Diagnosis, Therapy, and Prevention of Bacterial Diseases
8-1 Laboratory algorithms for identifying medically important gram-positive (A) and gram-negative (B) bacteria. Catalase, coagulase, and oxidase: +, organism produces the enzyme; –, it does not. Maltose and lactose: +, organism ferments the sugar; –, it does not. S, sensitive to indicated drug; R, resistant to drug. *Facultative aerobic
• Step 3: rinse with solvent that dissolves the outer membrane and washes out the crystal violet from the thin peptidoglycan layer of gram-negative bacteria but not from the thick peptidoglycan layer of gram-positive bacteria.
• 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)