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, 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 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
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.
