Pathogenic bacteria

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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, 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.


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.


Each pathogenic species has a characteristic spectrum of interactions with its human hosts. 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.[1] 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. 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.[2] 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.[3][4]


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.[5] 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.[6] 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: [7]

Clinically most important human pathogenic bacteria genuses[8] and species
Genus Important species Gram staining Shape Capsulation Bonding tendency Motility Respiration Growth medium Intra/Extracellular
Bacillus Gram-positive Blunt-ended bacilli Antiphagocytic capsule singly, in pairs or frequently in long chains Nonmotile Facultative or strictly aerobic Blood agar extracellular
Bordetella Gram-negative Small coccobacilli Encapsulated singly or in pairs aerobic Regan-Lowe agar extracellular
Borrelia Gram-negative, but stains poorly Long, slender, flexible, spiral- or corkscrew-shaped rods higly motile (difficult to culture) extracellular
Brucella Gram-negative Small coccobacilli Unencapsulated singly or in pairs aerobic Blood agar intracellular
Campylobacter Gram-negative Curved, spiral, or S-shaped
with single, polar flagellum
characteristic darting motion microaerophilic Blood agar inhibiting other fecal flora extracellular
Chlamydia (not Gram-stained) Small, round, ovoid motile Facultative or strictly aerobic Obligate intracellular
Clostridium Gram-positive Large, blunt-ended rods mostly motile Obligate aerobic Anaerobic blood agar extracellular
Corynebacterium Gram-positive (unevenly) Small, slender, pleomorphic rods unencapsulated clumps looking like Chinese characters or a picket fence nonmotile Mostly facultative anaerobic Aerobically on Tinsdale agar extracellular
Enterococcus Gram-positive Round to ovoid pairs or chains 6.5% NaCl, bile-esculin agar extracellular
Escherichia Gram-negative Short rods Facultative anaerobic MacConkey agar extracellular
Francisella Gram-negative Small, pleomorphic coccobacillus strictly aerobic (rarely cultured) Facultative intracellular
Haemophilus Gram-negative Ranging from small coccobacillus to long, slender filaments Chocolate agar with hemin and NAD+ extracellular
Helicobacter Gram-negative Curved or spiral rods
pultiple polar flagella
rapid, corkscrew motility Medium containing antibiotics against other fecal flora extracellular
Legionella Gram-negative, but stains poorly Slender rod in nature, cocobacillary in laboratory.
monotrichious flagella
unencapsulated motile Specialized medium facultative intracellular
Leptospira Gram-negative, but stains poorly Long, very slender, flexible, spiral- or corkscrew-shaped rods highly motile Specialized medium extracellular
Listeria Gram-positive, darkly Slender, short rods diplobacilli or short chains Distinct tumbling motility in liquid medium enriched medium intracellular
Mycobacterium (none) Long, slender rods nonmotile aerobic M. tuberculosis: Lowenstein-Jensen agar
M. leprae: (none)
Mycoplasma (none) Plastic, pleomorphic singly or in pairs (rarely cultured) extracellular
Neisseria Gram-negative Kidney bean-shaped diplococci aerobic Thayer-Martin agar Gonococcus: facultative intracellular
N. meningitidis
: extracellular
Pseudomonas Gram-negative rods encapsulated motile Obligate aerobic MacConkey agar extracellular
Rickettsia Gram-negative, but stains poorly Small, rod-like coccobacillary (rarely cultured) Obligate intracellular
Salmonella Gram-negative Facultative anaerobic MacConkey agar acellular
Shigella Gram-negative rods Facultative anaerobic Hektoen agar extracellular
Staphylococcus Gram-positive, darkly Round cocci in bunches like grapes Facultative anaerobic enriched medium (broth and/or blood) extracellular
Streptococcus Gram-positive ovoid to spherical pairs or chains nonmotile Facultative anaerobic blood agar extracellular
Treponema Gram-negative, but stains poorly Long, slender, flexible, spiral- or corkscrew-shaped rods highly motile none extracellular
Vibrio Gram-negative Short, curved, rod-shaped with single polar flagellum rapidly motile Facultative anaerobic blood- or MacConkey agar. Stimulated by NaCl extracellular
Yersinia Gram-negative, stains bipolarly Small rods encapsulated nonmotile MacConkey or CIN agar extracellular

Pathogenic species

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 [8]
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
In early infection:
  • Large, grayish, nonhemolytic colonies with irregular borders on blood agar
  • Direct immunofluorescence
Bordetella pertussis
  • Contact with respiratory droplets expelled by infected human hosts.


Macrolide antibiotics
  • Pertussis vaccine, DTP
Borrelia burgdorferi Ixodes ticks
reservoir in deer, mice and other rodents
Brucella abortus
Brucella canis
Brucella melitensis
Brucella suis
Campylobacter jejuni
Chlamydia pneumoniae
Chlamydia psittaci
Chlamydia trachomatis
Clostridium botulinum
Clostridium difficile
Clostridium perfringens
Clostridium tetani
  • [Tetanus]]
Corynebacterium diphteriae
Enterococcus faecalis
Enterococcus faecum
Escherichia coli
Francisella tularensis
Haemophilus influenzae
Helicobacter pylori
Legionella pneumophila
Leptospira interrogans
Listeria monocytogenes
Mycobacterium leprae
Mycobacterium 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

See also


  1. Fish D. "Optimal antimicrobial therapy for sepsis". Am J Health Syst Pharm. 59 Suppl 1: S13–9. PMID 11885408.
  2. Belland R, Ouellette S, Gieffers J, Byrne G (2004). "Chlamydia pneumoniae and atherosclerosis". Cell Microbiol. 6 (2): 117–27. PMID 14706098.
  3. Heise E. "Diseases associated with immunosuppression". Environ Health Perspect. 43: 9–19. PMID 7037390.
  4. Saiman, L. "Microbiology of early CF lung disease". Paediatr Respir Rev. volume = 5 Suppl A: S367&ndash, 369. Unknown parameter |yar= ignored (help) PMID 14980298
  5. Yonath A, Bashan A (2004). "Ribosomal crystallography: initiation, peptide bond formation, and amino acid polymerization are hampered by antibiotics". Annu Rev Microbiol. 58: 233–51. PMID 15487937.
  6. Khachatourians G (1998). "Agricultural use of antibiotics and the evolution and transfer of antibiotic-resistant bacteria". CMAJ. 159 (9): 1129–36. PMID 9835883.
  7. Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-8215-5.
  8. 8.0 8.1 Unless else specified then ref is: Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-8215-5. Pages 332 to 353

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