Introduction: Results: Methodology: Discussion: Conclusion: Bactericidal Activity of Multiple Combinations of Fosfomycin and Colistin against NDM-1 producing Enterobacteriaceae M. Albur, A. Noel, K. Bowker, A. MacGowan BCARE, Department of Microbiology, North Bristol NHS Trust, Bristol, UK E-793 Table 2. Area under the bacterial kill curve (AUBKC) of the isolates Introduction: Results: AUBKC (0-48hr) (log CFU/ml/hr) Fosfomycin sensitive strains (meanSD) AUBKC (0-48hr) (log CFU/ml/hr) Fosfomycin resistant strains (meanSD ) FM 0mg/L FM 5mg/L FM 20mg/L FM 250mg/L CS 0 mg/L 360.630.5 297.820.6 268.823.2 23.81.6 365.814.7 364.29.6 341.420.4 282.422.4 0.10 mg/L 340.534.5 208.7160 184.6123 24.310.8 350.123 36513 334.632.2 108.5157.2 0.16 mg/L 332.218.9 113.4148 102.181.8 34.27.9 358.218 341.934.2 323.946.1 134.3142.4 0.29 mg/L 307.45.7 37.213.2 29.23.3 21.39.3 311.720.1 316.922 168139.6 107.9156.6 CMS 365.319 295.928.9 265.913.5 27.27.8 371.214.1 360.925.9 34525.7 28030.8 2.1 mg/L 366.317.1 32.41.6 30.11.9 23.37.6 363.117.9 369.530.8 322.244.3 105.973.2 2.7 mg/L 325.215.1 30.63.6 29.53 27.22.3 352.337.6 326.561.3 282.357.1 168.2120.7 8.5 mg/L 242.646.1 22.37.2 21.56.6 2210.5 261.353.8 255.433 159.4126 20.47.5 Therapeutic options against NDM-1 producing Enterobactericeae are limited. Colistin, fosfomycin, and tigecycline are some of the agents where many strains still have MICs below the clinical breakpoint. Fosfomycin and colistin act on bacterial cells by different mechanisms. Hence there is a potential scope for both antagonism as well as synergism. The comparative mean TKC of both individual and various combinations of antimicrobials at different concentrations are shown in Fig 1, 2, & 3. Both colistin and FM showed concentration dependent bactericidal activity at all concentrations ( ≈ 3-4 log kill at 3hrs at peak concentration). Combinations of FM with CS/CMS produced lower AUBKC as compared to individual agent (i.e. increased cidality/synergy) [Table 2]. As expectedly, the degree of increased bacterial killing was more pronounced against FM sensitive strains compared to FM resistant strains. Methodology: Bactericidal activity of fosfomycin (FM), Colistin sulphate (CS), and Colistin methanesulfonate (CMS) was assessed by time-kill methodology. 6 well characterised strains of NDM-1 producing Enterobacteriaceae were used [Table 1]. Three of them were sensitive and three were resistant to FM. Pharmacokinetically achievable free drug serum concentrations were used. The following concentrations reflecting peak (Cmax), average (Css), and trough (Cmin) concentrations expressed in mg/L were used respectively for FM (250, 20, 5), CS (0.29, 0.16, 0.1) and CMS (8.5, 2.7, 2.1). A 44 drug exposure matrix of FM with CS and CMS were used along with a growth control (GC) at 0, 1, 3, 6, 12, 24 and 48 hr time-points. Fig. 1a Fig. 1b Discussion: Due to increase in antimicrobial resistance amongst Gram-negative bacteria, combination therapy is gaining importance. This is based on an assumption that combination therapy not only increases the efficacy (in terms of killing & spectrum) but also prevents emergence of secondary resistance on monotherapy. This study show that the addition of FM to colistin (the primary drug of choice against MDR Gram negative bacteria) produces increased bacterial killing at all therapeutically achievable concentrations even for FM resistant strains. There was no evidence of antagonism even against FM resistant strains. Fig. 2a Fig. 2b Table 1 MICs of the isolates Isol. No. Isolate ID FM MIC Colistin 45836 E. coli 1 0.38 45838 K. pneumoniae >128 0.125 45839 K. oxytoca 0.094 45840 45841 4 45842 0.5 Fig. 3a Fig. 3b Conclusion: Combination of fosfomycin and colistin produced increased bacterial killing against NDM-1 producing Enterobacteriaceae with no evidence of antagonism even against FM resistant strains.