bacte 1

• ENTEROBACTERIACEAE are Gram-negative, non sporeforming, facultatively anaerobic bacilli
• Most ENTEROBACTERIACEAE are present in the intestinal tract as commensal flora except Salmonella, Shigella and Yersinia
• ENTEROBACTERIACEAE


  • Gram-negative straight rods or coccobacilli with rounded ends
  • Large, smooth, gray colonies except Klebsiella and Enterobacter
  • Catalase (+); cytochrome oxidase (-) except P. shigelloides
  • Alginate liquefaction (-)
  • All members are motile with peritrichous flagella except Klebsiella, Shigella and Yersinia
  • All are nonencapsulated except Klebsiella, and Enterobacter
  • All members ferment glucose and reduce nitrate to nitrite
  • Some organisms may grow at low temperatures 1-5 C (Serratia and Yersinia)
• Genera of ENTEROBACTERIACEAE


  • Budvicia
  • Buttiauxella
  • Cedecea
  • Citrobacter
  • Edwarsiella
  • Enterobacter
  • Escherichia
  • Ewingella
  • Hafnia
  • Klebsiella
  • Kluyvera
  • Leclercia
  • Leminorella
  • Moellerella
  • Morganella
  • Obesumbacterium
  • Pragia
  • Pantoea
  • Photorhabdus
  • Proteus
  • Providencia
  • Rahnella
  • Salmonella
  • Serratia
  • Shigella
  • Tatumella
  • Trabulsiella
  • Xenorhabdus
  • Yersinia
  • Yokenella
• Two Groups of ENTEROBACTERIACEAE


  • Opportunistic Pathogens
  • Overt/True Pathogens
• Opportunistic Pathogens


  • They are part of the intestinal microbiota of both humans and animals
  • They generally do not initiate disease in healthy, uncompromised human hosts
  • Outside their normal body sites, they may produce serious extraintestinal
  • They produce significant virulent factors
• Opportunistic Pathogens


  • Citrobacter
  • Enterobacter
  • Klebsiella
  • Proteus
  • Serratia
• Overt/True Pathogens


  • They are not present as commensal flora of the GIT of humans
  • Only inhabit the bowel at the time of infection and are acquired by ingestion of contaminated food or water
  • Their discovery in clinical material should always be considered significant
• Overt/True Pathogens

  • Salmonella typhi
  • Shigella
  • Yersinia pestis
• Extended-spectrum-ß-lactamase-producing enterobacteria = E. coli, K. pneumonia and K. oxytoca
• E. coli, P. mirabilis and K. pneumoniae - isolated from urinary tract and cause bacteremia
• Citrobacter, Enterobacter and Serratia are antibiotic-resistant genera
• Shigella, Salmonella, E. coli and Yersinia - associated with diarrhea
• Flowchart for the presumptive identification of commonly encountered Enterobacteriaceae on triple sugar iron (TSI) agar
• K, alkaline; A, acid; PAD, phenylalanine deaminase; MMVC, Indole, methyl red, Voges-Proskauer, citrate; NLF, nonlactose fermenter
• This flowchart is adopted from the Textbook of Diagnostic Microbiology, Connie Mahon 4th ed., 2010
• Escherichia


  The most significant species in the genus Escherichia
• Escherichia coli (Colon bacillus)


  • Part of the normal bowel flora of humans and may also inhabit female genital tract
  • Invades the enterocytes lining the large intestine
  • Primary marker of fecal contamination in water purification
  • Leading cause of nosocomial infection - urinary tract infection
  • Flat, dry, with pink colonies (Mac Conkey); ẞ bemolytic on BAP
  • Some strains may be NLF or LF
  • "greenish metallic sheen" - EMB
  • E. coli "O" groups have shown cross reactivity with the "O" antigens of Shigella
  • Antigenic determinants: O, H and K antigens
  • Virulence factors: endotoxin, common pill, K1 antigen (neonatal meningitis), intimin
  • Has both the sex-pili and adhesive fimbriae
  • The K1 antigen has been found to be identical to the capsular antigen found on Neisseria meningitidis group B
  • IMVIC reaction: ++-
  • TSI reaction: A/A, (+) gas, (-) H₂S
• Escherichia hermanii


  • Formerly called E. coli atypical or enteric group-II
  • Has been isolated from CSF, wounds and blood
  • Yellow-pigmented colonies
• E. coli Strains


  • Enteropathogenic E. coli (EPEC)
  • Enterotoxigenic E. coli (ETEC)
  • Enteroinvasive E. coli (EIEC)
  • Enterohemorrhagic E. coli (EHEC serotype 0157:H7)
  • Enteroadherent E. coli (EAEC)
  • Diffusely adherent E. coli(DAEC)
  • Uropathogenic E. coli
• Enteropathogenic E. coli (EPEC)


  • Loss of microvilli; no toxin production
  • Attach to the brush border of the intestinal epithelial cells causing cell damage (adhesive property)
• Enterotoxigenic E. coli (ETEC)


  • Colonization occurs on the proximal small intestine
  • Infective dose: 10% to 100
• Enteroinvasive E. coli (EIEC)


  • Penetrates and multiplies within the intestinal epithelial cells
  • Infective dose: 10
• Enterohemorrhagic E. coli (EHEC serotype 0157:H7)


  • Associated with hemolytic uremic syndrome and thrombotic thrombocytopenic purpura
  • The toxins destroy vascular endothelial cells
  • Positive verotoxin (Shiga-like toxin) production
• Enteroadherent E. coli (EAEC)


  • Adheres to the surface of the intestinal mucosa
  • Adheres to Hep2 cells forming clumps of bacteria ("stacked brick appearance)
• Enterobacter


  Resembles Klebsiella when growing on Mac Conkey agar
• Enterobacter colonies on Mac Conkey agar


  • Pink colonies, maybe with mucoid colonies
• Significant Enterobacter species

  • E. aerogenes
  • E. cloacae (common isolate)
  • E. gergoviae
  • E. hormaechei
• Biochemical tests for Enterobacter

  • Ornithine decarboxylase test: +
  • Lysine decarboxylase test: + (except E. cloacae and E. gergoviae)
  • KCN: +
  • Gelatin liquefaction test: negative (E. aerogenes)
  • Sorbitol fermentation: +(E. aerogenes, E. cloacae)
  • Urease: +(E. cloacae)
  • Malonate test: +(E. cloacae)
  • IMVIC Reaction: -++ (E. aerogenes and E. cloacae)
  • TSI reaction: A/A, (+) gas, (-) H₂S (E. aerogenes and E.cloacae)