Pathogenic bacteria

Pathogenic bacteria are bacteria that cause infectious diseases. This article deals with human pathogenic bacteria.

Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic. The most common bacterial disease 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.

History

Koch's postulates, proposed by Robert Koch in 1890, are criteria designed to establish a causal relationship between a causative microbe and a disease. A pathogenic cause for a known medical disease may only be discovered many years after, as was the case with Helicobacter pylori and peptic ulcer disease.

Diseases

Each pathogenic species has a characteristic spectrum of interactions with its human hosts.

Conditionally pathogenic

Some organisms, such as Staphylococcus or Streptococcus, can cause skin infections, pneumonia, meningitis and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation and death.

Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all.

Intracellular

Other organisms invariably cause disease in humans, such as the Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms.

One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever.

Chlamydia, another phylum of obligate intracellular parasites, contains species that can cause pneumonia, or urinary tract infection and may be involved in coronary heart disease.

Mycobacterium and Brucella can exist intracellularly, though they are not obligate intracellular parasites.

Opportunistic

Finally, some species, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.

Treatment

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, and so 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. Infections can be prevented by antiseptic measures such as sterilizating 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 contamination and 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. Most bacteria in food are killed by cooking to temperatures above 60 °C (140 °F).

Pathogenic Genuses

The following Genuses contain the most important human pathogenic bacteria species:

Clinically most important human pathogenic bacteria genuses and species

Genus	Important species	Gram staining	Shape	Capsulation	Bonding tendency	Motility	Respiration	Growth medium	Intra/Extracellular	Bordetella	Bordetella pertussis	Gram-negative	Small coccobacilli	Encapsulated	singly or in pairs		aerobic	Regan-Lowe agar	extracellular
Borrelia	Borrelia burgdorferi	Gram-negative, but stains poorly			Long, slender, flexible, spiral- or corkscrew-shaped rods	highly motile	(difficult to culture)	extracellular
Brucella	Brucella abortus Brucella canis Brucella melitensis Brucella suis	Gram-negative	Small coccobacilli	Unencapsulated	singly or in pairs		aerobic	Blood agar	intracellular
Campylobacter	Campylobacter jejuni	Gram-negative	Curved, spiral, or S-shaped with single, polar flagellum			characteristic darting motion	microaerophilic	Blood agar inhibiting other fecal flora	extracellular
Chlamydia	Chlamydia pneumoniae Chlamydia psittaci Chlamydia trachomatis	(not Gram-stained)	Small, round, ovoid				motile	Facultative or strictly aerobic	Obligate intracellular
Clostridium	Clostridium botulinum Clostridium difficile Clostridium perfringens Clostridium tetani	Gram-positive	Large, blunt-ended rods		mostly motile	Obligate aerobic	Anaerobic blood agar	extracellular
Corynebacterium	Corynebacterium diphtheriae	Gram-positive (unevenly)	Small, slender, pleomorphic rods	unencapsulated	clumps looking lika Chinese characters or a picket fence	nonmotile	Mostly facultative anaerobic	Aerobically on Tinsdale agar	extracellular
Enterococcus	Enterococcus faecalis Enterococcus faecum	Gram-positive	Round to ovoid		pairs or chains		6.5% NaCl, bile-esculin agar	extracellular
Escherichia	Escherichia coli	Gram-negative	Short rods				Facultative anaerobic	MacConkey agar	extracellular
Francisella	Francisella tularensis	Gram-negative	Small, pleomorphic coccobacillus				strictly aerobic	(rarely cultured)	Facultative intracellular
Haemophilus	Haemophilus influenzae	Gram-negative	Ranging from small coccobacillus to long, slender filaments					Chocolate agar with hemin and NAD+	extracellular
Helicobacter	Helicobacter pylori	Gram-negative	Curved or spiral rods pultiple polar flagella			rapid, corkscrew motility		Medium containing antibiotics against other fecal flora	extracellular
Legionella	Legionella pneumophila	Gram-negative, but stains poorly	Slender rod in nature, cocobacillary in laboratory. monotrichious flagella	unencapsulated		motile		Specialized medium	facultative intracellular
Leptospira	Leptospira interrogans	Gram-negative, but stains poorly	Long, very slender, flexible, spiral- or corkscrew-shaped rods			highly motile		Specialized medium	extracellular
Listeria	Listeria monocytogenes	Gram-positive, darkly	Slender, short rods		diplobacilli or short chains	Distinct tumbling motility in liquid medium		enriched medium	intracellular
Mycobacterium	Mycobacterium leprae Mycobacterium tuberculosis	(none)	Long, slender rods			nonmotile	aerobic	M. tuberculosis: Lowenstein-Jensen agar M. leprae: (none)	extracellular
Mycoplasma	Mycoplasma pneumoniae	(none)	Plastic, pleomorphic		singly or in pairs			(rarely cultured)	extracellular
Neisseria	Neisseria gonorrhoeae Neisseria meningitidis	Gram-negative	Kidney bean-shaped		diplococci		aerobic	Thayer-Martin agar	Gonococcus: facultative intracellular N. meningitidis: extracellular
Pseudomonas	Pseudomonas aeruginosa	Gram-negative	rods	encapsulated		motile	Obligate aerobic	MacConkey agar	extracellular
Rickettsia	Rickettsia rickettsii	Gram-negative, but stains poorly	Small, rod-like coccobacillary					(rarely cultured)	Obligate intracellular
Salmonella	Salmonella typhi Salmonella typhimurium	Gram-negative					Facultative anaerobic	MacConkey agar	acellular
Shigella	Shigella sonnei	Gram-negative	rods				Facultative anaerobic	Hektoen agar	extracellular
Staphylococcus	Staphylococcus aureus Staphylococcus epidermidis Staphylococcus saprophyticus	Gram-positive, darkly	Round cocci		in bunches like grapes		Facultative anaerobic	enriched medium (broth and/or blood)	extracellular
Streptococcus	Streptococcus agalactiae Streptococcus pneumoniae Streptococcus pyogenes	Gram-positive	ovoid to spherical		pairs or chains	nonmotile	Facultative anaerobic	blood agar	extracellular
Treponema	Treponema pallidum	Gram-negative, but stains poorly	Long, slender, flexible, spiral- or corkscrew-shaped rods			highly motile		none	extracellular
Vibrio	Vibrio cholerae	Gram-negative	Short, curved, rod-shaped with single polar flagellum			rapidly motile	Facultative anaerobic	blood- or MacConkey agar. Stimulated by NaCl	extracellular
Yersinia	Yersinia pestis	Gram-negative, stains bipolarly	Small rods	encapsulated		nonmotile		MacConkey or CIN agar	extracellular

Pathogenic Genuses

This is a further description of the species presented in the previous section, containing transmission, diseases, treatment, prevention and laboratory diagnosis, which all can differ substantially among the species of the same genus.

Species of human pathogenic bacteria

Species	Transmission	Diseases	Treatment	Prevention	laboratory diagnosis
Bacillus anthracis	Contact with sheep, goats and horses Inhalation or skin penetration through abrasions of spor-contaminated dust	Cutaneous anthrax Pulmonary anthrax Gastrointestinal anthrax	In early infection: Penicillin Doxycycline Ciprofloxacin	Anthrax vaccine autoclaving of instruments	Large, grayish, nonhemolytic colonies with irregular borders on blood agar Direct immunofluorescence
Bordetella pertussis	Contact with respiratory droplets expelled by infected human hosts.	Whooping cough Complications: Secondary bacterial pneumonia	Macrolide antibiotics Azithromycin Erythromycin Clarithromycin	Pertussis vaccine, DTP	Direct immunofluorescence PCR amplification
Borrelia burgdorferi	Ixodes ticks reservoir in deer, mice and other rodents	Lyme disease	Early stages: cephalosporins amoxicillin doxycycline If arthritic symptoms have appeared: Longer courses of antibiotics	Lyme vaccine wearing clothing that limits skin exposure to ticks insect repellent	Microscopy using Giemsa or Wright stain PCR serology (low precision rate)
Brucella abortus Brucella canis Brucella melitensis Brucella suis	Direct contact with infected animal Oral, by ingestion of unpasteurized milk or milk products	Brucellosis	Combination therapy of: doxycycline streptomycin or gentamicin	-	Culture (difficult and time consuming) Agglutination serology
Campylobacter jejuni	Fecal/oral from animals (mammals and fowl) Contaminated meat (especially poultry) Contaminated water	Acute enteritis	Symptomatically by fluid and electrolyte replacement Ciprofloxacin in severe cases	No available vaccine Good hygiene Avoiding contaminated water Pasteurizing milk and milk products Cooking meat (especially poultry)	Finding campylobacter in feces
Chlamydia pneumoniae
Chlamydia psittaci
Chlamydia trachomatis
Clostridium botulinum		Botulism
Clostridium difficile
Clostridium perfringens
Clostridium tetani		Tetanus
Corynebacterium diphteriae		Diphtheria
Enterococcus faecalis
Enterococcus faecum
Escherichia coli
Francisella tularensis		Tularemia
Haemophilus influenzae
Helicobacter pylori
Legionella pneumophila		Legionnaire's Disease
Leptospira interrogans
Listeria monocytogenes
Mycobacterium leprae		Hansen's disease
Mycobacterium tuberculosis		Tuberculosis
Mycoplasma pneumoniae
Neisseria gonorrhoeae
Neisseria meningitidis
Pseudomonas aeruginosa
Rickettsia rickettsii
Salmonella typhi
Salmonella typhimurium
Shigella sonnei
Staphylococcus aureus
Staphylococcus epidermidis
Staphylococcus saprophyticus
Streptococcus agalactiae
Streptococcus pneumoniae
Streptococcus pyogenes
Treponema pallidum
Vibrio cholerae
Yersinia pestis		Plague

See also

• Pathogenic viruses

References