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  • Genus: Escherichia, Enterobacter, Klebsiella, Serratia, Citrobacter, Proteus
  • Species: many (Click here to view.)

  • Collectively, this group of Gram-negative bacilli (with the exception of Proteus) are referred to as "coliforms" because they share similar morphological and biochemical characteristics.
  • With the exception of Proteus, these organisms ferment lactose, which is a useful characteristic for differentiating them from Salmonella and Shigella.
  • Most of these organisms are members of the normal flora of humans and/or animals and are considered opportunistic pathogens.
  • Diseases produced by the coliforms and Proteus can be grouped into three general categories:

    1. Nosocomial or hospital-acquired infections: Forty percent of all nosocomial infections involve coliforms or Proteus. The primary sites for infection include the urinary tract (E. coli), surgical wound (E. coli), lower respiratory tract (Klebsiella), and primary bacteremia (E. coli).
    2. Infections in compromised patients: E. coli is responsible for 40% of neonatal bacterial meningitis infections.
    3. Community acquired infections: E. coli accounts for 85% of urethrocystitis cases, 80% of chronic bacterial prostatitis cases and 90% of acute pyelonephritis cases. Proteus, Klebsiella and Enterobacter may produce urinary tract infections. Proteus may also be responsible for some renal infection stones, due to the production of the enzyme urease and subsequent alkalinization and supersaturation of urine. In addition, K. pneumonia is responsible for approximately 3% of bacterial pneumonia cases and is more severe than that produced by S. pneumoniae. E. coli can also produce several different types of diarrheal disease (click here).

  • These bacteria are all facultative Gram negative bacilli. Except for Proteus, they all ferment lactose. Except for Klebsiella, they are all motile. Each genus has a typical Gram- negative cell wall containing LPS.
  • Antigens possessed by these organisms include the H (flagellar), K (capsular) and O (somatic).
  • Those organisms that possess fimbriae (common pili) use these appendages for adhesion purposes.
  • All genera produce the characteristic LPS endotoxin and some genera/species produce exotoxins.

  • Generally, these organisms are opportunistic pathogens. E. coli is perhaps the best studied.
  • The site of infection may be specific for a particular serotype. For example, Klebsiella capsular types 1 and 2 are most commonly associated with the production of bacterial pneumonia; types 8, 9, 10 and 24 are more commonly associated with urinary tract infections. This finding suggests that these antigenic specificities may provide a site-specific pathogenicity to the organisms. In support of this idea, E. coli displaying the K1 antigen have a propensity for producing neonatal meningitis. The K1 antigen provides the organism with an increased resistance to phagocytosis and the action of complement. The fact that this K1 antigen is cross reactive with the capsular antigen from group B meningococci (also capable of producing meningitis) suggests that the specific antigenic makeup of a particular organism may help to determine the sites where it can produce infections.

  • It is, in fact, the failure of host defense that leads to coliform disease.
  • The normal flora are antagonistic and help prevent infection. Prolonged antibiotic therapy can compromise normal defense mechanisms.
  • Loss of anatomic barriers or immunosuppression are also contributory to coliform disease.

  • Multiple reservoirs and modes of transmission exist for these organisms. They may be found in water (tap or distilled), soil, food and the intestinal tract as well as contaminated hospital food or containers, respiratory equipment, hemodialysis units, intravenous (IV) fluids or caps or staff members. Thus, there are both exogenous and endogenous sources for contamination by these organisms.


  • Clinical: Clinical diagnosis is difficult if not impossible.
  • Laboratory: The coliforms and Proteus are easily cultivated and identified by routine laboratory procedures. As a group, the Enterobacteriaceae are oxidase negative, capable of reducing nitrates to nitrites, ferment glucose, ferment lactose (except Proteus) and motile (except Klebsiella). Many conventional and rapid identification systems are available.


  • Sanitary: Frequent hand-washing by staff and a general awareness of microbial presence can reduce hospital-acquired infections. Disinfectants are not always effective.
  • Immunological: There is the possibility of anti-serum or vaccine against these organisms but none are currently in use.
  • Chemotherapeutic: Moderate or broad spectrum antibiotics are generally useful. Susceptibility tests should be performed when appropriate.

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