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 (ST) and clinically isolated antibiotic-resistant S. Typhimurium CCARM 8009 (ST) grown in single and mixture with 1 × MIC ceftriaxone (CEF) were used to determine the viability, β-lactamase activity, and gene expression. The MIC values of ST to CEF was increased by more than 5-fold with increasing inoculum densities from 10 to 10 CFU/mL. ST was resistant to 1 × MIC CEF in the mixed culture of ST and ST, showing the more than 10 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 (bla). These results shed new light on the cooperative resistance of antibiotic-sensitive bacteria within a heterogeneous population including β-lactamase-producing bacteria.