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Penicillin susceptibility testing of gonococci by disc diffusion
Abstract:
This study investigated the use of various beta-lactam discs in the detection of penicillin sensitive, intermediate resistant and resistant strains (minimal inhibitory concentrations of penicillin G of less than 0.06, 0.06 to 0.25, and greater than 0.25 mg/l respectively). Discs containing 0.15 and 0.6 microgram of penicillin G, and 5 microgram of methicillin were the most useful in separating the three groups. Cut-off zones between sensitive, intermediate resistant and resistant strains are 12 and 24 mm for 0.15 microgram penicillin discs; 24 and 32 mm for 0.6 microgram penicillin discs; and 20 and 30 mm for 5 microgram methicillin discs. Oxacillin discs (1 microgram) were useful in separating resistant from both sensitive and intermediate resistant strains at a cut-off zone of 8 mm. Use of a combination of discs provides the best index of susceptibility group, and at least two should be used
Insights
This study identifies optimal beta-lactam discs for detecting penicillin resistance in bacterial strains. Specific penicillin and methicillin disc concentrations and cut-off zones effectively differentiate sensitive, intermediate, and resistant strains.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Accurate detection of bacterial susceptibility to antibiotics is crucial for effective treatment.
- Penicillin resistance is a growing concern, necessitating reliable diagnostic methods.
- Beta-lactam discs are commonly used for antimicrobial susceptibility testing.
Purpose of the Study:
- To evaluate the efficacy of various beta-lactam discs in differentiating penicillin-sensitive, intermediate-resistant, and resistant bacterial strains.
- To determine optimal disc concentrations and zone diameter cut-offs for accurate susceptibility classification.
- To assess the utility of penicillin, methicillin, and oxacillin discs.
Main Methods:
- Investigated the performance of different beta-lactam discs against bacterial strains with known minimal inhibitory concentrations (MICs) of penicillin G.
- Determined zone diameters for various disc concentrations (0.15 µg and 0.6 µg penicillin G, 5 µg methicillin, 1 µg oxacillin).
- Established cut-off zone diameters to categorize strains into sensitive, intermediate, and resistant groups.
Main Results:
- 0.15 µg and 0.6 µg penicillin G discs, and 5 µg methicillin discs were most effective in separating the three susceptibility groups.
- Specific cut-off zones were defined: 12/24 mm for 0.15 µg penicillin, 24/32 mm for 0.6 µg penicillin, and 20/30 mm for 5 µg methicillin.
- 1 µg oxacillin discs distinguished resistant from sensitive/intermediate strains at an 8 mm cut-off.
Conclusions:
- A combination of at least two different beta-lactam discs provides the most reliable assessment of bacterial susceptibility to penicillin.
- The identified disc concentrations and cut-off zones offer a practical approach for routine antimicrobial susceptibility testing.
- Optimized disc selection enhances the accuracy of classifying bacterial resistance patterns.