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Chapter 21 Enterobacteriaceae

Tribe I


Tribe II


Tribe III


Tribe IV


Tribe V


Tribe VI


Tribe VII


Tribe VIII



E.coli; Kleiabsiella; Serratia; Enterobacter; Hafnia; Citrobacter

Noncoliforms opportunists

Proteus; Morganella; Providencia; Edwardsiella

True pathogens

Salmonella; Shigella; Yersinia

The Enterobacteriaceae are generally divided into 2 groups based upon their ability to ferment the sugar?


Characteristics for presumptive ID for Enterobacteriaceae; *Glucose; Oxidase; Nitrate reduction; Catalase

Glucose (++); Oxidase (--); Nitrate reduction (++)**; Catalase (++)

The exception for Nitrate reduction are?

Certain Enterobacter (Pantoea) agglomerans and some Serratia and Yersinia species

Components of TSI

Protein derivatives for enrichment, glucose, lactose, sucrose, ferrous sulfate, and phenol red indicator.

No carbohydrate fermentation

Alkaline slant/ Alkaline deep (K/K)

Glucose only fermented

Alkaline slant/ acid deep (K/A)

Glucose fermented, lactose not fermented, hydrogen sulfide produced

Alkaline slant/ acid deep (black) (K/A/H2S)

Glucose, lactose, and or sucrose fermented

Acid slant/ acid deep (A /A)

Tribe I: Genera

Escherichia; Shigella

Tribe II: Genera


Tribe III: Genera


Tribe IV: Genera


Tribe V: Genera

Klebsiella; Serratia; Enterobacter; Hafnia; Pantoea

Tribe VI: Genera

Proteus; Morganella; Providencia

Tribe VII: Genera


Tribe VIII: Genera


MacConkey Agar (MAC)

Bile salts and crystal violet inhibt gram- positive organisms. Lactose is the only carbohydrate. Neutral Red indicator

MacConkey positive and negative look like?

Bright pink colonies are Lactose positive; Colorless colonies are lactose negative

Eosin-Methylene Blue (EMB)

Dyes inhibit Gram-positivew organisms and are indicators of lactose fermentation E.coli exhibits a green metallic sheen, lactose fermenters are blue-violet, and non-lactose fermenters are transparent or colorless.

Salmonella Shigella Agar (SS)

Selective; Inhibits most coliforms due to bile salts and sodium citrate. Sodium thiosulfate is added as sulfur source and ferric ammonium sulfate for detection of H2S. Lactose is sole carbohydrate added with neutral red pH indicator.

Hektoen Enteric Agar (HE)

Increaes yield of Salmonella and Shigella from heavy concentrations of normal fecal flora. Significant ingredients include high bile salts concentration, lactose, sucrose, sodium thiosulfate and ferric ammonium sulfate used for H2S detection and acid fuschin-bromthymol blue for pH indicator.

Xylose Lysine Deoxycholate (XLD)

Less inhibitory than HE. Contains xylose, lactose, sucrose for acid production and phenol red indicator. Lysine is used to detect Salmonella species. Hydrogen sulfide detection is similar to compoinetns in HE.

Lactose utilization.

Fermenters produce acid from lactose; pH indicator. Non-fermenters lack enzymes to metabolize lactose

Hydrogen Sulfide production

Reacts with sulfure-containing amino acids in the media and H2S combines with the iron-based detector--> black ppt.
The ability to liberate sulfur from sulfur containing amino acids or other compounds.

Indole Production

Degradation end product from the metabgolism of the amino acid tryptophan by bacteria which produce the enzyme tryptophanase. Spot or tube test. RED= POS/ YELLOW= NEG.

Methyl Red (MR) test

Bacteria that utilize pyruvate (glucose) produce a lot of mixed acid during fermentation. INCUBATION TIME IS IMPORTANT & KEY. 48 hours. Detects the mixed acids. RED=POS/ NO COLOR= NEG.

Voges Proskauer Test (VP)

Used to detect the formation of acetylmethyl carbinol (acetoin) as a product of glucose fermentation. Detects the end product of acetoin during pyruvate (glucose) utilization.

Citrate Utilization

Requires O2; the organisms capability to turn the medium alkaline as they grow. Bromthymol blue is teh pH indicator.

Urease Test

Organisms possessing the enzyme Urease can hydrolyze urea in the media releasing ammonia as an end product. Hydrolysis of urea causes an alkalization of the medium and the pH indicator phenol red causes a HOT PINK color to form in the media= POSITIVE!

Decarboxylation of Ornithine, lysine, and arginine (all amino acids)

Many bacteria remove the carboxyl group (COOH) from an amino acid to form an alkaline reacting amine. Detects alkaline pH (pH indicators bromcresol purple and cresol red) shift in the medium.

Phenylalanine Deaminase Test

Certain bacteria area capable of deaminating phenylalanine (amino acid) with the formation of phenylpyruvic acid. Phenylpyruvic acid is detected by adding ferric chloride to the medium.

This test is used to ID Proteus, Morganella, and Providencia species.

Phenylalanine Deaminase Test


This test detects beta-galactosidase, the enzyme that cleaves lactose into glucose and galactose. ONPG (has orthonitrphenyl instead of glucose; bosth have galactose) closely resembles lactose. Detects late or slow lactose fermenters. YELLOW=POS/ CLEAR=NEG.

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