Diagnosis, Therapy, and Prevention of Bacterial Diseases

Published on 18/02/2015 by admin

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Last modified 18/02/2015

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Chapter 8

Diagnosis, Therapy, and Prevention of Bacterial Diseases

Laboratory Identification of Bacteria

Gram staining

1. Protocol

2. Results

• Purple (positive reaction): organisms with thick peptidoglycan cell wall

• Red (negative reaction): organisms with thin peptidoglycan layer and an outer membrane

• Gram-variable or gram-resistant: modified cell wall old cultures or cells treated with β-lactam antibiotics in which the peptidoglycan is weakened—therefore, poor or no color retention; modified cell wall (Box 8-1)

Growth and isolation of bacteria

1. Culture media

• Most bacteria will grow on blood agar or other nonselective media.

a. Table 8-1 lists common media used to isolate or identify particular bacteria.

• Selective medium inhibits growth of some bacteria (e.g., EMB agar inhibits gram-positive bacteria).

• Differential medium incorporates an identifying test.

• Special medium incorporates particular metabolites or provides specific culture conditions required by certain bacteria.

2. Colony characteristics (Table 8-2)

Biochemical tests

1. Metabolic tests for fermentation of various sugars and production of byproducts (e.g., acid or gases)

2. Tests for enzymes

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