|Classification and external resources|
Although most bacteria are harmless or often beneficial, several are pathogenic. One of the bacterial diseases with the highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and leprosy. Pathogenic bacteria are also the cause of high infant mortality rates in developing countries.
Each species has specific effect and causes symptoms in people who are infected. Some, if not most people who are infected with a pathogenic bacteria do not have symptoms. Immuno-compromised individuals are more susceptible to pathogenic bacteria.
Some pathogenic bacteria cause disease under certain conditions, such entry through the skin via a cut, through sexual activity or an compromised immune function.
Streptococcus and Staphylococcus are part of the normal skin microbiota and typically reside on healthy skin or in the nasopharangeal region. Yet these species can potentially initiate skin infections. They are also able to cause sepsis, pneumonia, meningitis. These infections can become quite serious creating a systemic inflammatory response resulting in massive vasodilation, shock, and death.
Other bacteria are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis Examples of these opportunistic pathogens include Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium.
Obligate intracellular parasites (e.g. Chlamydophila, Ehrlichia, Rickettsia) have the ability to only grow and replicate inside other cells. Even these intracellular infections may be asymptomatic, requiring an incubation period. An example of this Rickettsia which causes typhus. Another causes Rocky Mountain spotted fever.
Other groups of intracellular bacterial pathogens include: Salmonella, Neisseria, Brucella, Mycobacterium, Nocardia, Listeria, Francisella, Legionella, and Yersinia pestis. These can exist intracellularly, but can exist outside of host cells.
Bacterial pathogens often cause infection in specific areas of the body. Others are generalists.
Iron is required for humans, as well as the growth of most bacteria. To obtain free iron, some pathogens secrete proteins called siderophores, which take the iron away from iron-transport proteins by binding to the iron even more tightly. Once the iron-siderophore complex is formed, it is taken up by siderophore receptors on the bacterial surface and then that iron is brought into the bacterium.
Once pathogens attach to host cells, they can cause direct damage as the pathogens use the host cell for nutrients and produce waste products. As pathogens multiply and divide inside host cells, the cells usually rupture and the intercellular bacteria are released. Some bacteria such as E. coli, Shigella, Salmonella, and Neisseria gonorrhoeae, can induce host epithelial cells to engulf them in a process resembling phagocytosis. The pathogens can then disrupt host cells as they pass through them and be extruded from host cells by a reverse phagocytosis process, enabling them to enter other host cells. Some bacteria can also penetrate host cells by excreting enzymes and by their own motility; such penetration can itself damage the host cell.
Toxins are poisonous substances that are produced by certain microorganisms and are often the primary factor contributing to the pathogenic properties of the microorganisms. Endotoxins are the lipid portions of lipopolysaccharides that are part of the outer membrane of the cell wall of gram negative bacteria. Endotoxins are released when the bacteria lyses, which is why after antibiotic treatment symptoms can at first worsen as the bacteria are killed and they release their endotoxins. Exotoxins are proteins produced inside pathogenic bacteria as part of their growth and metabolism, most common in gram positive bacteria. The exotoxins are released when the bacteria die and the cell wall breaks apart. Exotoxins are highly specific in the effects on body tissues and work by destroying particular parts of the host cell or by inhibiting certain metabolic functions. Exotoxins are among the most lethal known substances, only 1 mg of the botulinum exotoxin is enough to kill one million guinea pigs. Diseases caused this way are often caused by minute amounts of exotoxins, not by the bacteria themselves.
Typically identification is done by growing the organism in a wide range of cultures which can take up to 48 hours. The growth is then visually or genomically identified. The cultured organism is then subjected to antibiotics to observe reaction to help further identify species and strain. 
Bacterial infections may be treated with antibiotics, which are classified as bacteriocidal if they kill bacteria or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics and each class inhibits a process that is different in the pathogen from that found in the host. For example, the antibiotics chloramphenicol and tetracyclin inhibit the bacterial ribosome but not the structurally different eukaryotic ribosome, so they exhibit selective toxicity. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth. Both uses may be contributing to the rapid development of antibiotic resistance in bacterial populations. Phage therapy can also be used to treat certain bacterial infections. Infections can be prevented by antiseptic measures such as sterilizing the skin prior to piercing it with the needle of a syringe and by proper care of indwelling catheters. Surgical and dental instruments are also sterilized to prevent infection by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection. Bacteria in food are killed by cooking to temperatures above 73 °C (163 °F).
|Genus||Species||Gram staining||Shape||Oxygen requirement||Intra/Extracellular|
|Borrelia||Negative, stains poorly||spirochete||Anaerobic||Extracellular|
|Chlamydia and Chlamydophila||(not Gram-stained)||Small, round, ovoid||Facultative or strictly aerobic||Obligate intracellular|
|Clostridium||Positive||Large, blunt-ended rods||Obligate anaerobic||extracellular|
|Corynebacterium||Positive (unevenly)||bacilli||Mostly facultative anaerobic||extracellular|
|Escherichia||Negative||Bacillus||Facultative anaerobic||extracellular or intracellular|
|Francisella||Negative||coccobacillus||strictly aerobic||Facultative intracellular|
|Haemophilus||Negative||coccobacilli to long and slender filaments||extracellular|
|Legionella||Negative, stains poorly||cocobacilli||aerobic||facultative intracellular|
|Leptospira||Negative, stains poorly||Spirochete||Strictly aerobic||extracellular|
|Listeria||Positive, darkly||Slender, short rods||Facultative Anaerobic||intracellular|
|Mycobacterium||(none)||Long, slender rods||aerobic||extracellular|
|Mycoplasma||(none)||'fried egg' appearance, no cell wall||Mostly facultative anaerobic; M. pneumoniae strictly aerobic||extracellular|
|Neisseria||Negative||Kidney bean-shaped||aerobic||Gonococcus: facultative intracellular
N. meningitidis: extracellular
|Rickettsia||Negative, stains poorly||Small, rod-like coccobacillary||Aerobic||Obligate intracellular|
|Salmonella||Negative||Bacillus shape||Facultative anaerobica||Facultative intracellular|
|Staphylococcus||Positive, darkly||Round cocci||Facultative anaerobic||extracellular, facultative intracellular|
|Streptococcus||Positive||ovoid to spherical||Facultative anaerobic||extracellular|
|Treponema||Negative, stains poorly||Spirochete||Aerobic||extracellular|
|Ureaplasma||Stains poorly||indistinct, 'fried egg' appearance, no cell wall||anaerobic||extracellular|
|Vibrio||Negative||Spiral with single polar flagellum||Facultative anaerobic||extracellular|
|Yersinia||Negative, bipolarly||Small rods||Facultative Anaerobe||Intracellular|
This is description of the more common genera and species presented with their clinical characteristics and treatments.
|Actinomyces israelii||Oral flora||Actinomycosis: painful abscesses in the mouth, lungs, or gastrointestinal tract.||Prolonged penicillin G and drainage|
|Bacillus anthracis||In early infection:
|Bacteroides fragilis||Gut flora||Abscesses in gastrointestinal tract, pelvic cavity and lungs||metronidazole||Wound care
Contact with respiratory droplets expelled by infected human hosts.
|Macrolides such as erythromycin, before paroxysmal stage|
Early stages of Lyme disease:
Chronic Lyme disease:
|Chlamydia||C. pneumoniae||Atypical pneumonia||None|
|Chlamydophila psittaci||Inhalation of dust with secretions or feces from birds (e.g. parrots)||Psittacosis||-|
|Clostridium||C. botulinum||Spores from soil, persevere in canned food, smoked fish and honey||
Proper food preservation techniques
|C. difficile||Fecal bacteriotherapy
|C. perfringens||Gas gangrene:||Appropriate food handling|
No vaccine Hand washing and other nosocomial prevention
|Escherichia||E. coli (generally)||UTI:
(resistance-tests are required first)
|(no vaccine or preventive drug)|
|Enterotoxigenic E. coli (ETEC)|
|Enteropathogenic E. coli||
|Enteroinvasive E.coli (EIEC)|
|Enterohemorrhagic (EHEC), including E. coli O157:H7||
|Francisella tularensis||Tularemia: Fever, ulceration at entry site and/or lymphadenopathy. Can cause severe pneumonia.|
(resistance-tests are required first)
|Helicobacter pylori||(No vaccine or preventive drug)|
|Legionella pneumophila||(no vaccine or preventive drug)
||Vaccine not widely used
Prevention of exposure
|Listeria monocytogenes||(no vaccine)
Standard "short" course:
|Neisseria||N. gonorrhoeae||Uncomplicated gonorrhea:
|Pseudomonas aeruginosa||Opportunistic; Infects damaged tissues or people with immunodeficiency.||Pseudomonas infection:||(no vaccine)|
|Nocardia asteroides||In soil||Nocardiosis||TMP/SMX|
|Rickettsia rickettsii||(no preventive drug or approved vaccine)|
|Other Salmonella species||
||(No vaccine or preventive drug)|
|Staphylococcus||aureus||Coagulase-positive staphylococcal infections:||(no vaccine or preventive drug)
|epidermidis||Human flora in skin, anterior nares and mucous membranes||None|
|saprophyticus||Part of normal vaginal flora||None|
|Streptococcus||agalactiae||Human flora in vagina, urethral mucous membranes, rectum||None|
|viridans||Oral flora, penetration through abrasions||Penicillin G|
|Treponema pallidum||No preventive drug or vaccine|