Determination of vancomycin minimum inhibitory concentration for ceftazidime resistant Streptococcus pneumoniae in Iran

BACKGROUND: In the context of growing health concerns over antibiotic resistance, the evaluation of the minimum inhibitory concentration (MIC) of vancomycin for Streptococcus pneumoniae (S. pneumoniae) strains resistant to ceftazidime becomes important for guiding health policy makers. The aim of this study was to determine vancomycin MIC of ceftazidime resistant S. pneumoniae strains. METHODS: Fifty identified serotypes of ceftazidime resistant S. pneumoniae strains were included in the study. The vancomycin MIC of the above mentioned bacteria was determined based on the 0.5 McFarland standards, by using a microdilution broth and the Etest method. RESULTS: The results showed that out of 50 ceftazidime resistant strains of S. pneumoniae, 46 strains (92%) have shown a vancomycin MIC ≤0.19 − 0.1.5 μg/ml and only four strains (8%) have shown a vancomycin MIC equal to 1.5 μg/ml and the related maximum zone of inhibition was of 10 millimeter diameters. CONCLUSIONS: The results of this investigation point out the emergence of S. pneumoniae strains with a vancomycin MIC ≥1.5 μg/ml, which were resistant to ceftazidime. This finding uncovers a major health concern: a vancomycin MIC higher than 1.5 μg/ml and maximum zone of inhibition of only 10 millimeter. These findings represent an important warning for health authorities globally, concerning the treatment of patients, as the occurrence of S. pneumoniae strains with decreased vancomycin susceptibility has been demonstrated.