Fosfomycin-daptomycin and other fosfomycin combinations as alternative therapies in experimental foreign-body infection by methicillin-resistant Staphylococcus aureus

Abstract

The efficacy of daptomycin, imipenem, or rifampin with fosfomycin was evaluated and compared with that of daptomycin-rifampin in a tissue cage model infection caused by methicillin-resistant Staphylococcus aureus (MRSA). Strain HUSA 304 was used. The study yielded the following results for MICs (in μg/ml): fosfomycin, 4; daptomycin, 1; imipenem, 0.25; and rifampin, 0.03. The study yielded the following results for minimum bactericidal concentration (MBC) (in μg/ml): fosfomycin, 8; daptomycin, 4; imipenem, 32; and rifampin, 0.5. Daptomycin-rifampin was confirmed as the most effective therapy against MRSA foreign-body infections. Fosfomycin combinations with high doses of daptomycin and rifampin were efficacious alternative therapies in this setting. Fosfomycin-imipenem was relatively ineffective and did not protect against resistance.

Figures

Fig 1

MICs, MBCs, and time-kill curves with the antibiotics alone and in combination with fosfomycin (FOS). Powdered antibiotics were resuspended in sterile water in all cases but rifampin (in methanol). In experiments with FOS and daptomycin (DAP), Mueller-Hinton broth was supplemented with 25 mg/liter of glucose-6-phosphate and with 50 mg/liter of calcium, respectively. MICs (µg/ml) were 1 for DAP, 4 for FOS, 0.25 for imipenem (IMI), and 0.03 for rifampin (RIF). MBCs (µg/ml) in log phase with an inoculum of 105 CFU per ml, in log phase with 108 CFU per ml, and in stationary phase were, respectively, 4, 20, and 24 for DAP; 8, >128, and >128 for FOS; 32, >128, and >128 for IMI; and 0.5, >8, and >8 for RIF. For the time-kill curves, fixed concentrations of antibiotics (in μg/ml) were used: (i) for log phase, all concentrations included in the range from 0.25× to 512× the MIC, and (ii) for stationary phase, concentrations equivalent to the peak and trough total levels achieved in the TCF (Table 1). Bactericidal activity was equal to a ≥3 log 10 decrease in the initial inoculum in CFU/ml at 24 h. Synergy, indifference, and antagonism were equal to a ≥2 log increase, a <2 log change (increase or decrease), and a ≥2 log decrease, respectively, in killing of the combination in comparison with that of the most active single drug. To avoid carry-over antimicrobial agent interference, the sample was placed on the plate in a single streak down the center and allowed to absorb into the agar until the plate surface appeared dry; then it was spread over the plate. This method has been checked by comparing the results with bacterial counts obtained after the centrifugation of sample and resuspension with physiological serum from pelleting (28). (a) Log phase (inoculum of 105 CFU/ml). FOS antagonized the bactericidal effect of RIF. (b) Log phase (inoculum of 108 CFU/ml). FOS plus IMI had a synergistic and bactericidal effect against standard inocula but was indifferent and did not protect the emergence of FOS resistance when using high inocula. (c) Stationary phase. DAP plus FOS showed a bactericidal activity, but the effect was indifferent regarding DAP in isolation.

Fig 2

Decreases in bacterial counts from TCF samples (mean of log CFU/ml) at the end of therapy (days 8 and 11). Analysis of variance and an unpaired Student t test with Bonferroni correction were used to determine statistical significance (defined as a P value of <0.05). Lower limit of detection of bacterial counts, 10 CFU/ml. Abbreviations: FOS, fosfomycin 500 mg/kg/12 h; D45, daptomycin 45 mg/kg/day; D100, daptomycin 100 mg/kg/day; IMI, imipenem 120 mg/kg/12 h; RIF, rifampin 25 mg/kg/12 h; CON, control group. (a) Day 8. The combination of D100 and RIF (D100+RIF) was the most effective treatment (##, P < 0.05 versus all therapies). D45, D100, and RIF in combination with FOS were better than either monotherapy or FOS+IMI (#, P < 0.05). Results for all drugs were significantly improved by their use in combination (versus their use as monotherapies): D45, D100, and RIF were the most effective (**, P < 0.05 versus IMI and FOS), FOS had modest efficacy (*, P < 0.05 versus IMI), and IMI was ineffective. (b) Day 11. The efficacy of all groups except IMI, RIF, and D45 was greater than on day 8. D100+RIF had greater efficacy than all therapies but FOS+RIF (¥, P < 0.05). FOS+RIF was more effective than all monotherapies, FOS+IMI and FOS+D45 (# #, P < 0.05), and FOS+D100 (# #, P = 0.06). FOS plus daptomycin (both dosages) was better than all monotherapies and FOS+IMI (#, P < 0.05). D45, D100, RIF, and FOS+IMI were more effective than IMI and FOS (**, P < 0.05). *, P < 0.05 for FOS versus IMI.

Fig 3

(a and b) Bacterial counts in CV (mean log CFU/ml) (a) and cure rates of infection (expressed as percentages of samples with bacterial counts under the limit of detection with respect to the total samples) (b) for different groups. (a) Results for all groups were better than results for controls (P < 0.05). D45, D100, RIF, and FOS+IMI were better than IMI alone (*, P < 0.05). All FOS combinations (except FOS+IMI) and D100+RIF were the best therapies (# #, P < 0.05 versus all therapies). (b) Results for all groups except IMI (cure rate, 0%; not represented) were better than results for controls (P < 0.05). D100+RIF (# #, P < 0.05 versus all monotherapies, FOS+IMI, and FOS+D45) was the most effective treatment (94%), and it had greater efficacy than FOS+RIF (79%) and FOS+D100 (82%) (#, P < 0.05 versus D45, D100, FOS, and FOS+IMI). Among monotherapies, RIF was the most active therapy (*, P < 0.05 versus D45, D100, and FOS+IMI). Abbreviations are defined in Fig. 2.