Gram- Anaerobic Rods (Self-Study) - 4/29/16

Question 1

What type of anaerobes are most commonly isolated as causes of infection?

Answer 1

Gram-negative anaerobic rods

Question 2

Where are gram (-) anaerobic rods most commonly found (where do they dominate)?

Answer 2

• Dominate normal flora of large intestine, gingival crevices, and dental plaque (most abundant)
• Present in smaller numbers in small intestine, vagina, distal urethra

Question 3

The most important members of the group fall into three closely-related genera, rod-shaped or coccobacillary. All were placed in the genus Bacteroides until recently.

Answer 3

1. Bacteroides - predominantly intestinal, tend to produce abdominal infections
2. Bacteroides fragilis - the most important species of the entire group
3. Porphyromonas and Prevotella - predominantly oral; tend to prodcure infections of the head, neck, respiratory tract, bite wounds (porphyromonas makes black pigments; colonies fluoresce red under UV light)

Two less-important genera, not closely-related biologically to the first group of reach other, but similar biology:

• Fusobacterium (slender rods with taper ends; mostly oral)
• Veillonella - cocci; oral flora

Question 4

Infections with anaerobes:

Answer 4

1. Often spread from endogenous sources
2. Often contain multiple species of both anaerobic and facultative bacteria (the latter facilitate growth of anaerobes by using up ambient oxygen)

Question 5

Types of infections produced by G-anaerobes reflect their normal distribution

Answer 5

• Peridontal disease
• Abdominal abscesses
• Infections of the female reproductive tract
• Pneumonia due to aspiration of oropharyngeal contents

Brain abscesses may form by spread of organisms from sites of infection in the URT.
Septicemia occurs when organisms spread to the bloodstream from the intestine or a focus of infection [B. fragilis = one of the organisms most commonly isolated from blood cultures]

Question 6

What has recently been identified as a cause of diarrhea?

Answer 6

B. fragilis

Secretes an exotoxin - protease which cleaves E-cadherin, loosens epithelial zona adherens junctions between enterocytes, and causes cytoskeletal changes in them - allows paracellular flow of fluid into intestinal lumen

Question 7

Pathogenesis of B. fragilis

Answer 7

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