Overview of Bacteria
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, Rickettsia, Chlamydia, and Chlamydophila spp) preferentially reside and replicate intracellularly. Some bacteria such as chlamydiae, Chlamydophila sp, 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, Brucellasp, 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: Classification of Common Pathogenic Bacteria).
The most common stain for general bacterial identification is Gram stain. 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 Nocardia and Cryptosporidia spp. 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.
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—see also Overview of Anaerobic Bacteria) 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.