Related Experiment Videos
Screening pneumococci for penicillin resistance
Abstract:
Eighty-four pneumococci with various MICs of penicillin (38 with MICs of less than or equal to 0.06 micrograms/ml [susceptible], 35 with MICs of 0.12 to 1.0 micrograms/ml [relatively resistant], and 11 with MICs of greater than 1.0 micrograms/ml [resistant] ) were screened by a disk diffusion test using oxacillin and methicillin to see how well they distinguished penicillin-susceptible strains from those with decreased susceptibility to penicillin. The effects of Mueller-Hinton agar plus 5% sheep blood and Trypticase soy agar plus 5% sheep blood and two atmospheres, ambient air and a candle extinction jar (increased CO2), were compared. There were no obvious differences between the effects of the two media, but zones were generally larger in ambient air than in increased CO2. Although the oxacillin test can separate penicillin-susceptible and -resistant strains, it cannot separate penicillin-resistant from relatively penicillin-resistant strains by using the breakpoint of less than 20 mm recommended by the National Committee for Clinical Laboratory Standards. When the 20-mm breakpoint was applied to methicillin, 12% of the relatively resistant strains tested were erroneously classified as susceptible. When different breakpoints were used for methicillin, there was better separation of the two classes of penicillin-resistant isolates, but a few relatively resistant strains were still classified as susceptible. We recommend that oxacillin, not methicillin, be used as the screening agent with Mueller-Hinton sheep blood agar and ambient air incubation and that the breakpoint be less than 20 mm to indicate resistance or relative resistance.
Insights
Oxacillin disk diffusion effectively screens penicillin-susceptible pneumococci from resistant strains. However, it struggles to differentiate between resistant and relatively resistant strains using standard breakpoints.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Diagnostic Testing
Background:
- Penicillin resistance in Streptococcus pneumoniae is a growing public health concern.
- Accurate and efficient laboratory methods are crucial for identifying resistant strains.
- Disk diffusion tests are commonly used for antimicrobial susceptibility testing.
Purpose of the Study:
- To evaluate the effectiveness of oxacillin and methicillin disk diffusion tests in distinguishing penicillin-susceptible from penicillin-resistant pneumococci.
- To compare the performance of different agar media and incubation atmospheres.
- To assess the suitability of current breakpoints for classifying resistance levels.
Main Methods:
- Eighty-four pneumococcal isolates with varying penicillin minimum inhibitory concentrations (MICs) were tested.
- Disk diffusion tests were performed using oxacillin and methicillin on two different agar media.
- Incubation was carried out in ambient air and increased carbon dioxide (CO2) conditions.
Main Results:
- Oxacillin disk diffusion could differentiate susceptible from resistant strains but not resistant from relatively resistant strains using the 20 mm breakpoint.
- Methicillin testing with a 20 mm breakpoint misclassified 12% of relatively resistant strains as susceptible.
- Adjusting breakpoints for methicillin improved separation but still resulted in some misclassification of relatively resistant strains.
Conclusions:
- Oxacillin is recommended over methicillin as a screening agent for penicillin resistance in pneumococci.
- Mueller-Hinton agar with 5% sheep blood incubated in ambient air is the preferred method.
- A breakpoint of less than 20 mm for oxacillin disk diffusion should indicate resistance or relative resistance.