A Conjugative IncI1 Plasmid Carrying erm(B) and blaCTX-M-104 That Mediates Resistance to Azithromycin and Cephalosporins.

In this study, an IncI1 plasmid encoding resistance to both cefotaxime and azithromycin was recovered from a clinical Klebsiella pneumoniae strain. The azithromycin resistance was confirmed to be mediated by the erm(B) gene. This plasmid could be readily conjugated to strains of Escherichia coli and Salmonella, promoting rapid dissemination of azithromycin- and ceftriaxone-resistance-encoding elements among Gram-negative bacterial pathogens. Transmission of this plasmid in Salmonella is of particular concern, since it could mediate expression of phenotypic resistance to azithromycin and ceftriaxone, which are the current choices for treatment of Salmonella infections. Our findings suggest a need to monitor the efficiency and pattern of transmission of this plasmid among key Gram-negative bacterial pathogens. IMPORTANCE Since the approval by the FDA of azithromycin for treatment of Salmonella infections, efforts have been made to monitor the development of resistance to azithromycin in these organisms. In this study, we report an IncI1 plasmid from a clinical K. pneumoniae strain that encodes resistance to both cefotaxime and azithromycin. This plasmid could be readily conjugated to strains of Escherichia coli and Salmonella, promoting rapid dissemination of azithromycin- and ceftriaxone-resistance-encoding elements among Gram-negative bacterial pathogens. Furthermore, data from this study confirmed for the first time the role of the erm(B) gene in mediating resistance to azithromycin in various bacterial species, particularly Salmonella.