This study was designed to evaluate the stability of chloramphenicol, erythromycin, tetracycline, cephalothin, ciprofloxacin, and tobramycin against antibiotic-sensitive Salmonella Typhimurium (ASST) and antibiotic-resistant S. Typhimurium (ARST) during the broth microdilution assay. The antimicrobial activity in association with antibiotic stability was measured by using antibiotic susceptibility, time-delayed inoculation, time-extended incubation, and inoculum effect assays. The loss of the antimicrobial activity of cephalothin against ASST exposed to 1 MIC was observed for the 10 h delayed inoculation. The antimicrobial activities of tetracycline and ciprofloxacin against ASST and ARST exposed to ½ MIC were significantly decreased after the 10 h delayed inoculation. All antibiotics used in this study, except for ciprofloxacin, showed the considerable losses of antimicrobial activities against ASST and ARST after 40 h of incubation at 37 °C when compared to the 20 h of incubation during AST. Compared to the standard inoculum level (6 log CFU/mL), the MIC0.1 values of bactericidal antibiotics, ciprofloxacin and tobramycin against ASST were increased by more than 4-fold at the high inoculum level of 9 log CFU/mL. This would provide practical information for better understanding the clinical efficacy of the currently used antibiotics by considering the antibiotic stability during incubation time at different inoculum levels.
This study was designed to assess the effect of β-lactam/β-lactamase inhibitor combinations on the inhibition of biofilm formation of Salmonella Typhimurium. The anti-planktonic and anti-biofilm activities of ampicillin (AMP), ceftriaxone (CEF), and combination treatments of antibiotics and sulbactam (AMP + SUL and CEF + SUL) were evaluated against antibiotic-sensitive S. Typhimurium ATCC 19585 (STAS) and clinically isolated multidrug-resistant (MDR) S. Typhimurium CCARM 8009 (STMDR). Compared to the control, the minimum inhibitory concentrations (MICs) of AMP against STAS and CEF against STMDR were decreased from 32 to 16 μg/mL and 0.25 to 0.125 μg/mL, respectively, in the presence of SUL. The numbers of STMDR treated with AMP + SUL and CEF + SUL were effectively reduced by more than 2 logs after 4 h of incubation at 37 °C. The β-lactamase activities of STAS and STMDR treated with AMP and CEF were reduced from 3.3 to 2.6 μmol/min/mL and from 8.3 to 3.4 μmol/min/mL, respectively, in the presence of SUL. The biofilm cell numbers of STAS and STMDR were reduced at all treatments after 24 h of incubation at 37 °C. The biofilm cell numbers of STAS and STMDR were reduced by more than 2 logs in the presence of SUL compared to the AMP and CEF alone. The lowest relative fitness level was 0.6 in STAS treated with AMP + SUL, while no significant differences in the relative fitness were observed in STMDR. This study suggests that β-lactamase inhibitors (BLIs) could be used for controlling biofilm formation of β-lactamase-producing multidrug-resistant S. Typhimurium.
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