Frequency of 16S rRNA Methylase and Aminoglycoside-Modifying Enzyme Genes among Clinical Isolates of Acinetobacter baumannii in Iran

Multidrug-resistant Acinetobacter baumannii (MDR-AB) is an important nosocomial pathogen which is associated with significant morbidity and mortality, particularly in high-risk populations. Aminoglycoside-modifying enzymes (AMEs) and 16S ribosomal RNA (16S rRNA) methylation are two important mechanisms of resistance to aminoglycosides. The aim of this study was to determine the prevalence of 16S rRNA methylase (armA, rmtA, rmtB, rmtC, and rmtD), and the AME genes [aac(6′)-Ib, aac(3)-I, ant(3′′)-I, aph(3′)-I and aac(6')-Id], among clinical isolates of A. baumannii in Tehran, Iran.
Between November 2015 to July 2016, a total of 110 clinical strains of A. baumannii were isolated from patients in two teaching hospitals in Tehran, Iran. Antimicrobial susceptibility testing was performed according to Clinical and Laboratory Standards Institute guidelines. The presence of genes encoding the AMEs and16S rRNA methylases responsible for resis­tance was investigated by multiplex polymerase chain reaction.
The results showed that colistin was an effective antibiotic and could be used as a last-resort treatment of infections caused by MDR-AB. The resistance rate to aminoglycosides were 100%, 96.36% and 90.9% for tobramycin, gentamicin and amikacin, respectively. In this study, AME genes of aac(6′)-Ib, aac(3)-I and ant(3′′)-I were most prevalent among the isolated strains.
Conclusion Markedly high resistance to tobramycin, gentamicin and amikacin was noted in current study. Our results suggested that modifying enzyme genes in conjunction with methylation of 16S rRNA might contribute to aminoglycoside resistance induced in vivo in A. baumannii.Further studies are required to determine the prevalence of the aminoglycoside resistance genes in other hospitals of Iran.