Stability of four carbapenem antibiotics in discs used for antimicrobial susceptibility testing

Abstract

In low-to middle-income countries, microbiological laboratories often use disc diffusion for antimicrobial susceptibility testing (AST). Reliable AST of carbapenem antibiotics is crucial for treatment decisions and surveillance purposes. Transport and storage conditions of materials used for AST are critical and may be challenging in some settings, where temperature cannot always be controlled. This study aimed to test the stability of four carbapenems in discs for AST under unfavourable conditions, i.e., at room temperature and 35{degrees}C for up to 72 hours. Imipenem, meropenem, ertapenem, and doripenem discs from three brands, Oxoid, Becton Dickinson, and HiMedia, containing 10 g of antibiotic were included. Discs were exposed to six unfavourable conditions and the recommended storage-condition as control. Subsequently, disc diffusion testing following the EUCAST guidelines was performed with four well-defined strains of Escherichia coli with different susceptibility profiles to carbapenems. The inhibition zone diameters were measured after 16-18 hours of incubation at 35{+/-}2{degrees}C. All experiments were executed in triplicate. In parallel, the carbapenems degradation was observed using a spectrophotometric method. Our study revealed that carbapenem discs were generally stable for AST although the concentration of most carbapenem antibiotics in discs decreased over time. Overall, imipenem (Oxoid and Becton Dickinson) discs were the most stable. Meropenem discs were less stable when exposed to 35{degrees}C than at room temperature. Concentrations of carbapenems in HiMedia discs were higher than those in Oxoid and Becton Dickinson. For carbapenem AST using disc diffusion in a rural area, we recommend using imipenem discs from Oxoid or Becton Dickinson. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=82 SRC="FIGDIR/small/599257v1_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@1ca4d5aorg.highwire.dtl.DTLVardef@9c54bborg.highwire.dtl.DTLVardef@17ee220org.highwire.dtl.DTLVardef@12fcfc0_HPS_FORMAT_FIGEXP M_FIG C_FIG HIGHLIGHTSO_LIIn remote areas, transportation conditions of AST discs may be challenging C_LIO_LIDiscs with four carbapenems from three brands were tested using six conditions C_LIO_LICarbapenem discs were generally stable for AST C_LIO_LIIn unfavourable conditions, concentrations in the discs degraded over time C_LIO_LIImipenem (Oxoid and Becton Dickinson) discs were the most stable C_LI