JMM Editor’s Choice: does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?

Posted on September 7, 2020   by Microbiology Society

The Journal of Medical Microbiology (JMM) is a journal published by the Microbiology Society, focused on providing comprehensive coverage of medical, dental and veterinary microbiology and infectious diseases, including bacteriology, virology, mycology and parasitology. This month, Dr Steve Michell discusses the paper ‘Does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?’ which was chosen as Editor’s Choice for the August issue of JMM.

The determination of accurate minimum inhibitory concentrations (MICs) is critical to inform correct clinical decisions. In severe cases, failure of antibiotic therapy to treat bacterial infections, including methicillin-resistant Staphylococcus aureus (MRSA), can be fatal. In this issue of JMM, Kanesaka and colleagues highlight that the inclusion of sub-culturing in MIC determination can lead to lower values that are problematic in clinical use compared to values determined from blood cultures.
I have highlighted this paper as its publication is timely given that in the same week, EUCAST published the results of their pan-European trial of rapid antimicrobial susceptibility testing (RAST) from blood cultures. Data like these will lead to a change in protocols for more accurate and relevant MIC determination, eventually resulting in less treatment failure.

Does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?

Methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infections are one of the most frequent healthcare-associated infections, particularly amongst the elderly. Vancomycin is the typical treatment for bloodstream MRSA infections, but susceptibility to this antibiotic is decreasing in MRSA communities. Accurate determination of the minimum inhibitory concentration (MIC) of vancomycin required for MRSA infection is vital for patient survival.

This study compared vancomycin MICs from direct susceptibility testing of MRSA-positive blood cultures and MRSA sub-cultured from positive blood cultures to determine the effect of sub-cultures on MIC values.