Deletion of TnAbaR23Results in both Expected and Unexpected Antibiogram Changes in a Multidrug-Resistant Acinetobacter baumannii Strain

Abstract

ABSTRACTSince the 2006 discovery of theAcinetobacter baumanniistrain AYE AbaR1 resistance island, similar elements have been reported in numerous members of this species. As AbaR1 is distantly related to Tn7, we have renamed it TnAbaR1. TnAbaRtransposons are known to carry multiple antibiotic resistance- and efflux-associated genes, although none have been experimentally studieden bloc. We deleted the TnAbaRtransposon inA. baumanniiA424, which we have designated TnAbaR23, and characterized independent deletion mutants DCO163 and DCO174. The NotI pulsed-field gel electrophoresis (PFGE) profile of strain DCO174 was consistent with targeted deletion of TnAbaR23alone, but strain DCO163 apparently harbored a second large genomic deletion. Nevertheless, “subtractive amplification” targeting 52 TnAbaRand/or resistance-associated loci yielded identical results for both mutants and highlighted genes lost relative to strain A424. PCR mapping and genome sequencing revealed the entire 48.3-kb sequence of TnAbaR23. Consistent with TnAbaR23carrying two copies ofsul1, both mutants exhibited markedly increased susceptibility to sulfamethoxazole. In contrast, loss oftetAR(A) resulted in only a minor and variable increase in tetracycline susceptibility. Despite not exhibiting a growth handicap, strain DCO163 was more susceptible than strain DCO174 to 9 of 10 antibiotics associated with mutant-to-mutant variation in susceptibility, suggesting impairment of an undefined resistance-associated function. Remarkably, despite all three strains sharing identicalgyrAandparCsequences, the ciprofloxacin MIC of DCO174 was >8-fold that of DCO163 and A424, suggesting a possible paradoxical role for TnAbaR23in promoting sensitivity to ciprofloxacin. This study highlights the importance of experimental scrutiny and challenges the assumption that resistance phenotypes can reliably be predicted from genotypes alone.