Development of de novo resistance in Salmonella Typhimurium treated with antibiotic combinations

Phenotypic and Genotypic Responses of Foodborne Pathogens to Sublethal Concentrations of Lactic Acid and Sodium Chloride

The aim of this study was to evaluate the phenotypic and genotypic responses of Salmonella Typhimurium ATCC 19585 (ST) and Staphylococcus aureus KACC 13236 (SA) preadapted to sublethal concentrations of lactic acid (LA) and sodium chloride (NaCl) for 48 hr at 37°C, followed by re-exposure to lethal concentrations of LA and NaCl for 24 hr at 37°C. ST and SA treated in a sequential and ordered manner with LA and NaCl were assigned as LA-LA, LA-NaCl, NaCl-LA, and NaCl-NaCl. The treatments, LA-LA, LA-NaCl, NaCl-LA, and NaCl-NaCl, were evaluated by antimicrobial susceptibility, bacterial fluctuation, relative fitness, zeta potential, and gene expression. The MICt/MICc ratios of LA, NaCl, CIP, GEN, and TET against ST treated with LA-LA were 1.0 to 0.8, 0.8, 0.3, 0.4, and 0.5, respectively. The MICt/MICc ratios of NaCl, CIP, GEN, and TET were between 0.5-0.8 for SA treated with LA-LA. ST treated with LA-LA and SA treated with LA-NaCl exhibited the highest coefficient of variance. The lowest relative fitness was observed at ST treated with LA-LA (0.5). ST and SA treated with LA-LA showed the lowest zeta potential. The transporter-, toxin-antitoxin system-, chaperone protein-, and SOS response-related genes were suppressed at ST and SA treated with LA-LA. The transporter-, toxin-antitoxin system-, and chaperone protein-related genes were overexpressed in SA treated with LA-NaCl, NaCl-LA, and NaCl-NaCl. The results suggest that ST and SA treated with LA-LA, LA-NaCl, NaCl-LA, and NaCl-NaCl could induce collateral sensitivity and cross-resistance.

Ciprofloxacin and Tetracycline Resistance Cause Collateral Sensitivity to Aminoglycosides in Salmonella Typhimurium

The objective of this study was to evaluate collateral sensitivity and cross-resistance of antibiotic-induced resistant Salmonella Typhimurium to various antibiotics. S. Typhimurium ATCC 19585 (STWT) was exposed to ciprofloxacin, gentamicin, kanamycin, and tetracycline to induce antibiotic resistance, respectively, assigned as STCIP, STGEN, STKAN, and STTET. The susceptibilities of the antibiotic-induced resistant mutants to cefotaxime, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, polymyxin B, streptomycin, tetracycline, and tobramycin were determined in the absence and presence of CCCP and PAβN. STCIP showed the cross-resistance to tetracycline and collateral sensitivity to gentamicin (1/2 fold) and kanamycin (1/4 fold). STTET was also cross-resistant to ciprofloxacin (128-fold) and collateral sensitive to gentamicin (1/4-fold) and kanamycin (1/8-fold). The cross-resistance and collateral sensitivity of STCIP and STTET were associated with the AcrAB-TolC efflux pump and outer membrane porin proteins (OmpC). This study provides new insight into the collateral sensitivity phenomenon, which can be used for designing effective antibiotic treatment regimens to control antibiotic-resistant bacteria.

Assessment of the potential of phage-antibiotic synergy to induce collateral sensitivity in Salmonella Typhimurium

This study was designed to evaluate the synergistic effect of phage and antibiotic on the induction of collateral sensitivity in Salmonella Typhimurium. The synergistic effects of Salmonella phage PBST32 combined with ciprofloxacin (CIP) against S. Typhimurium KCCM 40253 (ST^KCCM) were evaluated using a fractional inhibitory concentration (FIC) assay. The CIP susceptibility of ST^KCCM was increased when combined with PBST32, showing 16-fold decrease at 7 log PFU/mL. The combination of 1/2 × MIC of CIP and PBST32 (CIP[1/2]+PBST32) effectively inhibited the growth of ST^KCCM up to below the detection limit (1.3 log CFU/mL) after 12 h of incubation at 37 °C. The significant reduction in bacterial swimming motility was observed for PBST32 and CIP[1/4]+PBST32. The CIP[1/4]+PBST32 increased the fitness cost (relative fitness = 0.57) and decreased the cross-resistance to different classes of antibiotics. ST^KCCM treated with PBST32 alone treatment exhibited the highest coefficient of variation (90%), followed by CIP[1/4]+PBST32 (75%). These results suggest that the combination of PBST32 and CIP can be used to control bacterial pathogens.

The Evolution of Fluoroquinolone Resistance in Salmonella under Exposure to Sub-Inhibitory Concentration of Enrofloxacin

The evolution of resistance in Salmonella to fluoroquinolones (FQs) under a broad range of sub-inhibitory concentrations (sub-MICs) has not been systematically studied. This study investigated the mechanism of resistance development in Salmonella enterica serovar Enteritidis (S. Enteritidis) under sub-MICs of 1/128×MIC to 1/2×MIC of enrofloxacin (ENR), a widely used veterinary FQ. It was shown that the resistance rate and resistance level of S. Enteritidis varied with the increase in ENR concentration and duration of selection. qRT-PCR results demonstrated that the expression of outer membrane porin (OMP) genes, ompC, ompD and ompF, were down-regulated first to rapidly adapt and develop the resistance of 4×MIC, and as the resistance level increased (≥8×MIC), the up-regulated expression of efflux pump genes, acrB, emrB amd mdfA, along with mutations in quinolone resistance-determining region (QRDR) gradually played a decisive role. Cytohubba analysis based on transcriptomic profiles demonstrated that purB, purC, purD, purF, purH, purK, purL, purM, purN and purT were the hub genes for the FQs resistance. The 'de novo' IMP biosynthetic process, purine ribonucleoside monophosphate biosynthetic process and purine ribonucleotide biosynthetic process were the top three biological processes screened by MCODE. This study first described the dynamics of FQ resistance evolution in Salmonella under a long-term selection of sub-MICs of ENR in vitro. In addition, this work offers greater insight into the transcriptome changes of S. Enteritidis under the selection of ENR and provides a framework for FQs resistance of Salmonella for further studies.