Resistance Development of Cystic Fibrosis Respiratory Pathogens When Exposed to Fosfomycin and Tobramycin Alone and in Combination under Aerobic and Anaerobic Conditions

Abstract

Although antibiotics from different classes are frequently prescribed in combination to prevent the development of resistance amongst Cystic Fibrosis (CF) respiratory pathogens, there is a lack of data as to the efficacy of this approach. We have previously shown that a 4∶1 (w/w) combination of fosfomycin and tobramycin (F∶T) has excellent activity against CF pathogens with increased activity under physiologically relevant anaerobic conditions. Therefore, the aim of this study was to determine whether F∶T could delay or prevent the onset of resistance compared to either fosfomycin or tobramycin alone under aerobic and anaerobic conditions. The frequency of spontaneous mutants arising following exposure to fosfomycin, tobramycin and F∶T was determined for clinical Pseudomonas aeruginosa and MRSA isolates under aerobic and anaerobic conditions. The effect of sub-inhibitory concentrations of fosfomycin, tobramycin and F∶T on the induction of resistance was also investigated, with the stability of resistance and fitness cost associated with resistance assessed if it developed. P. aeruginosa and MRSA isolates had a lower frequency of spontaneous mutants to F∶T compared to fosfomycin and tobramycin under both aerobic and anaerobic conditions. There was a maximum two-fold increase in F∶T MICs when P. aeruginosa and MRSA isolates were passaged in sub-inhibitory F∶T for 12 days. In contrast, sequential resistance to fosfomycin and tobramycin developed quickly (n = 3 days for both) after passage in sub-inhibitory concentrations. Once developed, both fosfomycin and tobramycin resistance was stable and not associated with a biological fitness cost to either P. aeruginosa or MRSA isolates. The results of this study suggest that F∶T may prevent the development of resistance compared to fosfomycin or tobramycin alone under aerobic and physiologically relevant anaerobic conditions. F∶T may be a potential treatment option in CF patients chronically colonised by MRSA and/or P. aeruginosa.

Efficacy of Fosfomycin Compared to Vancomycin in Treatment of Implant-Associated Chronic Methicillin-Resistant Staphylococcus aureus Osteomyelitis in Rats

“…were found (25). In the present study, the mean number of CFU obtained from bones of the no-treatment group (not weight based) was 2.7 ϫ 10 6 .…”

Antimicrobial Activity of Fosfomycin-Tobramycin Combination against Pseudomonas aeruginosa Isolates Assessed by Time-Kill Assays and Mutant Prevention Concentrations

“…This fact reinforces the need to administer tobramycin in combination with other compounds. Fosfomycin has demonstrated that it can enhance the active transport of tobramy- cin into the cell (10); furthermore, its activity does not decrease in anaerobiosis, and some studies have reported high activity under these conditions (16). It has also been reported that fosfomycintobramycin association is advantageous in anaerobiosis, as it downregulates the expression of nitrate reductase genes that are essential for the growth of P. aeruginosa (28).…”

“…Fosfomycin, tobramycin, and fosfomycin-tobramycin combination MPCs were calculated for three tobramycin-susceptible (Pa2, MIC ϭ 2 g/ml; Pa4, MIC ϭ 4 g/ml; Pa5, MIC ϭ 4 g/ml) and two tobramycin-resistant (Pa7, MIC ϭ 64 g/ml; Pa8, MIC ϭ 32 g/ml) isolates. MPCs were determined in aerobic and anaerobic conditions (16). Briefly, an initial inoculum of approximately 10 10 CFU/ml was placed onto Mueller-Hinton agar plates with twofold-increasing fosfomycin concentrations (16 to 2,048 g/ml) and tobramycin concentrations (0.5 to 512 g/ml) and with the fosfomycin-tobramycin combination.…”

Validation of the mutant selection window hypothesis with fosfomycin against Escherichia coli and Pseudomonas aeruginosa: an in vitro and in vivo comparative study

“…These results are consistent with previous studies that have examined the activity of fosfomycin alone. 20 Under both aerobic and anaerobic conditions, inhibitory activity of fosfomycin lasted for 12 h but there was re-growth thereafter for both strains at antibiotic concentrations of × 4, × 8 and × 16 aerobic MIC (Figure 1). However, no emergence of fosfomycin resistance was detected when the antibiotic concentration below the MIC (at concentration of × 0.5 aerobic MIC) or above the MPC (at concentration of × 32 aerobic MIC).…”

“…20 Selection of resistant mutants were obtained by plating the high inocula (ca. 10 10 CFU) of parental strains on MH agar plates containing 25 μg ml -1 of glucose-6-phosphate and × 8 MIC of fosfomycin, and MICs were determined to confirm resistance.…”