In-vitro activity of β-lactams/trimethoprim-sulfamethoxazole combinations against different strains of Burkholderia pseudomallei

Trimethoprim-sulfamethoxazole is an active agent against Burkholderia pseudomallei and is being used in intensive and maintenance phases of melioidosis therapy. In this study, we evaluated the bactericidal activities of beta-lactams (imipenem, ceftazidime and amoxicillinclavulanate) alone and in combinations with trimethoprim-sulfamethoxazole against B. pseudomallei. Four clinical strains of B. pseudomallei were selected based on different genotypes that are frequently found in Malaysia. The minimum inhibitory concentrations of trimethoprim-sulfamethoxazole, ceftazidime, imipenem and amoxicillin-clavulanate were determined using microdilution broth method. The bactericidal activities and synergy effects of beta-lactams and/or trimethoprim-sulfamethoxazole were evaluated by checkerboard and static time-kill analyses at 1xMIC concentration of each antibiotic. Using checkerboard method, the beta-lactam/trimethoprim-sulfamethoxazole combinations exhibited SFIC of 0.75-4.00. In time-kill analysis, ceftazidime/trimethoprim-sulfamethoxazole combination demonstrated synergy against three strains (less 2.25-2.41 log(10)CFU/mL compared to the most active antibiotic monotherapy) whereas imipenem/ trimethoprim-sulfamethoxazole combination regimen showed synergy against one strain (less 3.32 log(10)CFU/mL). No antagonist effect or major re-growth was observed in all combination regimens, whereas 11 out of 12 of beta-lactam monotherapy regimens were associated with re-growth of bacteria. However, all beta-lactam monotherapy regimens exhibited rapid and stronger killing activities against BUPS/07/14, in the initial 12 hours compared to beta-lactam/ trimethoprimsulfamethoxazole combination regimens. The combination of beta-lactams with trimethoprimsulfamethoxazole demonstrated better killing effect at 24 hours compared to monotherapy and no major bacterial regrowth was observed. Nevertheless, delay in killing activities of beta-lactam/ trimethoprim-sulfamethoxazole combination regimens against BUPS/07/14 need further examination because this phenomenon can lead to treatment failure in some patients.