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General Characteristics :

  • often referred to as “enterics”
  • gram negative rods or coccobacilli
  • facultative anaerobes
  • typical colony on nonselective medium is wet, gray, circular, convex, and smooth with definite edges
  • most are non-hemolytic
  • ferment glucose with production of acid
  • reduce nitrates to nitrites
  • oxidase negative (except of Plesiomonas)
  • high antigenic variability
  • can be resistant to multiple antibiotics (unpredictable antibiogram) due to plasmids with antimicrobial resistance genes

Clinical Significance :

  • found everywhere in nature
  • reside in GI tract …. only Salmonella, Shigella, and Yersinia are NOT part of normal flora
  • based on clinical infections caused, divided into 3 groups :
    • Primary pathogens : not present as normal flora… Salmonella, Shigella, Yersinia
    • Opportunistic pathogens : part of normal flora of the GI tract and only cause disease when they get outside the GI tract
    • Others : newer groups whose role not yet defined

Pathogens :

  • Salmonella
  • Shigella
  • Edwardsiella
  • Yersinia

Opportunists :

  • Esherichia
  • Klebsiella
  • Enterobacter
  • Serratia
  • Proteus
  • Providencia
  • Morganella
  • Citrobacter

Others :

  • Hafnia
  • Pleiseiomonas

Virulence and Antigenic Factors :

  • ability to adhere
  • ability to colonize
  • ability to produce toxins
  • ability to invade tissue

Antigens used to ID :

  • O Antigen or Somatic antigen is heat-stable on cell wall
  • H Antigen or Flagellar antigen is heat-labile and on flagella
  • K Antigen or Capsular antigen is heat-labile and found only on certain encapsulated species
  • Vi Antigen is capsular antigen that is produced primarily by Salmonella 

Serological Identification :

  • Antigenic Formula
    • always listed in the same sequence
    • sequence is O, K, H
    • Salmonella typhi (Salmonella serotype Typhi)
      • has O antigens 9 and 12; Vi antigen; H antigen phase 1 flagellar antigen d
      • Antigenic formula : 9,12:(Vi):d

Pathogenic Toxins :

  • LPS is cell wall : Lipid A (endotoxin)
  • LPS is the major factor associated with morbidity and mortality in patients infected with Enterobacteriaceae
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