Pathogenic bacteria

Bacterial infection
File:Clostridium tetani 01.png
Clostridium tetani is a pathogenic bacterium that causes tetanus
Classification and external resources
NCI Pathogenic bacteria
Patient UK Pathogenic bacteria
MeSH D001424

Pathogenic bacteria are bacteria that can cause infection. This article deals with human pathogenic bacteria.

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.

Koch's postulates are criteria designed to establish a causal relationship between a causative microbe and a disease.


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

Conditionally pathogenic

Conditionally pathogenic bacteria are only pathogenic under certain conditions, such as a wound that allows for entry into the blood, or a decrease in immune function.

For example, Staphylococcus or Streptococcus are also part of the normal human flora and usually exist on the skin or in the nose without causing disease, but can potentially cause skin infections, pneumonia, meningitis, and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation and death.[1]

Some species of bacteria, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.[2][3]


Other organisms invariably cause disease in humans, such as obligate intracellular parasites (e.g. Chlamydophila, Ehrlichia, Rickettsia) that are able to grow and reproduce only within the cells of other organisms. Still, infections with intracellular bacteria may be asymptomatic, such as during the incubation period. An example of intracellular bacteria is Rickettsia. 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.[4]

Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, and Yersinia pestis can exist intracellularly, though they are facultative (not obligate) intracellular parasites.

By location

Following is a list of bacterial infections classified by location in the body:



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.[13]

Direct damage

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 can itself damage the host cell.[13]

Toxin production

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.[13]


Main article: Antibiotics
See also overview list below

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.[14] 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.[15] Phage therapy can also be used to treat certain bacterial infections.[16] 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).

List of genera of pathogenic bacteria and microscopy features

Following are the genera that contain the most important human pathogenic bacteria species, as well as their characteristics that are used for identification by microscopy:[17]

Genus Important species Gram staining Shape Capsulation Bonding tendency Motility Oxygen requirement Growth medium Intra/Extracellular
Bacillus Positive Rods Yes Singly Motile Facultative anaerobic Mannitol salt agar Extracellular
Bartonella Negative Rods No Singly Motile Aerobic Blood agar with lithium heparin Facultative intracellular
Bordetella Negative Small coccobacilli Yes Singly or in pairs Non-motile Aerobic Regan-Lowe agar[18][19] Extracellular
Borrelia Negative, stains poorly Flat-wave spirochete No Singly Highly motile Anaerobic Barbour-Stoenner-Kelly (BSK) medium Extracellular
Brucella Negative Small coccobacilli No Singly or in pairs Non-motile Aerobic Ruiz Castaneda medium Intracellular
Campylobacter Negative Curved
bacillus shape with a single, polar flagellum
No Singly characteristic darting motion microaerophilic Blood agar inhibiting other gut flora extracellular
Chlamydia and Chlamydophila (not Gram-stained) Small, round, ovoid No motile Facultative or strictly aerobic Obligate intracellular
Clostridium Positive Large, blunt-ended rods Normally yes mostly motile Obligate anaerobic Anaerobic blood agar extracellular
Corynebacterium Positive (unevenly) Small, slender, pleomorphic rods No clumps looking like Chinese characters or a picket fence nonmotile Mostly facultative anaerobic Loeffler agar[20] or Tinsdale agar[19][20] extracellular
Enterococcus Positive Cocci pairs or chains Non-motile Facultative Anaerobic Bile-esculin agar[19][21] extracellular
Escherichia Negative Bacillus shape Variable Normally motile Facultative anaerobic MacConkey agar[19][22] extracellular or intracellular
Francisella Negative Small, pleomorphic coccobacillus Yes Non-motile strictly aerobic (rarely cultured) Facultative intracellular
Haemophilus Negative Variable, from small coccobacilli to long and slender filaments Variable Non-motile Chocolate agar[19][23] extracellular
Helicobacter Negative Spirillum with multiple polar flagella rapid, corkscrew motility Microaerophile H. pylori special peptone agar (HPSPA)[24] extracellular
Legionella Negative, stains poorly Slender rod in nature, cocobacilli in laboratory.
single flagella
No motile aerobic Specialized medium facultative intracellular
Leptospira Negative, stains poorly Spirochete Highly motile Strictly aerobic Ellinghausen-McCullough-Johnson-Harris (EMJH) medium[19][25] extracellular
Listeria Positive, darkly Slender, short rods diplobacilli or short chains Distinct tumbling motility in liquid medium Facultative Anaerobic Enriched medium intracellular
Mycobacterium (none) Long, slender rods No nonmotile aerobic M. tuberculosis: Lowenstein-Jensen agar
M. leprae: (none)
Mycoplasma (none) Plastic, pleomorphic Yes singly or in pairs Mostly facultative anaerobic; M. pneumoniae strictly aerobic M. pneumoniae: Eaton's agar extracellular
Neisseria Negative Kidney bean-shaped Variable Diplococcus Non-motile aerobic Thayer-Martin agar[19][26] Gonococcus: facultative intracellular
N. meningitidis
: extracellular
Pseudomonas Negative rods Yes motile Obligate aerobic MacConkey agar[19][27] extracellular
Rickettsia Negative, stains poorly Small, rod-like coccobacillary Slime/microcapsule Non-motile Aerobic (rarely cultured) Obligate intracellular
Salmonella Negative Bacillus shape Yes Normally motile Facultative anaerobica MacConkey agar Facultative intracellular
Shigella Negative rods Variable Non-motile Facultative anaerobic Hektoen agar[19][28] extracellular
Staphylococcus Positive, darkly Round cocci Variable in bunches like grapes Non-motile Facultative anaerobic Enriched medium (broth and/or blood) extracellular, facultative intracellular
Streptococcus Positive ovoid to spherical Variable pairs or chains nonmotile Facultative anaerobic Blood agar extracellular
Treponema Negative, stains poorly Spirochete highly motile Aerobic none extracellular
Vibrio Negative Spiral with single polar flagellum No rapidly motile Facultative anaerobic Blood agar,[19] MacConkey agar[19] or TCBS agar[29] extracellular
Yersinia Negative, bipolarly Small rods Yes nonmotile Facultative Anaerobe MacConkey agar[19][30] Intracellular

List of species of pathogenic bacteria and clinical characteristics

This is a rather clinical description of the species presented in the previous section, containing the main examples of transmission, diseases, treatment and prevention, which all can differ substantially among the species of the same genus.

Species of human pathogenic bacteria[17]
Species Transmission Diseases Treatment Prevention
Bacillus anthracis
  • Contact with cattle, sheep, goats and horses[31]
  • Inhalation or skin penetration through abrasions of spores in dust[19]
In early infection:[33]


Bordetella pertussis
  • Contact with respiratory droplets expelled by infected human hosts.


Macrolide antibiotics
Ixodes ticks
reservoir in deer, mice and other rodents
  • Direct contact with infected animal
  • Oral, by ingestion of unpasteurized milk or milk products
Combination therapy of: -
Campylobacter jejuni
  • Fecal/oral from animals (mammals and fowl)
  • Contaminated meat (especially poultry)
  • Contaminated water
  • 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)
Chlamydia pneumoniae
  • Respiratory droplets
Community-acquired respiratory infection None
Chlamydia trachomatis
  • Sexual (NGU, LGV)
  • Direct or contaminated surfaces and flies (trachoma)
  • Passage through birth canal (ICN)
No vaccine
  • Erythromycin or silver nitrate in newborn's eyes
  • Safe sex
Chlamydophila psittaci Inhalation of dust with secretions or feces from birds (e.g. parrots) Psittacosis -
Clostridium botulinum Spores from soil,[19][32] persevere in canned food, smoked fish and honey[32]
  • Proper food preservation techniques
Clostridium difficile None[19]
Clostridium perfringens Gas gangrene:

Food poisoning:

  • Supportive care is sufficient[19]
Appropriate food handling[19]
Clostridium tetani
  • Spores in soil, skin penetration through wounds[19][32]
Corynebacterium diphtheriae
  • Respiratory droplets
  • Part of human flora
Enterococcus faecalis and Enterococcus faecium No vaccine
Escherichia coli (generally)
  • Part of gut flora, spreading extraintestinally or proliferating in the GI tract

(resistance-tests are required first)



  • Antibiotics above shorten duration
  • Electrolyte and fluid replacement
(no vaccine or preventive drug)
  • Food and water preparation
    • Cooking ground beef and pasteurizing milk against O157:H7
  • Hand washing and disinfection
Enterotoxigenic Escherichia coli (ETEC)
  • Fecal-oral through food and water
  • Direct physical contact
Enteropathogenic E. coli
  • Diarrhea in infants
E. coli O157:H7
  • Reservoir in cattle
Francisella tularensis
  • vector-borne by arthropods
  • Infected wild or domestic animals, birds or house pets
  • Avoiding insect vectors
  • Precautions when handling wild animals or animal products
Haemophilus influenzae
  • Droplet contact
  • Human flora of e.g. upper respiratory tract

(resistance-tests are required first)

Helicobacter pylori
  • Colonizing stomach
  • Unclear person-to-person transmission
(No vaccine or preventive drug)
Klebsiella pneumoniae
Legionella pneumophila (no vaccine or preventive drug)

Heating water

Leptospira species
  • Food and water contaminated by urine from infected wild or domestic animals. Leptospira survives for weeks in fresh water and moist soil..
Vaccine not widely used

Prevention of exposure

Listeria monocytogenes
(no vaccine)[19]
  • Proper food preparation and handling[19]
Mycobacterium leprae
  • Prolonged human-human contact, e.g. through exudates from skin lesions to abrasion of other person
Tuberculoid form:

Lepromatous form:

Mycobacterium tuberculosis
  • Droplet contact

(difficult, see Tuberculosis treatment for more details)

Standard "short" course:

Mycoplasma pneumoniae
  • Human flora
  • Droplet contact
Neisseria gonorrhoeae Uncomplicated gonorrhea:

Ophthalmia neonatorum:

(No vaccine)
Neisseria meningitidis
  • Respiratory droplets
Pseudomonas aeruginosa Infects damaged tissues or people with reduced immunity.

Localized to eye, ear, skin, urinary, respiratory or gastrointestinal tract or CNS, or systemic with bacteremia, secondary pneumonia bone and joint infections, endocarditis, skin, soft tissue or CNS infections.

(no vaccine)
Rickettsia rickettsii (no preventive drug or approved vaccine)
  • Vector control, such as clothing
  • Prompt removal of attached ticks
Salmonella typhi
Salmonella typhimurium
  • Fecal-oral
  • Food contaminated by fowl (e.g. eggs), pets and other animals
  • Fluid and electrolyte replacement for severe diarrhea
  • Antibiotics (in immunocompromised to prevent systemic spread)
(No vaccine or preventive drug)
  • Proper sewage disposal
  • Food preparation
  • Good personal hygiene
Shigella sonnei
  • Fecal-oral
  • Flies
  • Contaminated food or water
  • Protection of water and food supplies
  • Vaccines are in trial stage[37]
Staphylococcus aureus Coagulase-positive staphylococcal infections: (no vaccine or preventive drug)
  • Barrier precautions, washing hands and fomite disinfection in hospitals
Staphylococcus epidermidis Human flora in skin,[19][32] anterior nares[19] and mucous membranes[32] None
Staphylococcus saprophyticus Part of normal vaginal flora None
Streptococcus agalactiae Human flora in vagina,[19][32] urethral mucous membranes,[19] rectum[19] None
Streptococcus pneumoniae
  • 23-serotype vaccine for adults (PPV)[19][32]
  • Heptavalent conjugated vaccine for children (PCV)[19]
Streptococcus pyogenes
  • Respiratory droplets
  • Direct physical contact with impetigo lesions
No vaccine
  • Rapid antibiotic treatment helps prevent rheumatic fever
Streptococcus viridans Oral flora,[32] penetration through abrasions Penicillin G[32]
Treponema pallidum
  • Sexual
No preventive drug or vaccine
  • Safe sex
  • Antibiotics to pregnant women at risk of transmitting congenital syphilis
Vibrio cholerae
  • Contaminated water and food, especially raw seafood
  • Fluid and electrolyte replacement
  • e.g. doxycycline to shorten duration
  • Preventing fecal contamination of water supplies and food
  • Adequate food preparation
Yersinia pestis
  • Fleas from animals
  • Ingestion of animal tissues
  • Respiratory droplets
  • Formalin-killed plague vaccine
  • Minimize exposure to rodents and fleas

See also


  1. ^ Fish DN (February 2002). "Optimal antimicrobial therapy for sepsis". Am J Health Syst Pharm 59 (Suppl 1): S13–9. PMID 11885408. 
  2. ^ Heise E (1982). "Diseases associated with immunosuppression". Environ Health Perspect 43: 9–19. JSTOR 3429162. PMC 1568899. PMID 7037390. doi:10.2307/3429162. 
  3. ^ Saiman L (2004). "Microbiology of early CF lung disease". Paediatr Respir Rev 5 (Suppl A): S367–9. PMID 14980298. doi:10.1016/S1526-0542(04)90065-6. 
  4. ^ Belland R, Ouellette S, Gieffers J, Byrne G (2004). "Chlamydia pneumoniae and atherosclerosis". Cell Microbiol 6 (2): 117–27. PMID 14706098. doi:10.1046/j.1462-5822.2003.00352.x. 
  5. ^ Terri Warren, RN (2010). "Is It a Yeast Infection?". Retrieved 2011-02-23. 
  6. ^ Ferris DG, Nyirjesy P, Sobel JD, Soper D, Pavletic A, Litaker MS (March 2002). "Over-the-counter antifungal drug misuse associated with patient-diagnosed vulvovaginal candidiasis". Obstetrics and Gynecology 99 (3): 419–425. PMID 11864668. doi:10.1016/S0029-7844(01)01759-8. 
  7. ^ "Urinary Tract Infections". Retrieved 2010-02-04. 
  8. ^ "Adult Health Advisor 2005.4: Bacteria in Urine, No Symptoms (Asymptomatic Bacteriuria)". Archived from the original on 2007-07-12. Retrieved 2007-08-25. 
  9. ^ NHS Impetigo
  10. ^ Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; & Mitchell, Richard N. (2007). Robbins Basic Pathology (8th ed.). Saunders Elsevier. pp. 843 ISBN 978-1-4160-2973-1
  11. ^ "erysipelas" at Dorland's Medical Dictionary
  12. ^ "cellulitis" at Dorland's Medical Dictionary
  13. ^ a b c Tortota, Gerard (2013). Microbiology an Introduction. ISBN 978-0-321-73360-3. 
  14. ^ 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. doi:10.1146/annurev.micro.58.030603.123822. 
  15. ^ Khachatourians GG (November 1998). "Agricultural use of antibiotics and the evolution and transfer of antibiotic-resistant bacteria". CMAJ 159 (9): 1129–36. PMC 1229782. PMID 9835883. 
  16. ^ Keen, E. C. (2012). "Phage Therapy: Concept to Cure". Frontiers in Microbiology 3. PMC 3400130. PMID 22833738. doi:10.3389/fmicb.2012.00238.  edit
  17. ^ a b Unless else specified then ref is: Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. (2007). Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstown, MD: Lippincott Williams & Wilkins. pp. 332–353. ISBN 0-7817-8215-5. 
  18. ^ Kurzynski TA, Boehm DM, Rott-Petri JA, Schell RF, Allison PE (1988). "Comparison of modified Bordet-Gengou and modified Regan-Lowe media for the isolation of Bordetella pertussis and Bordetella parapertussis". J. Clin. Microbiol. 26 (12): 2661–3. PMC 266968. PMID 2906642. 
  19. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs bt bu bv bw bx by bz ca cb cc cd ce cf cg ch ci cj ck cl cm cn co cp cq cr cs ct cu cv cw cx cy Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. (2007). Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstown, MD: Lippincott Williams & Wilkins. pp. 332–353. ISBN 0-7817-8215-5. 
  20. ^ a b Baron, Samuel (1996). Medical Microbiology, 4th edition. University of Texas Medical Branch at Galveston, Galveston, Texas. ISBN 0-9631172-1-1. 
  21. ^ Rollins, David M. (2000). "BSCI424 Laboratory Media". University of Maryland. Retrieved 2008-11-18. 
  22. ^ Cain, Donna (January 14, 2015). "MacConkey Agar (CCCCD Microbiology". Collin College. 
  23. ^ Gunn, B.A. "Chocolate agar: A differential medium for gram positive cocci". PubMed. Retrieved 28 September 2012. 
  24. ^ Stevenson TH, Castillo A, Lucia LM, Acuff GR (2000). "Growth of Helicobacter pylori in various liquid and plating media". Lett. Appl. Microbiol. 30 (3): 192–6. PMID 10747249. 
  25. ^ Johnson RC, Harris VG (1967). "Differentiation of Pathogenic and Saprophytic Leptospires I. Growth at Low Temperatures". J. Bacteriol. 94 (1): 27–31. PMC 251866. PMID 6027998. 
  26. ^ "Thayer Martin Agar (Modified) Procedure" (PDF). University of Nebraska-Medical Center, Clinical Laboratory Science Program. Retrieved 2015-05-03. 
  27. ^ Allen, Mary E. (Created: 30 September 2005. Last update: 01 April 2013). "MacConkey Agar Plates Protocols". American Society for Microbiology.  Check date values in: |date= (help)
  28. ^ "Hektoen Enteric Agar". Austin Community College District. Retrieved 2015-05-03. 
  29. ^ Pfeffer, C.; Oliver, J.D. (2003). "A comparison of thiosulphate-citrate-bile salts-sucrose (TCBS) agar and thiosulphate-chloride-iodide (TCI) agar for the isolation of Vibrio species from estuarine environments". Letters in Applied Microbiology 36 (3): 150–151. PMID 12581373. doi:10.1046/j.1472-765X.2003.01280.x. 
  30. ^ "Yersinia pestis" (PDF). Wadsworth Center. 2006. 
  31. ^ "Anthrax in animals". Food and Agriculture Organization. 2001. 
  32. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs bt bu bv bw bx by bz ca cb cc cd ce cf cg "Bacteria Table" (PDF). Creighton University School of Medicine. Retrieved 2015-05-03. 
  33. ^ "CDC Anthrax Q & A: Treatment". Retrieved 4 April 2011. 
  34. ^ "FDA approves raxibacumab to treat inhalational anthrax". Retrieved 14 December 2012. 
  35. ^ Ryan, KJ; Ray, CG, eds. (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9. 
  36. ^ "Klebsiella pneumoniae in Healthcare Settings". Centers for Disease Control and Prevention. Page last reviewed: November 24, 2010. Page last updated: August 27, 2012.  Check date values in: |date= (help)
  37. ^ Institut Pasteur Press Office - Vaccine against shigellosis (bacillary dysentery):a promising clinical trial 15 January 2009. Retrieved on 27 February 2009
  38. ^ Levinson, W. (2010). Review of Medical Microbiology and Immunology (11th ed.). pp. 94–9. 

External links

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