9.1

• Family Enterobacteriaceae

  • Gram negative
  • Facultative anaerobic
  • Non-sporeforming
  • Predominantly facultative flora of the human bowel
  • Motile with peritrichous flagella except Tutamella, Shigella & Klebsiella

  
• Gram stain of urine positive for E. coli
• The short, plump gram-negative rods are typical of any member of the Enterobacteriaceae

  
• Biochemical Characteristics of Enterobacteriaceae

  • Catalase (+)
  • All members ferment glucose
  • Reduce nitrate to nitrite
  • Oxidase (-)
• Oxidase (-)


  Differentiates Enterobacteriaceae from Vibrio which are oxidase (+)
• Colonies on BAP

  • Luxurious
  • Moist
  • Smooth
  • Gray
  • Shiny
  • Entire
  • Convex
  • Opaque
• Virulence factors of Enterobacteriaceae

  • Ability to adhere
  • Colonize
  • Produce toxins
  • Invade tissue
• Some Enterobacteriaceae harbor plasmids that can provide antimicrobial resistance genes
• Antigens of Enterobacteriaceae

  • O or somatic antigen
  • H or flagellar antigen
  • K or capsular antigen
• Endotoxins present within the cell walls of Enterobacteriaceae are responsible for much of the morbidity and mortality resulting from infections
• Effects of endotoxins


  • Fever
  • Chills
  • Hypotension
  • Granulocytosis
  • Thrombocytopenia
  • Disseminated intravascular coagulation
  • Activation of both classic and alternate complement pathways
• Categories of Enterobacteriaceae based on clinical infections

  • Opportunistic pathogens
  • Primary pathogens

  
• True Intestinal Pathogens

  • Salmonella
  • Shigella
  • Escherichia
  • Yersinia
• Opportunistic Pathogens

  • Escherichia
  • Citrobacter
  • Proteus
  • Edwardsiella
  • Morganella
  • Hafnia
  • Providencia
  • Enterobacter
  • Serratia
  • Klebsiella
• Gram Negative Bacilli MAC - Positive

  • Enterobacteriaceae
  • Acinetobacter
  • Stenotrophomonas
• MAC is the most frequently used primary selective and differential agar
• MAC contains crystal violet dye
• The pH indicator in MAC is neutral red
• Bacterial fermentation of lactose results in acid production
• Oxidase – Negative indicates cytochrome oxidase participates in electron transport and in the nitrate metabolic pathways of certain bacteria
• 1% tetramethyl-p-phenylenediamine dihydrochloride is the reagent used for oxidase testing
• Rapid Lactose fermenters

  • Escherichia
  • Klebsiella
  • Enterobacter

  
• Late Lactose fermenters

  • Salmonella arizonae
  • Shigella sonnei
  • Serratia
  • Hafnia
  • Yersinia
• Non Lactose fermenters

  • Salmonella
  • Shigella
  • Proteus
  • Providencia
  • Edwardsiella
  • Erwinia
  • Morganella
• Eosin Methylene Blue (EMB) Fermentable CHO


  • Lactose (Levine’s)
  • Lactose & sucrose (Holt Harris)
• Gram (+) bacteriostatic agents in EMB

  • Eosin Y
  • Methylene blue
• Indicators in EMB

  • Eosin Y
  • Methylene blue
• Color of Colonies on EMB

  • Lactose fermenter: pink colonies
  • Non LF: colorless colonies
• Color of Colonies on EMB for specific organisms

  • Escherichia coli: Pink/purple colonies with greenish metallic sheen
  • Klebsiella: Pink mucoid colonies
  • Enterobacter: "Fish eye colonies" with pink colonies and dark center
• Dark center is seen surrounded by a wide, light-colored, mucoid rim in Enterobacter colonies
• McConkey agar Gram(+) bacteria inhibitors

  • Crystal violet
  • Bile salts
• CHO in McConkey agar

  • Lactose
• pH indicator in McConkey agar

  • Neutral red
• Color of colony in McConkey agar

  • Lactose fermenter: pink
  • NonLF: colorless colonies
• Salmonella Shigella agar (SSA) characteristics

  • Selective & differential for Salmonella & Shigella
  • CHO: lactose
  • Gram (+) bacteriostatic agent: Bile salts
  • pH indicator: neutral red
  • H2S indicator: ferric citrate
• Color of colonies on SSA

  • Salmonella: NLF (colorless colonies), H2S (+)
  • Shigella: NLF (colorless colonies), H2S (-)
• H2S (+) organisms

  • Salmonella
  • Proteus
  • Arizona
  • Citrobacter
  • Edwardsiella
• Hektoen Enteric agar (HEA) characteristics

  • Gram (+) bacteriostatic agent: bile salts
  • CHO: lactose, sucrose & salicin
  • pH indicator: bromthymol blue
  • H2S indicator: ferric ammonium citrate
• Color of colonies on HEA

  • H2S (+) colonies: colonies with black center
• Triple Sugar Iron (TSI) media components

  • CHON
  • CHO source: Lactose (10 parts), Sucrose (10 parts), Glucose (1 part)
  • pH indicator: phenol red
  • H2S indicator: ferrous sulphate
  • Source of sulphur: Na thiosulfate