Pathogenic bacteria

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Bacterial infection
Classification and external resources
Clostridium tetani 01.png
Clostridium tetani is a pathogenic bacterium that causes tetanus
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Bacterial infection
Classification and external resources
Clostridium tetani 01.png
Clostridium tetani is a pathogenic bacterium that causes tetanus

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[edit]

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[edit]

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[edit]

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[edit]

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 1mg 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 pathogenic bacteria by basic laboratory characteristics[edit]

Following are the genera that contain the most important human pathogenic bacteria species:[17]

GenusImportant speciesGram stainingShapeCapsulationBonding tendencyMotilityRespirationGrowth mediumIntra/Extracellular
BordetellaGram-negativeSmall coccobacilliEncapsulatedSingly or in pairsNon-motileAerobicRegan-Lowe agarExtracellular
BorreliaGram-negative, but stains poorlySpirocheteUnencapsulatedLong, slender, flexible, spiral- or corkscrew-shaped rodsHighly motileAnaerobic(Difficult to culture)Extracellular
BrucellaGram-negativeSmall coccobacilliUnencapsulatedSingly or in pairsNon-motileAerobicRuiz Castaneda mediumIntracellular
CampylobacterGram-negativeCurved, spiral, or S-shaped
with single, polar flagellum
UnencapsulatedSinglycharacteristic darting motionmicroaerophilicBlood agar inhibiting other fecal floraextracellular
Chlamydia and Chlamydophila(not Gram-stained)Small, round, ovoidUnencapsulatedmotileFacultative or strictly aerobicObligate intracellular
ClostridiumGram-positiveLarge, blunt-ended rodsNormally encapsulatedmostly motileObligate anaerobicAnaerobic blood agarextracellular
CorynebacteriumGram-positive (unevenly)Small, slender, pleomorphic rodsUnencapsulatedclumps looking like Chinese characters or a picket fencenonmotileMostly facultative anaerobicAerobically on Tinsdale agarextracellular
EnterococcusGram-positiveRound to ovoid (cocci)pairs or chainsNon-motileFacultative Anaerobic6.5% NaCl, bile-esculin agarextracellular
EscherichiaGram-negativeShort rods (bacilli)Encapsulated and UnencapsulatedNormally motileFacultative anaerobicMacConkey agarextracellular or intracellular
FrancisellaGram-negativeSmall, pleomorphic coccobacillusEncapsulatedNon-motilestrictly aerobic(rarely cultured)Facultative intracellular
HaemophilusGram-negativeRanging from small coccobacillus to long, slender filamentsEncapsulated or UnencapsulatedNon-motileChocolate agar with hemin and NAD+extracellular
HelicobacterGram-negativeCurved or spiral rods
with multiple polar flagella
rapid, corkscrew motilityMicroaerophileMedium containing antibiotics against other fecal floraextracellular
LegionellaGram-negative, but stains poorlySlender rod in nature, cocobacillary in laboratory.
monotrichious flagella
unencapsulatedmotileaerobicSpecialized mediumfacultative intracellular
LeptospiraGram-negative, but stains poorlyLong, very slender, flexible, spiral- or corkscrew-shaped rodsHighly motileStrictly aerobicSpecialized mediumextracellular
ListeriaGram-positive, darklySlender, short rodsdiplobacilli or short chainsDistinct tumbling motility in liquid mediumFacultative Anaerobicenriched mediumintracellular
Mycobacterium(none)Long, slender rodsUnencapsulatednonmotileaerobicM. tuberculosis: Lowenstein-Jensen agar
M. leprae: (none)
Mycoplasma(none)Plastic, pleomorphicEncapsulatedsingly or in pairsMostly facultative anaerobic; M.pneumoniae strictly aerobic(rarely cultured)extracellular
NeisseriaGram-negativeKidney bean-shapedEncapsulated or UnencapsulateddiplococciNon-motileaerobicThayer-Martin agarGonococcus: facultative intracellular
N. meningitidis
: extracellular
PseudomonasGram-negativerodsencapsulatedmotileObligate aerobicMacConkey agarextracellular
RickettsiaGram-negative, but stains poorlySmall, rod-like coccobacillarySlime/microcapsuleNon-motileAerobic(rarely cultured)Obligate intracellular
SalmonellaGram-negativeBacilliEncapsulatedNormally motileFacultative anaerobicMacConkey agarFacultative intracellular
ShigellaGram-negativerodsUnencapsulatedNon-motileFacultative anaerobicHektoen agarextracellular
StaphylococcusGram-positive, darklyRound cocciEncapsulated or Unencapsulatedin bunches like grapesNon-motileFacultative anaerobicenriched medium (broth and/or blood)extracellular, facultative intracellular
StreptococcusGram-positiveovoid to sphericalEncapsulated or Unencapsulatedpairs or chainsnonmotileFacultative anaerobicblood agarextracellular
TreponemaGram-negative, but stains poorlyLong, slender, flexible, spiral- or corkscrew-shaped rodshighly motileAerobicnoneextracellular
VibrioGram-negativeShort, curved, rod-shaped with single polar flagellumUnencapsulatedrapidly motileFacultative anaerobicblood- or MacConkey agar. Stimulated by NaClextracellular
YersiniaGram-negative, stains bipolarlySmall rodsencapsulatednonmotileFacultative AnaerobeMacConkey or CIN agarIntracellular

List of pathogenic bacteria by clinical characteristics[edit]

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

Species of human pathogenic bacteria[17]
SpeciesTransmissionDiseasesTreatmentPreventionlaboratory diagnosis
Bacillus anthracis
  • Contact with sheep, goats and horses
  • Inhalation or skin penetration through abrasions of spore-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
Borrelia burgdorferiIxodes 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
  • 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
  • Respiratory droplets
Community-acquired respiratory infectionNoneNone for routine use
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 psittaciInhalation of dust with secretions or feces from birds (e.g. parrots)Psittacosis-
  • Rise in antibody titre
    • Complement fixation
    • indirect immunofluorescence
Clostridium botulinumSpores from soil and aquatic sediments contaminating vegetables, meat and fish
  • Proper food preservation techniques
  • Mouse inoculation detects toxin from food, intestinal contents or serum
  • Culture in standard aerobic culture
Clostridium difficile
  • Spores both indoors and outdoors
  • Human flora, overgrowing when other flora is depleted
Clostridium perfringensGas gangrene:

Food poisoning:

  • Self-limiting; Supportive care is sufficient
Appropriate food handling
  • Microscopically
  • Blood agar culture, forming double-zone β-hemolysis
  • Sugar fermentation
  • Organic acid production
Clostridium tetani
  • Spores in soil infecting puncture wounds, severe burns or surgery
Corynebacterium diphtheriae
  • Respiratory droplets
  • Part of human flora
(no rapid)
Enterococcus faecalis and Enterococcus faecium
  • Part of human flora, opportunistic or entering through GI tract or urinary system wounds
  • Nosocomial infections
No vaccine
  • Culture in 6.5% NaCl
  • Can hydrolyze esculin in presence of bile
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
  • Culture on MacConkey agar and study carbohydrate fermentation patterns:
    • Lactose fermentation (most E. coli strains)
    • Gas production in glucose fermentation
    • Mannitol fermentation
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
(rarely cultured)
  • Serology
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)
  • Microscopically
    • Corkscrew movement
  • Urease-positivity by radioactively labeled urea
  • Serology by ELISA
Legionella pneumophila(no vaccine or preventive drug)

Heating water

  • Culture from respiratory secretions on buffered charcoal yeast extract enriched with L-cysteine, iron and α-ketoglutarate
  • Serology, including direct immunofluorescence and radioimmunoassay for antigen in urine
  • Hybridization to ribosomal RNA using DNA probe
Leptospira interrogans
  • Food and water contaminated by e.g. urine from wild or domestic animals. Leptospira survives for weeks in stagnant water.
(no vaccine)

Prevention of exposure

  • Rodent control
Listeria monocytogenes
  • Dairy products, ground meats, poultry
  • Vertical to newborn or fetus
(no vaccine)
  • Proper food preparation and handling
Isolation from e.g. blood and CSF
  • Beta-hemolysis and catalase production on blood agar
  • Microscopy for morphology and motility
Mycobacterium leprae
  • Prolonged human-human contact, e.g. through exudates from skin lesions to abrasion of other person
Tuberculoid form:

Lepromatous form:

Tuberculoid form:
Hard to isolate (diagnosis on clinical findings and histology of biopsies)

Lepromatous form:

Mycobacterium tuberculosis
  • Droplet contact

(difficult, see Tuberculosis treatment for more details)

Standard "short" course:

Mycoplasma pneumoniae
  • Human flora
  • Droplet contact
(difficult to culture)
Neisseria gonorrhoeaeUncomplicated gonorrhea:

Ophthalmia neonatorum:

(No vaccine)
Neisseria meningitidis
  • Respiratory droplets
  • Microscopy showing gram-negative diplococci, often with PMNs
  • Culture on chocolate agar, giving positive oxidase test and fermentation of glucose and maltose in 5% CO2 in air
Pseudomonas aeruginosaInfects 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 typhiHuman-human
  • Fecal-oral through food or water
  • Ty21a and ViCPS vaccines
  • Hygiene and food preparation
  • Isolation from blood, feces, bone marrow, urine or rose spots on skin
  • Colorless, non-lactose fermenting colonies on MacConkey agar
  • Serology for antibodies against O antigen
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[18]
  • Culture on Hektoen agar or other media for intestinal pathogens
Staphylococcus aureus
  • Human flora on mucosae in e.g. anterior nares and vagina, entering through wound
Coagulase-positive staphylococcal infections: (no vaccine or preventive drug)
  • Barrier precautions, washing hands and fomite disinfection in hospitals
Staphylococcus epidermidisHuman flora in skin and anterior nares
  • Infections of implanted prostheses, e.g. heart valves and catheters
Staphylococcus saprophyticusPart of normal vaginal floraNone
Streptococcus agalactiaeHuman flora in vagina or urethral mucous membranes, rectumNone
Streptococcus pneumoniae
  • Respiratory droplets
  • Often human flora in nasopharynx (spreading in immunocompromised)
  • 23-serotype vaccine for adults (PPV)
  • Heptavalent conjugated vaccine for children (PCV)
Streptococcus pyogenes
  • Respiratory droplets
  • Direct physical contact with impetigo lesions
No vaccine
  • Rapid antibiotic treatment helps prevent rheumatic fever
Treponema pallidum
  • Sexual
No preventive drug or vaccine
  • Safe sex
  • Antibiotics to pregnant women at risk of transmitting congenital syphilis
Cannot be cultured or viewed in gram-stained smear
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

See also[edit]


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  2. ^ Heise E (1982). "Diseases associated with immunosuppression". Environ Health Perspect 43: 9–19. doi:10.2307/3429162. JSTOR 3429162. PMC 1568899. PMID 7037390. 
  3. ^ Saiman L (2004). "Microbiology of early CF lung disease". Paediatr Respir Rev 5 (Suppl A): S367–9. doi:10.1016/S1526-0542(04)90065-6. PMID 14980298. 
  4. ^ Belland R, Ouellette S, Gieffers J, Byrne G (2004). "Chlamydia pneumoniae and atherosclerosis". Cell Microbiol 6 (2): 117–27. doi:10.1046/j.1462-5822.2003.00352.x. PMID 14706098. 
  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. doi:10.1016/S0029-7844(01)01759-8. PMID 11864668. 
  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. doi:10.1146/annurev.micro.58.030603.123822. PMID 15487937. 
  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. doi:10.3389/fmicb.2012.00238. PMC 3400130. PMID 22833738.  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. ^ Institut Pasteur Press Office - Vaccine against shigellosis (bacillary dysentery):a promising clinical trial 15 January 2009. Retrieved on 27 February 2009

External links[edit]