Evidence for an Efflux Pump Mediating Multiple Antibiotic Resistance in Salmonella enterica Serovar Typhimurium

Abstract

The mechanism of multiple antibiotic resistance in six isolates of Salmonella enterica serovar Typhimurium recovered from a patient treated with ciprofloxacin was studied to investigate the role of efflux in the resistance phenotype. Compared to the patient's pretherapy isolate (L3), five of six isolates accumulated less ciprofloxacin, three of six isolates accumulated less chloramphenicol, and all six accumulated less tetracycline. The accumulation of one or more antibiotics was increased by carbonyl cyanide m-chlorophenylhydrazone to concentrations similar to those accumulated by L3 for all isolates except one, in which accumulation of all three agents remained approximately half that of L3. All isolates had the published wild-type sequences of marO and marR. No increased expression of marA, tolC, or soxS was observed by Northern blotting; however, three isolates showed increased expression of acrB, which was confirmed by quantitative competitive reverse transcription-PCR. However, there were no mutations within acrR or the promoter region of acrAB in any of the isolates.

AcrB drug-binding pocket substitution confers clinically relevant resistance and altered substrate specificity

“…The same association was previously made for L18 (18); indeed, increased expression of acrB has been confirmed in this study. However, based upon our recent data showing that the bacterial cell responds to lack of efflux (irrespective of mechanism) (31), we now postulate that increased transcription of acrB relative to L3 is in response to altered efflux owing to the G288D substitution.…”

“…Antibiotic resistance, particularly to ciprofloxacin [minimum inhibitory concentration (MIC) ≥0.5 μg/mL], occurred and increased in the isolates along the time course of infection (Table 1; susceptibility data for other antibiotics have been previously published and are shown in Table S2) (14,15,18). Previously, the MDR was attributed to increased expression of the acrB transcript (18). Indeed, measurement of acrB expression using an acrAB promoter-GFP fusion confirmed increased acrB expression in all strains isolated after the onset of therapy but the expression level did not correlate with the MDR phenotype.…”

Emergence of Salmonella epidemics: The problems related to Salmonella enterica serotyp Enteritidis and multiple antibiotic resistance in other major serotypes

“…FQ resistance in S. Typhimurium has also been attributed to active efflux mechanisms [57,119], and especially to the participation of the AcrAB-TolC efflux system. Indeed, inactivation of the genes coding for either the AcrB inner membrane multidrug transporter or the TolC outer membrane channel in S. Typhimurium DT204 strains resulted in a 16-to 32-fold reduction of resistance level to several FQ (Cip MIC of 2 µg/mL) [10,11].…”

Mechanisms of quinolone resistance in Salmonella

“…In Salmonella, evidence for the participation of active efflux in quinolone resistance has recently been provided [18,58]. Although no direct evidence has been provided for the role of the AcrAB-TolC efflux system in resistance, the production level of the AcrA protein in in vitro quinoloneselected mutants correlated well with the resistance levels to nalidixic acid and fluoroquinolones [18].…”