9.3.4 Penicillin-resistant Streptococcus pneumoniae
Penicillin resistance of S. pneumoniae is commonly screened for with an oxacillin disk in a simple disk diffusion test. This detects non-wild type organisms if the zone size is reduced implying an increased MIC. If necessary, the MIC is then confirmed by a gradient diffusion test (Figure 18). Normally this would only be done for organisms causing an invasive infection. In general, it is seldom necessary to know the specific mechanism.
Activity 13 Interpreting an E-test to help inform a clinical decision
What is the MIC reading for the S. pneumoniae isolate in Figure 18? Refer to Table 5 and determine whether the standard dose of benzylpenicillin is suitable for treating a patient with a) pneumonia and b) meningitis.
| Penicillins | MIC breakpoints (mg/L) | |||
|---|---|---|---|---|
| S ≤ | R > | |||
| Benzyl penicillin (indications other than meningitis) | 0.06 | 2 | ||
| Benzyl penicillin (meningitis) | 0.06 | 0.06 | ||
Discussion
The pneumococcus in Figure 18 has an MIC of 0.5 mg/L. The MIC breakpoint for benzyl penicillin is
- a.according to Table 5 it is possible to use benzyl penicillin to treat pneumonia caused by this organism as the MIC is
- b.It would not be appropriate to use benzyl penicillin if this was a case of meningitis, because the blood-brain barrier reduces the amount of drug that reaches the meninges resulting in a lower level of the antibiotic in the brain than in the lungs. Therefore, a lower MIC cut-off is used for infections of the central nervous system. In this case, different antimicrobials would have to be tested to use for treatment.
How you report this isolate for surveillance will depend on the breakpoints set out in the reporting instructions.
9.3.3 Vancomycin-resistant Enterococcus
