Evaluation of Efficacy of Fosfomycin Combined with other Antimicrobials against MDR Proteus Mirabilis In vitro and In vivo

Aims: The aim of this study is to evaluate antibiotic combinations against MDR Proteus mirabilis.

Study Design: This was a cross sectional study.

Place and Duration of Study: Department of Microbiology Dhaka Medical College Dhaka Bangladesh from June 2019 to July 2020.

Methodology: Total 570 urine, blood or wound swab and pus samples were collected from the patients admitted in Dhaka Medical College Hospital after taking informed written consent. Proteus mirabilis were isolated and identified by observing pale of colorless colonies in MacConkey’s agar media and biochemical tests. Antimicrobial susceptibility test of various drugs were done by modified Kirby–Bauer disc diffusion method [1] and zones of inhibition were interpreted according to CLSI guidelines [2]. Escherichia coli ATCC 29212 was used as control strain to assess the performance of the method. Minimum inhibitory concentrations (MIC) of various drugs were determined using agar dilution method [3,4]. To prepare bacterial inoculum, the turbidity of bacterial suspension in normal saline was compared with 0.5 McFarland turbidity standards. Antibiotic combinations of various drugs were used to see synergistic, additive, indifferent or antagonistic effects by agar dilution method both in vivo and In vitro [5].

Results: Out of 44 proteus mirabilis 29 were multidrug resistant (MDR). Among the MDR proteus mirabilis 7(24.14%), 20(68.97%), 11(37.93%) and 13(44.83%) were resistant to fosfomycin, amikacin, piperacilin- tazobactam and tigecycline, respectively. The MIC value for fosfomycin, amikacin, piperacilin- tazobactam and tigecycline ranged from 64 µg/ml to 4096µg/ml, from 256 µg/ml to 16,384µg/ml, from 128/4 µg/ml to 1024/4µg/ml and from 8 µg/ml to 64µg/ml, respectively. Out of 4 fosfomycin and amikacin resistant P. mirabilis, one (25%) had 8 fold reduction of MIC, 3 (75%) had 4 fold reduction of MIC (Table 5). Out of 4 fosfomycin and amikacin resistant strains, all showed synergism in combination as their FICI value were ≤0.50. Out of 4 fosfomycin and tigecycline resistant strains, one had FICI value 0.50 (synergistic), two had FICI value 1 (additive) and one had FICI value 0.25 (synergistic). Out of 4 fosfomycin and piperacillin-tazobactam resistant strains, 2 had FICI value 0.50 (synergistic) and one had FICI value 0.25 (synergistic), one had FICI value 1 (additive). All the mice in the positive control group were bacteramic. All the mice in the negative control were blood culture negative. In the group treated with fosfomycin, 20% were culture negative. In the group treated with tigecycline, piperacillin- tazobactam none was culture negative. In the group treated with amikacin, 20% were culture negative. In the group treated with fosfomycin and amikacin, 100% were culture negative. In the group treated with fosfomycin and tigecycline, 80% were culture negative. In the group treated with fosfomycin and Piperacilin- tazobactam 80% were culture negative. Comparison between synergism of different antibiotic combinations in MDR P. mirabilis in vitro and in vivo was showed in (Table 10). While combining fosfomycin with amikacin, they showed 100% synergistic effect both In vitro and In vivo, while combining tigecycline with fosfomycin, they showed 50% synergistic effect in vitro and 80% synergistic effect in vivo and while combining fosfomycin with piperacillin-tazobactam, they showed 75% synergistic effect in vitro and 80% synergistic effect in vivo.

Conclusion: Combination therapy is good treatment option for MDR P. mirabilis both in vitro and in vivo. Fosfomycin and amikacin was the most effective combination in both in vitro and in vivo which showed 100% synergism. From the present study it appeared that combination of fosfomycin and amikacin may be a good option for treating infection by MDR P. mirabilis.