| Summary: | The objective of this study was to evaluate collateral sensitivity and cross-resistance of antibiotic-induced resistant <i>Salmonella</i> Typhimurium to various antibiotics. <i>S</i>. Typhimurium ATCC 19585 (ST<sup>WT</sup>) was exposed to ciprofloxacin, gentamicin, kanamycin, and tetracycline to induce antibiotic resistance, respectively, assigned as ST<sup>CIP</sup>, ST<sup>GEN</sup>, ST<sup>KAN</sup>, and ST<sup>TET</sup>. 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. ST<sup>CIP</sup> showed the cross-resistance to tetracycline and collateral sensitivity to gentamicin (1/2 fold) and kanamycin (1/4 fold). ST<sup>TET</sup> 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 ST<sup>CIP</sup> and ST<sup>TET</sup> 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.
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