Genetic characterization of tigecycline and carbapenem resistant Acinetobacter baumannii in a tertiary care hospital of Northern India.

BACKGROUND: Acinetobacter baumannii has emerged as a predominant cause of hospital-acquired infections worldwide, posing a significant challenge due to its extraordinary ability to acquire resistance to multiple antibiotics. According to recent data, infections caused by multidrug-resistant A. baumannii lead to increased morbidity, longer hospital stays, and higher healthcare costs. Carbapenems and tigecycline have historically provided reliable treatment for A. baumannii infections; however, recent global trends indicate a significant increase in carbapenem and tigecycline resistance among circulating isolates. OBJECTIVES: The study aimed to determine the prevalence and genetic characterization of tigecycline resistance in A. baumannii (TRAB) among carbapenem-resistant isolates and to assess the transferability of carbapenem and tetracycline resistance genes between isolates. METHODS: All 160 clinical isolates were obtained from admitted patients in SGPGIMS, Lucknow (India). Identification of the isolates was performed using MALDI-TOF MS, followed by antimicrobial susceptibility testing (AST), determination of minimum inhibitory concentrations (MICs), and PCR-based detection of carbapenem and tetracycline resistance genes. The transferability of resistance genes was further assessed through conjugation assays, and the AST of transconjugant isolates was evaluated accordingly. RESULTS: A total of 32 out of 160 isolates (20%) were resistant to both carbapenem and tigecycline. All these isolates were multidrug-resistant (MDR), exhibiting resistance to more than three classes of antibiotics. Genetic screening showed the blaNDM and blaOXA-48 carbapenemase genes in 25% and 18.75% of the respective isolates; blaKPC was not detected. Among tetracycline resistance genes, only tet(B) was present in 15.63% (5/32) of the isolates; other genes such as tet(A), tet(K), tet(M), and tet(S) were absent. Both carbapenemase and tetracycline resistance genes were plasmid-mediated, and often co-existed in the same isolates. However, only the carbapenemase genes were transferable by conjugation; the tetracycline resistance genes were not transferable. CONCLUSIONS: The increasing prevalence of carbapenem and tigecycline-resistant A. baumannii is primarily linked to plasmid-mediated carbapenemase genes, which facilitate their spread. In contrast, tigecycline resistance, although present, appears to be non-transferable under the conditions studied.