Clarithromycin Resistance Mechanisms of Epidemic β-Lactamase-Nonproducing Ampicillin-Resistant Haemophilus influenzae Strains in Japan

Abstract

The aim of this study was to clarify the clarithromycin resistance mechanisms of ␤-lactamase-nonproducing ampicillin-resistant Haemophilus influenzae strains. In all clarithromycin-resistant strains, the transcript level of acrB was significantly elevated, and these strains had a frameshift mutation in acrR. Introduction of the acrR mutation into H. influenzae Rd generated a clarithromycin-resistant transformant with the same MIC as the donor strain. Our results indicate that the acrR mutation confers clarithromycin resistance by the increasing the transcription of acrB.

Amino Acid Substitution in the Major Multidrug Efflux Transporter Protein AcrB Contributes to Low Susceptibility to Azithromycin in Haemophilus influenzae

“…Although all of the isolates with a nonsense mutation in acrR showed resistance to CLR (MICs of Ն32 g/ml), almost all of the isolates showed susceptibility to azithromycin (AZM) (MICs of Յ4 g/ml) and the AZM MICs were even slightly reduced, indicating that this mutation could not be involved in resistance to AZM. However, one of the CLR-resistant isolates with the acrR mutation showed nonsusceptibility to AZM (5). Therefore, the aim of the present study was to clarify the difference between AZM-nonsusceptible and -susceptible strains.…”

“…S1 in the supplemental material). No other CLR-resistant and AZM-susceptible strains (n ϭ 6) tested had this substitution (data not shown) (5).…”

“…In particular, clarithromycin (CLR)-resistant strains have been increasing in Japan (1). We reported previously that these strains harbored a nonsense mutation in the negative regulator gene acrR that resulted in overexpression of the chromosomal efflux pump AcrAB, which likely contributed to the CLR resistance (MICs of Ն32 g/ml) (5). Although all of the isolates with a nonsense mutation in acrR showed resistance to CLR (MICs of Ն32 g/ml), almost all of the isolates showed susceptibility to azithromycin (AZM) (MICs of Յ4 g/ml) and the AZM MICs were even slightly reduced, indicating that this mutation could not be involved in resistance to AZM.…”

“…The transcription levels of the chromosomal multidrug efflux pump acrAB were determined by semiquantitative reverse transcription (RT)-PCR, as described previously (5). The acr operons of isolates 2011-70 and 2012-42 were analyzed according to the sequencing results obtained in the previous study (5).…”

“…The acr operons of isolates 2011-70 and 2012-42 were analyzed according to the sequencing results obtained in the previous study (5).…”