Enterobacteriaceae: Classification & Characteristics

• Enterobacteriaceae are mostly located in the intestine or digestive system
• Genera of Enterobacteriaceae include: Citrobacter, Edwardsiella, Enterobacter, Escherichia, Hafnia, Klebsiella, Morganella, Pleisiomonas, Proteus, Providencia, Salmonella, Serratia, Shigella, and Yersinia

Classification of Enterobacteriaceae that infects human:

• Opportunistic pathogens: Citrobacter spp, Edwardsiella spp, Enterobacter spp, Ewingella spp, Hafnia spp, Klebsiella spp, Morganella, Pantoea, Plesiomonas, Providencia, Proteus spp, and Serratia spp
• Overt pathogens (True pathogens): Salmonella spp, Shigella spp, Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis
• In between: Escherichia coli
• Pathogenic Bacteria are Enterobacteriaceae that infects humans

General Characteristics

• Gram (-), bacilli/rods or coccobacilli bacteria
• Non-spore forming
• Motile with peritrichous flagella, except Shigella and Klebsiella, Yersinia enterocolitica at 37°C
• Facultative anaerobe, ferments glucose
• Catalase (+), except Shigella dysenteriae type 1
• Oxidase (-), except Pleisiomonas
• All are non-capsulated, except Klebsiella and Enterobacter and presence of mucoid colonies
• Reduces nitrate to nitrite, except for Erwinia and Pantoea agglomerans
• Fecal-oral route, except Yersinia pestis (black plague) that can be transmitted through the bite of a vector

Niches

• Human GIT
• Animal GIT
• Various environmental sites

Laboratory Diagnosis

• Specimen Collection should be collected from the first 4 days from onset and before antimicrobials
• Media is not required for transport
• Stool samples for Shigella spp may be with preservative or none, then keep in 4°C
• Bacteria, Fungi, Parasite should be transported at ROOM TEMPERATURE
• Rectal Swab: Aimes, Stuart may be used but Cary-blair is preferred then keep in 4°C for Shigella spp

Microscopy

• General coccobacilli or straight rods with rounded ends
• For Yersinia pestis, use a closed safety pin

Culture

• General Media: SBA, CAP, MAC
• Broth: Thioglycollate broth, brain-heart infusion broth
• Enrichment broth: Gram negative (GN) broth and Selenite broth
• Selective General media: Hektoen enteric agar (HEA), Xylose-lysine deoxycholate (XLD) agar
• For Klebsiella granulomatis: Agar with egg-yolk growth factors

Media, Purpose, Other Remarks

• Blood Agar Plate purpose is for screening oxidase of Enterobacteriaceae from other gram (-) .
• Bismuth Sulfite Agar purpose Salmonella spp and inhibits most G(+) and G(-).
• Brilliant Green Agar purpose is Salmonella spp and inhibits most G(+) and G(-).
• Cefsulodin Irgasan Novobiocin (CIN) agar purpose against Yersinia enterocolitica and it inhibits most G(+) and G(-).
• Citrate Agar purpose is for screening citrate (+) members of the Enterobacteriaceae and it uses bromothymol blue as pH indicator, with citrate as carbon source. Incubate to 4 days
• Decarboxylases-containing media purpose is for differentiation of fermentative and non-fermentative Enterobacteriaceae
• Eosin Methylene Blue purpose is to identify Escherichia coli via greenish metallic sheen, and uses Eosin Y and Methylene Blue to inhibit G(+
  • For other coliform (G-) fermenters: pink colony
  • For other non-coliform fermenters: amber or colorless, has lactose and subcrose.
• Gram negative broth enhances recovery of enteric pathogens from fecal specimens and has mannitol which favors growth of mannitol-fermenting G(-) rods
• Hektoen Enteric Agar facilitates the detection of enteric pathogens from feces or from selective enrichment broth and is best used after broth
  • Fermenters appear bright orange/salmon pink
  • Non-fermenters appear green/ blue green or lack color change
• Lysine Iron Agar identifies members of Enterobacteriaceae that are; lysine decarboxylase +, lysine deaminase +, H2S production and results are best read after 18-24 hours with a slant/butt result
• MacConkey Agar is for selection of Gram (-) and differentiation among Enterobacteriaceae
  • Bile salt and crystal violet inhibits most G(+)
  • Lactose fermenter bacteria colonies are pink or red
  • Non-lactose fermenter bacteria colonies are colorless or transparent
• MAC-SOR (MacConkey +Sorbitol) is where lactose is substituted for D-sorbitol
  • E. coli 0157:H7 is sorbitol negative hence clear
  • Others show pink colonies