In vitro effect of cefoxitin and amikacin combination on M. fortuitum

The possible in vitro synergistic effect between amikacin and cefoxitin against 29 clinical isolates of M.fortuitum was studied either by the agar dilution checkerboard technique and by the killing curve on Mueller Hinton agar plus OADC enrichment and on 7H11. As evaluated by FIC index on MHA a synergistic effect was noted on 20 (68.96%) strains tested, an additive effect on 8 (27.60%) isolates, and a possible antagonistic effect only on 1 (3.44%) isolate. On 7H11 a synergistic effect was shown on 10 (38.50%) isolates, an additive effect on 12 (46.10%) and an antagonistic effect on 4 (15.40%) isolates. The killing curve observed on three strains (562, 565 and 505), specially selected as representative of synergistic (562), additive (565) and antagonistic effect (505) on the basis of their checkerboard results, showed synergistic effect on strains 562 and 565 as well as on strain 505 where complete killing was obtained in 24 h by a combination of two antibiotics used at 1/2 Mic. By the killing curve technique both antibiotics were found to be bactericidal but the effect was earlier for amikacin than for cefoxitin.