Gram-Negative Enteric Bacilli (Enterobacteriaceae) – Part A

Vocabulary flashcards covering classification, culture media, biochemical tests, antigenic markers, and virulence factors of Enterobacteriaceae as discussed in the lecture.

Enterobacteriaceae

A family of Gram-negative, facultative anaerobic rods that ferment glucose, reduce nitrate to nitrite, are oxidase-negative, and inhabit the intestinal tract.

Coliforms

Lactose-fermenting Enterobacteriaceae normally found in the colon; key genera include Escherichia, Klebsiella, and Enterobacter.

Non-lactose fermenters (NLF)

Enterobacteriaceae that do not ferment lactose; divided into invasive pathogens (e.g., Salmonella, Shigella, Yersinia) and opportunists (e.g., Proteus, Providencia, Morganella).

Seven tribes of Enterobacteriaceae

Escherichieae, Edwardsiellae, Salmonelleae, Citrobacteriaceae, Klebsielleae, Proteae, and Yersiniae.

Gram-negative bacilli (GNB)

Rod-shaped bacteria with a thin peptidoglycan layer and outer membrane, staining pink on Gram stain.

Facultative anaerobe

An organism that grows with or without oxygen, switching metabolic pathways accordingly.

Peritrichous flagella

Flagella distributed over the entire bacterial surface, providing motility to most Enterobacteriaceae.

SKY organisms

The three non-motile Enterobacteriaceae: Shigella, Klebsiella, Yersinia.

EK organisms

The two encapsulated Enterobacteriaceae: Enterobacter and Klebsiella.

EMB (Eosin Methylene Blue) agar

Primary isolation medium; Aeosin Y and methylene blue inhibit Gram-positives and indicate lactose fermentation (E. coli produces a green-metallic sheen).

MAC (MacConkey) agar

Primary isolation medium with crystal violet and bile salts; lactose fermenters appear pink/red, NLF are colorless.

DCA (Deoxycholate Citrate Agar)

Primary medium containing sodium deoxycholate/citrate; differentiates lactose fermenters (pink) from NLF (colorless).

HEA (Hektoen Enteric Agar)

Selective medium with bile salts; lactose/sucrose fermenters are yellow, NLF appear green or blue-green (black centers if H₂S+).

XLD (Xylose Lysine Deoxycholate) agar

Selective medium where lactose/sucrose fermenters are yellow, NLF are red; detects H₂S (black centers).

SSA (Salmonella-Shigella Agar)

Selective medium for Salmonella/Shigella; lactose fermenters are pink/red, NLF are colorless; H₂S producers show black centers.

TSIA (Triple Sugar Iron Agar)

Biochemical test containing glucose (0.1 %), lactose (1 %), sucrose (1 %); reports slant/butt reactions (A or K), gas, and H₂S production.

KIA (Kligler Iron Agar)

Carbohydrate test with glucose (0.1 %) and lactose (1 %); interpreted similarly to TSIA but lacks sucrose.

Oxidase test

Detects cytochrome c oxidase; Enterobacteriaceae are oxidase-negative (no color change).

Nitrate reduction test

Assesses ability to reduce nitrate to nitrite; almost all Enterobacteriaceae are positive except Pantoea agglomerans and some Erwinia spp.

IMViC reactions

Series of tests—Indole, Methyl Red, Voges-Proskauer, Citrate—used to differentiate Enterobacteriaceae.

Indole test

Detects tryptophanase converting tryptophan to indole; positive shows a red ring after adding Kovac’s or Ehrlich’s reagent.

Methyl Red test

Detects stable mixed-acid fermentation from glucose; red color after adding methyl red reagent indicates positivity.

Voges-Proskauer (VP) test

Detects butylene glycol pathway producing acetoin; red color after reagents (α-naphthol & KOH) indicates positivity.

Citrate utilization test

Determines ability to use citrate as sole carbon source on Simmons citrate agar; positive = growth with blue color.

Urease test

Detects hydrolysis of urea to ammonia using Christensen’s urea agar; positive turns medium pink/magenta.

Phenylalanine deaminase (PAD) test

Uses 10 % ferric chloride to detect phenylpyruvic acid; green color indicates Proteus, Providencia, Morganella group.

Lysine decarboxylase (LDC) test

Detects decarboxylation of lysine to cadaverine; positive tubes change from yellow to purple (bromcresol purple indicator).

LIA (Lysine Iron Agar)

Differentiates Enterobacteriaceae by lysine decarboxylation/deamination and H₂S production.

DNase test

Assesses DNA hydrolysis; Serratia spp. are uniquely DNase-positive among Enterobacteriaceae.

Type III secretion system

Needle-like apparatus that injects effector proteins into host cells; a major virulence factor of many enteric pathogens.

Endotoxin (Lipid A)

Heat-stable component of the outer membrane LPS of Gram-negative bacteria; induces fever and shock.

Enterotoxin

Exotoxin acting on the small intestine, causing fluid/electrolyte loss and diarrhea (e.g., ETEC heat-labile and heat-stable toxins).

Shiga toxin

Potent cytotoxin produced by Shigella dysenteriae and some E. coli (STEC); inhibits protein synthesis in host cells.

O antigen

Heat-stable somatic polysaccharide portion of LPS; used in serotyping of Enterobacteriaceae.

H antigen

Heat-labile antigen located on flagella; present only in motile Enterobacteriaceae.

K antigen

Capsular polysaccharide antigen; found in encapsulated species like Klebsiella, Salmonella, and some E. coli.

H₂S production

Black precipitate in media (TSIA, XLD, HEA) due to ferrous sulfide formation; characteristic of Salmonella, Proteus, Edwardsiella, etc.

Mixed-acid fermenter

Bacterium producing stable acidic end products (e.g., E. coli) yielding a positive Methyl Red test.

Butylene glycol fermenter

Organism converting glucose to neutral end products like acetoin, producing a positive VP test (e.g., Enterobacter, Klebsiella).

Proteus-Providencia-Morganella group

PAD-positive, urease-positive, often motile NLF opportunists; associated with UTIs and wound infections.