Transition of MICs of linezolid against methicillin-resistant Staphylococcus aureus at our hospital and their comparison using different antimicrobial susceptibility testing methods and measuring instruments

Abstract

At Teikyo University Hospital, antimicrobial susceptibility testing was conducted by the prompt inoculation method using the WalkAway 96 Plus system. In recent years, the frequency of the isolation of methicillin-resistant Staphylococcus aureus (MRSA) strains with a minimum inhibitory concentration (MIC) of 4 μg/mL for linezolid (LZD) has seemed to be increasing. This MIC of LZD indicates the upper limit of sensitivity. Therefore, we investigated the transition in the isolation rate of MRSA strains that showed a MIC of 4 μg/mL for LZD. The isolation frequency of these strains increased from 1.4% (8/568 strains) in 2010 to 24.4% (75/307 strains) in 2014 and was later maintained at 10–15%. In addition, the MIC for 103 strains was reevaluated using both the turbidity inoculation method and the agar plate dilution method. The MIC for all the strains was 2 μg/mL or lower in both methods. We also reevaluated the MIC of LZD for 40 selected strains using three types of antimicrobial susceptibility testing equipment. The MIC for all the strains was 2 μg/mL or lower when remeasured using the DPS192iX and BD Phoenix M50 systems; however, that for 39 strains was 2 μg/mL or lower when remeasured using the VITEK2 system. These results indicate that the MIC of LZD for MRSA strains measured by the prompt inoculation method using the WalkAway 96 Plus system is approximately 1 doubling dilution higher than that of the other methods and equipment. In most cases, a difference of ±1 doubling dilution is considered acceptable. However, the difference would increase to over 2 doubling dilutions when other confounding factors are involved. In conclusion, it is important to perform various evaluations to understand the characteristics of various antimicrobial susceptibility testing methods and equipment.