Chapter 21 - Medical Bacteriology

1. Overview of Enterobacteriaceae

I found that these organisms share several "universal" characteristics that you should memorize for identification:

  • Gram Stain: Gram-negative rods.

  • Oxidase Test: Always Oxidase negative (this distinguishes them from Pasteurellaceae).

  • Metabolism: Facultative anaerobes; all ferment glucose.

  • Virulence: All possess Lipid A (endotoxin) in their cell walls, which can trigger fever, inflammation, and DIC (disseminated intravascular coagulation).

2. The Major Pathogens

I have broken these down by their clinical significance and unique "letter" identifiers as requested.

A. Escherichia coli (E. coli)

  • Identification: A "coliform" (Lactose fermenter). It typically shows a metallic green sheen on EMB agar.

  • Clinical: The #1 cause of urinary tract infections (UTIs). Specific strains like EHEC (O157:H7) cause bloody diarrhea and Hemolytic Uremic Syndrome (HUS).

B. Klebsiella pneumoniae

  • Identification: Non-motile. It produces a very large polysaccharide capsule, giving colonies a "mucoid" or slimy appearance.

  • Clinical: Causes healthcare-associated pneumonia, often resulting in "currant jelly" sputum.

C. Serratia marcescens

  • Identification: Known for producing a red/pink pigment (prodigiosin) when grown at room temperature. It is DNAse and gelatinase positive.

  • Clinical: An opportunistic pathogen often associated with UTIs from indwelling catheters and burn infections.

D. Proteus mirabilis

  • Identification: Highly motile. It exhibits "swarming" motility on agar, forming concentric wavelike rings. It also has a distinct "burnt chocolate" odor.

  • Clinical: Associated with UTIs and the development of kidney stones (due to urease production).

3. Key Biochemical Grouping

I noticed your slides distinguish between Coliforms and Non-coliforms. This is a high-yield distinction for exams:

  • Coliforms (Lactose Fermenters): Generally part of the normal intestinal flora.

    • Examples: E. coli, Klebsiella, Enterobacter.

  • Non-coliforms (Non-Lactose Fermenters): Often includes the "true" pathogens.

    • Examples: Salmonella, Shigella, Proteus, Serratia.

Summary Identification Table

Organism

Lactose Fermentation

Key Identifying Feature

Common Disease

E. coli

Positive

Green sheen on EMB

UTI, Diarrhea

Klebsiella

Positive

Large Capsule (Mucoid)

Pneumonia

Serratia

Negative

Red pigment; DNAse (+)

Opportunistic / Catheters

Proteus

Negative

Swarming motility

Kidney stones