Mechanisms of Heteroresistance and Resistance to Imipenem in Pseudomonas aeruginosa

BACKGROUND: Heteroresistance is a phenomenon that occurs in all bacteria and can cause treatment failure. Yet, the exact mechanisms responsible for heteroresistance still remain unknown. The following study investigated the mechanisms of imipenem-heteroresistance and -resistance in Pseudomonas aeruginosa clinical isolates from Wenzhou, China. METHODS: Imipenem resistance was detected by the agar dilution method; heteroresistance was determined by population analysis profiles. Biofilm formation assay and modified carbapenem inactivation methods were also performed. Polymerase chain reaction (PCR) was conducted to detect oprD, and quantitative real-time PCR was used to determine expression levels of oprD, ampC and four efflux pump coding genes (mexB, mexD, mexE and mexY). RESULTS: Six imipenem-heteroresistant and -resistant P. aeruginosa isolates were selected respectively. Deficient oprD was detected in all resistant isolates and two heteroresistant isolates. No strains produced carbapenemases. Expression levels of oprD were down-regulated in heteroresistant isolates. Transcription levels of the mexE and mexY were significantly increased in all heterogeneous subpopulations compared with their respective native ones. Compared with the susceptible group, increased mean relative expression levels of mexE and mexY or the decreased mean relative expression levels of oprD were observed in the resistant group (P < 0.05), whereas transcription levels of the mexB and mexD remained unchanged. CONCLUSION: Down-regulation of oprD contributed to the resistance and heteroresistance of imipenem in our P. aeruginosa clinical isolates. In addition, the marginal up-regulation of efflux systems may indirectly affect imipenem resistance. Contrarily, defective oprD was less common in our experimental heteroresistant strains than resistant strains.