Genetic Characterization of a Conjugative Plasmid That Encodes Azithromycin Resistance in Enterobacteriaceae

Mechanisms of azithromycin resistance have rarely been reported. In this study, an IncFIB/IncHI1B plasmid that confers resistance to azithromycin was recovered from a clinical Klebsiella pneumoniae strain. This plasmid could be efficiently disseminated to Escherichia coli, Salmonella, and other Gram-negative bacterial pathogens through conjugation. This plasmid was shown to carry three macrolide resistance genes: erm(B), a novel erm(42) gene, and mph(A). The functions of erm(42) were confirmed by direct cloning of this gene and determination of the MIC of azithromycin in strains of various bacterial species which have acquired this gene. Of particular concern is the potential transmission of azithromycin-resistance to extensively drug-resistant (XDR) Salmonella, which causes infections for which treatment options are extremely limited. Monitoring and preventing dissemination of this azithromycin resistance-encoding conjugative plasmid in Enterobacteriaceae is of utmost importance.