Lecture 3: Gram-Negative Rods

Question 1

What are the key characteristics of the Enterobacteriaceae family?

Answer 1

Gram-negative rods, facultative anaerobes, catalase-positive, oxidase-negative, ferment sugars to acid, non-sporulating.

Question 2

What are the lab identification features of S. typhimurium?

Answer 2

Gram-negative bacilli, gamma-hemolytic (BA), non-lactose fermenter (yellow on MAC), xylose fermenter, indole-negative, black colonies on XLD agar.

Question 3

How does E. coli O157:H7 cause damage?

Answer 3

Shiga toxin binds to Gb3 receptors → hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS). Uses Tir/intimin binding to evade autophagy.

Question 4

What is the role of SPI-1 and SPI-2 in Salmonella infection?

Answer 4

SPI-1: T3SS for epithelial cell invasion (intestinal phase).

SPI-2: T3SS for survival/replication in phagocytes (systemic phase).

Question 5

How do S. typhimurium and E. coli appear on MacConkey agar?

Answer 5

E. coli: Pink (lactose fermenter).

S. typhimurium: Yellow (non-lactose fermenter).

Question 6

Why is E. coli O157:H7 clinically significant?

Answer 6

Produces Shiga toxin → HC (bloody diarrhea) and HUS (kidney failure); fatal in severe cases.

Question 7

Why does S. typhimurium turn black on XLD agar?

Answer 7

Ferments xylose and decarboxylates lysine → H₂S production (reacts with iron salts).

Question 8

How is the indole test performed, and what does it indicate?

Answer 8

Tryptophan broth + Kovac’s reagent → pink = positive (e.g., E. coli); yellow = negative (e.g., Salmonella).

Question 9

How is E. coli O157:H7 identified on SMAC?

Answer 9

Colorless/yellow colonies (non-sorbitol fermenter); non-pathogenic E. coli ferment sorbitol → pink.

Question 10

Name 3 measures to prevent Salmonella/E. coli infections.

Answer 10

Handwashing, avoiding cross-contamination (raw meat), proper cooking, hygiene with pets/livestock.