Bacteria are microorganisms that have circular double-stranded DNA and (except for mycoplasmas) cell walls. Most bacteria live extracellularly. Some bacteria (eg, Salmonella typhi Overview of Salmonella Infections The genus Salmonella is divided into 2 species, S. enterica and S. bongori, which include > 2500 known serotypes. Some of these serotypes are named. In such cases, common... read more ; Neisseria gonorrhoeae Gonorrhea Gonorrhea is caused by the bacteria Neisseria gonorrhoeae. It typically infects epithelia of the urethra, cervix, rectum, pharynx, or conjunctivae, causing irritation or pain and purulent... read more 
; Legionella Legionella Infections Legionella pneumophila is a gram-negative bacillus that most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic or urine antigen... read more 
, Mycobacteria, Rickettsia Overview of Rickettsial and Related Infections Rickettsial diseases (rickettsioses) and related diseases (anaplasmosis, ehrlichiosis, Q fever, scrub typhus) are caused by a group of gram-negative, obligately intracellular coccobacilli. All... read more 
, Chlamydia Chlamydia Three species of Chlamydia cause human disease, including sexually transmitted infections and respiratory infections. All are susceptible to macrolides (eg, <span class="disableDrug"... read more , 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 Brucellosis Brucellosis is caused by Brucella species, which are gram-negative bacteria. Symptoms begin as an acute febrile illness with few or no localized signs and may progress to a chronic stage... read more species, Francisella tularensis Tularemia Tularemia is a febrile disease caused by the gram-negative bacterium Francisella tularensis; it may resemble typhoid fever. Symptoms are a primary local ulcerative lesion, regional lymphadenopathy... read more 
, N. gonorrhoeae, Neisseria meningitidis Meningococcal Diseases Meningococci (Neisseria meningitidis) are gram-negative cocci that cause meningitis and meningococcemia. Symptoms, usually severe, include headache, nausea, vomiting, photophobia, lethargy... read more , Legionella and Listeria Listeriosis Listeriosis is bacteremia, meningitis, cerebritis, dermatitis, an oculoglandular syndrome, intrauterine and neonatal infections, or rarely endocarditis caused by Listeria species. Symptoms... read more species, Mycobacterium tuberculosis Tuberculosis (TB) Tuberculosis (TB) is a chronic, progressive mycobacterial infection, often with a period of latency following initial infection. TB most commonly affects the lungs. Symptoms include productive... read more 
) 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 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 Factors Facilitating Microbial Invasion Microbial invasion can be facilitated by the following: Virulence factors Microbial adherence Resistance to antimicrobials Defects in host defense mechanisms read more .)
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 Nocardiosis Nocardiosis is an acute or chronic, often disseminated, suppurative or granulomatous infection caused by various aerobic soil saprophytes of the gram-positive bacilli genus Nocardia.... read more 
and the protozoans Cryptosporidia Cryptosporidiosis Cryptosporidiosis is infection with the protozoan Cryptosporidium. The primary symptom is watery diarrhea, often with other signs of gastrointestinal distress. Illness is typically self-limited... read more 
, Cyclospora Cyclosporiasis Cyclosporiasis is infection with the protozoan Cyclospora cayetanensis. Symptoms include watery diarrhea with gastrointestinal and systemic symptoms. Diagnosis is by detection of characteristic... read more 
, and Cystoisospora Cystoisosporiasis Cystoisosporiasis is infection with the protozoan Cystoisospora (Isospora) belli. Symptoms include watery diarrhea with gastrointestinal and systemic symptoms. Diagnosis is by detection... read more 
. 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 Streptococcal Infections Streptococci are gram-positive aerobic organisms that cause many disorders, including pharyngitis, pneumonia, wound and skin infections, sepsis, and endocarditis. Symptoms vary with the organ... read more 
, Haemophilus influenzae Haemophilus Infections The gram-negative bacteria Haemophilus species cause numerous mild and serious infections, including bacteremia, meningitis, pneumonia, sinusitis, otitis media, cellulitis, and epiglottitis... read more ), 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 Overview of Anaerobic Bacteria Bacteria can be classified by their need and tolerance for oxygen: Facultative: Grow aerobically or anaerobically in the presence or absence of oxygen Microaerophilic: Require a low oxygen concentration... read more ) 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.
