Assessment of cooperative antibiotic resistance of Salmonella Typhimurium within heterogeneous population

초록

This study was designed to investigate the cooperative resistance in the mixed culture of antibiotic-sensitive and antibiotic-resistant Salmonella Typhimurium. Strains of S. Typhimurium ATCC 19585 (STS) and clinically isolated antibiotic-resistant S. Typhimurium CCARM 8009 (STR) grown in single and mixture with 1 x MIC ceftriaxone (CEF) were used to determine the viability, beta-lactamase activity, and gene expression. The MIC50 values of STR to CEF was increased by more than 5-fold with increasing inoculum densities from 10(2) to 10(7) CFU/mL. STS was resistant to 1 x MIC CEF in the mixed culture of STS and STR, showing the more than 10(8) CFU/mL after 20 h of incubation at 37 degrees C. The highest beta-lactamase activity was 18 mu mol/min/mL in the mixed culture, corresponding to the highest relative expression of beta-lactamase-related genes (bla(TEM)). These results shed new light on the cooperative resistance of antibiotic-sensitive bacteria within a heterogeneous population including beta-lactamase-producing bacteria.

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