Microscopy.)
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%).
Microaerophilic bacteria can grow in environments where oxygen is limited. Examples include Campylobacter species and Helicobacter pylori.
Obligate intracellular parasites, such as chlamydiae, acquire energy from the host cell and do not produce it themselves.
Classification of Common Pathogenic Bacteria
Type | Bacteria |
|---|---|
Obligate aerobic | |
Gram-negative cocci | |
Gram-positive bacilli | Corynebacterium jeikeium |
Acid-fast bacilli | Mycobacterium avium complex, M. kansasii, M. leprae, M. tuberculosis Nocardia species |
Nonfermentative gram-negative bacilli, non-Enterobacterales (formerly Enterobacteriaceae) | Acinetobacter calcoaceticus Elizabethkingia meningoseptica (formerly Flavobacterium meningosepticum) Pseudomonas aeruginosa,* P. alcaligenes, other Pseudomonas species Stenotrophomonas maltophilia |
Fastidious gram-negative coccobacilli and bacilli | Brucella species Bordetella species Francisella species Legionella species |
Leptospiraceae (spiral bacteria) | Leptospira species |
Obligate anaerobic | |
Bacteroides fragilis, other Bacteroides species Fusobacterium species Prevotella species | |
Gram-negative cocci | Veillonella species |
Gram-positive cocci | Peptococcus niger Peptostreptococcus species |
Non–spore-forming gram-positive bacilli | Actinomyces species Bifidobacterium species Eubacterium species Cutibacterium (formerly Propionibacterium) species |
Endospore-forming gram-positive bacilli | Clostridium botulinum, C. perfringens, C. tetani, other Clostridium species |
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 | Erysipelothrix rhusiopathiae Gardnerella vaginalis (gram-variable) |
Enterobacterales (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) | |
Fermentative, non-Enterobacterales | Aeromonas hydrophila Chromobacterium violaceum Pasteurella multocida |
Fastidious gram-negative coccobacilli and bacilli | Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans Eikenella corrodens Haemophilus influenzae, other Haemophilus species |
Microaerophilic | |
Curved bacilli | Campylobacter jejuni Vibrio cholerae, V. vulnificus |
Spirochaetaceae (spiral bacteria) | |
Obligate intracellular parasitic | |
Chlamydiaceae | Chlamydia trachomatis, C. psittaci, Chlamydophila pneumoniae |
Coxiellaceae | |
Rickettsia prowazekii, R. rickettsii, R. typhi Anaplasma phagocytophilum | |
* Although usually considered obligate aerobes, these bacteria are able to grow in the absence of oxygen by using nitrite as an alternative electron acceptor. | |
