This study was designed to assess the interactions between antibiotics in combination treatments of Salmonella enterica serovar Typhimurium in association with the development of antibiotic heteroresistance. Salmonella Typhimurium ATCC 19585 (STWT), ciprofloxacin (CIP)-induced S. Typhimurium ATCC 19585 (STCIP), and clinically isolated antibiotic-resistant S. Typhimurium CCARM 8009 (STCLI) treated with antibiotic alone (cephalothin [CEP], CIP, ceftriaxone [CEF], and tobramycin [TOB]) and combination antibiotics (CEP-CIP and CEF-TOB) were used to compare the antibiotic susceptibility, resistance fitness, and cross-resistance. The susceptibilities of STWT, STCIP, and STCLI to CEP were not significant differences between CEP and CEP-CIP treatments, whereas those of STWT, STCIP, and STCLI to TOB were significant differences between TOB and CEF-TOB treatments. The interactions between CEP and CIP in the combination treatment showed mutually synergistic activities against STWT and STCLI. For the CEF-TOB combination treatments, TOB helped enhance the antibiotic activity of CEF against STWT, showing directional synergistic effect. The CEF-TOB combination treatment increased bactericidal activity against STWT, STCIP, and STCLI without causing injured cells. The combination antibiotic treatments (CEP-CIP and CEF-TOB) increased the fitness cost (relative fitness = 0.7) and decreased the cross-resistance of STWT, STCIP, and STCLI when exposed to different classes of antibiotics. This study provides new insight for designing combination antibiotic regimens that can synergistically enhance the antimicrobial activity against antibiotic-resistant Salmonella and inhibit the emergence of cross-resistance to different classes of antibiotics.
Keywords: Salmonella; combination therapy; cross-resistance; fitness; synergistic activity.