Staphylococcus aureus is one of the most well-known and widespread bacterial pathogens. The preferred antibiotic for treating infections caused by penicillin-susceptible S. aureus isolates is penicillin. To determine the susceptibility of S. aureus to penicillin, EUCAST recommends using the disk diffusion method. However, a large inhibition zone diameter around the disk does not rule out the possibility of the presence of blaZ gene, so at least one confirmation method is necessary. As part of our master's thesis, we wanted to determine an optimal and operationally simple procedure for reliably identifying penicillin-susceptible S. aureus isolates. We evaluated the zone edge test, the clover-leaf test, polymerase chain reaction for detection of blaZ gene and mass spectrometry with the MBT STAR-BL test. 98 % of S. aureus isolates with an inhibition zone diameter indicating to penicillin-susceptible isolates were actually susceptible to penicillin. The zone edge test had the highest sensitivity, correctly identifying all isolates with blaZ gene, but we found that our evaluations were too strict, which resulted in low specificity of the method. The clover-leaf test and detection of ampicillin resistance with MBT STAR-BL test are more specific methods, but less sensitive and time-consuming compared to the zone edge test. The most reliable method is the use of a reference method for determining the presence of blaZ gene with PCR.
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