Assessment of cooperative antibiotic resistance of Salmonella Typhimurium within heterogeneous population.

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 × MIC ceftriaxone (CEF) were used to determine the viability, β-lactamase activity, and gene expression. The MIC50 values of STR to CEF was increased by more than 5-fold with increasing inoculum densities from 102 to 107 CFU/mL. STS was resistant to 1 × MIC CEF in the mixed culture of STS and STR, showing the more than 108 CFU/mL after 20 h of incubation at 37 °C. The highest β-lactamase activity was 18 μmol/min/mL in the mixed culture, corresponding to the highest relative expression of β-lactamase-related genes (blaTEM). These results shed new light on the cooperative resistance of antibiotic-sensitive bacteria within a heterogeneous population including β-lactamase-producing bacteria.