What is the significance of an Oxacillin (antibiotic) disk diffusion test in Streptococcus pneumoniae?

From the Guidelines

The oxacillin disk diffusion test is not recommended for determining beta-lactam susceptibility in Streptococcus pneumoniae, and instead, clinicians should use either the penicillin G disk (1 μg) for screening or perform minimum inhibitory concentration (MIC) testing with penicillin. When considering the management of community-acquired pneumonia, including those caused by S. pneumoniae, it's crucial to understand the limitations of various susceptibility testing methods, as highlighted in guidelines such as those from the Infectious Diseases Society of America 1.

Key Considerations

• The susceptibility of S. pneumoniae to penicillin is defined by the National Committee for Clinical Laboratory Standards (NCCLS), with susceptible isolates inhibited by 0.06 mg/mL, intermediate resistance by 0.1–1.0 mg/mL, and resistant isolates by >2.0 mg/mL 1.
• The use of oxacillin disk diffusion for S. pneumoniae is not recommended due to its original design for staphylococci, which have different resistance mechanisms than pneumococci.
• For S. pneumoniae, a more appropriate approach involves using the penicillin G disk (1 μg) for initial screening, with MIC testing for confirmation, especially in cases of serious infections like meningitis 1.

Clinical Implications

• The increasing resistance of S. pneumoniae to penicillin and other antimicrobial drugs necessitates accurate susceptibility testing to guide effective treatment 1.
• Clinicians must be aware of the variations in resistance rates among different populations and regions, as well as the clinical implications of intermediate versus high-grade resistance in the context of pneumococcal infections 1.
• The choice of antibiotic therapy should be based on the most current and reliable susceptibility data, considering the specific infection site and the patient's clinical condition 1.

From the FDA Drug Label

Oxacillin is indicated in the treatment of infections caused by penicillinase producing staphylococci which have demonstrated susceptibility to the drug. Cultures and susceptibility tests should be performed initially to determine the causative organism and its susceptibility to the drug

The drug label does not mention Streptococcus pneumoniae or the use of an oxacillin disk diffusion test in this context.

• The label discusses the use of oxacillin in treating infections caused by penicillinase producing staphylococci.
• It does mention the importance of susceptibility tests, but does not provide information on using oxacillin disk diffusion tests for Streptococcus pneumoniae 2. The FDA drug label does not answer the question.

From the Research

Oxacillin Disk Diffusion Test in Streptococcus pneumoniae

• The oxacillin disk diffusion test is used to detect penicillin susceptibility in Streptococcus pneumoniae isolates 3, 4, 5, 6, 7.
• Studies have shown that the oxacillin disk diffusion test can be a reliable method for detecting penicillin resistance in S. pneumoniae, with good correlation between the zones of inhibition and penicillin minimum inhibitory concentrations (MICs) 3, 7.
• However, some studies have also reported limitations of the oxacillin disk diffusion test, including the potential for false-positive results and the inability to distinguish between penicillin-resistant and intermediate strains 5.
• The test has been evaluated in various laboratory settings, including a study that found good intra- and interlaboratory reproducibility of the test results 6.
• The oxacillin disk diffusion test has been proposed as a useful tool for predicting penicillin resistance in cases of meningeal strains of S. pneumoniae, particularly in low- and middle-income countries where MIC determination is not routinely available 7.

Comparison with Other Methods

• The oxacillin disk diffusion test has been compared with other methods, including the E-test and agar dilution, and has been found to be a good screening test for S. pneumoniae isolates with decreased susceptibility to penicillin 3.
• The test has also been compared with broth microdilution and disk diffusion antimicrobial susceptibility testing, and has been found to have good correlation with these methods 6.

Limitations and Future Directions

• Further studies are needed to fully evaluate the limitations and potential biases of the oxacillin disk diffusion test in detecting penicillin resistance in S. pneumoniae 5.
• The development of standardized interpretive criteria and quality control limits for the oxacillin disk diffusion test is necessary to ensure accurate and reproducible results 6.