Enterobacteria

• Lecture objectives include listing members of Enterobacteria, general characteristics, host diversity, major infections, and infection control
• Enterobacteria


  A heterogenous group of Gram negative bacilli associated with GIT either as pathogens or normal flora
• Examples of Enterobacteria

  • E. coli
  • Shigella
  • Salmonella
  • Proteus
  • Klebsiella
  • Enterobacter
  • Citrobacter
  • Yersinia
• General characteristics of Enterobacteria

  • Gram -ve rods
  • Oxidase –ve
  • Ferment glucose
  • Reduce nitrates to nitrites
  • Aerobic and facultative anaerobic
  • Catalase +ve, with rare exceptions
  • Have antigens (O, H, K, Vi)
• Antigens of Enterobacteria

  • O antigen (cell envelope)
  • H antigen (flagella)
  • K antigen (capsule)
  • Vi antigen (Salmonella Typhi)
• Culture of Enterobacteria

  1. On ordinary media

  2. On differential medium

  3. On enrichment medium

  4. On selective media
• Growth on differential media

  • MacConkey agar
  • Lactose fermenters – pink/red
  • Non-lactose fermenters - pale
• Selective media for Enterobacteria

  • Salmonella Shigella agar (SS)
  • Bismuth sulphite agar
• Black colonies on Bismuth sulphite agar indicate Salmonella
• Enrichment of Enterobacteria

  1. Selenite F broth

  2. Cold enrichment
• Identification


  Process by which a microbe’s key features are delineated
• Identification methods

  • Morphology (colonial & Gram)
  • Physiological
  • Genetic
• Definitive identification biochemical tests

  • Indole
  • TSI (triple sugar iron)
  • Urea
  • Citrate test
  • H2S
  • Motility
  • MR/VP (Methyl Red, Voges-Proskauer)
• Serology is important in the identification of enterobacteria
• Virulence factors of Enterobacteria

  • Endotoxin (LPS)
  • Capsule
  • Antigenic phase variation
  • Type III Secretion system
  • Siderophores
  • Resistance to serum killing
• Spectrum of Diseases caused by Enterobacteria

  • Overt pathogens (S. Typhi, Shigella, Y. pestica)
  • Opportunistic pathogens (Klebsiella, Enterobacter, Citrobacter)
  • Normal flora (E. coli)
• Diseases associated with Enterobacteria

  • Central nervous system (E. coli, Citrobacter)
  • Lower respiratory tract (Klebsiella, Enterobacter, E. coli)
  • Blood stream (E. coli, Klebsiella, Enterobacter)
  • Gastrointestinal tract (Salmonella, Shigella, Yersinia)
  • Urinary tract (E. coli, Proteus, Klebsiella)
• E. coli is a common organism associated with Gram negative sepsis
• E. coli accounts for >80% of all community acquired UTI and hospital acquired infections
• Gastroenteritis caused by E. coli has about 6 different pathotypes
• E. coli categories

  • EPEC (Infant diarr in impoverished countries)
  • ETEC (Traveler’s, & infant diarr)
  • EAEC (Infant, traveler’s, persistent watery diarr)
  • EHEC (haemorrhagic colitis & haemolytic uremic syndrome)
  • EIEC (may progress to dysentery)