The in vitro activity of mecillinam alone and in combination with other beta lactam antibiotics

The in-vitro activity of mecillinam, a 6 amidinopenicillin, was evaluated alone and in combination with ampicillin, carbenicillin, cephalothin, cefamandole, and cefoxitin. Using disc-diffusion and agar-dilution methods, 431 strains of Enterobacteriaceae and 171 strains of non-fermentative gram-negative rods were studied. Mecillinam was as active or more active than the other agents against strains of Escherichia coli, Klebsiella pneumoniae, Enterobacter species, Proteus mirabilis, and Citrobacter species. Against Serratia marcescens and indole-positive Proteus species, carbenicillin was the most active beta-lactam agent. Non-fermentative gram-negative rods were uniformly resistant to mecillinam. Mecillinam was as active against enteric pathogens as ampicillin. Combination studies revealed that a synergistic or additive effect was demonstrated most frequently when a cephalosporin was combined in a fixed ratio with mecillinam - 50% with cefoxitin, 42% with cephalothin, and 35% with cefamandole - although this finding could not be demonstrated against the non-fermentative gram-negative rods. Mecillinam may be an effective single agent in certain infections with gram-negative rods and when combined with other beta lactam antibiotics may produce a synergistic or additive effect.