Detection of Colistin Heteroresistance in Carbapenem-Resistant Pseudomonas aeruginosa Clinical Isolates in Iran.

Background: Since antimicrobial resistance is a rising danger and a serious threat to the health of humans, recent studies have revealed that routine tests, such as minimum inhibitory concentration (MIC), are not enough to detect heteroresistance (HR) phenotype (i.e., phenotypic heterogeneity in antibiotic susceptibility in the identical bacterial population), and medical laboratories require awareness raising to face it. Colistin is regarded as the final therapeutic option for infections caused by carbapenem-resistant gram-negative bacteria. In this study, we explored the presence of HR to colistin in carbapenem-resistant Pseudomonas aeruginosa isolates in Iran. Methods: From 2019 to 2020, 100 P. aeruginosa clinical isolates were gathered from hospitals in various regions in Iran. In the present study, antibiotic susceptibility testing was used to determine antibiotic resistance. The population analysis profile (PAP) test was conducted to measure HR. Additionally, PCR was carried out to detect metallo-β-lactamase genes, including bla NDM-1 and bla IMP, and bla VIM genes of colistin heteroresistant isolates. Results: As a result, of the 100 P. aeruginosa isolates that were tested by AST, 66 were resistant to carbapenem antibiotics. Here, we found that 62 carbapenem-resistant Pseudomonas aeruginosa (CRPA) isolates were susceptible to colistin, while four isolates were resistant to colistin. Regarding the PAP assay, we identified eight heteroresistant isolates, of which three and two isolates carried bla NDM-1 and bla IMP genes, respectively. Furthermore, the HR isolates demonstrated a stable phenotype after seven subcultures in Mueller Hinton agar (MHA) without antibiotics. Conclusions: Finally, our study highlights the interplay between HR and its diagnosis methods. Since it is difficult to detect heteroresistant isolates by routine tests in clinical laboratories, it might be misclassified as susceptible and lead to challenges for clinicians and their patients.