In vitro combination of mecillinam with cephradine or amoxycillin for organisms resistant to single agents.

Of 36 multi-resistant Gram-negative bacilli isolated from different cases of urinary tract infection, 18 strains of Escherichia coli, klebsiella, enterobacter, proteus, providencia and serratia with the exception of Acinetobacter species showed synergy when mecillinam was combined with cephradine or amoxycillin in the ratio of 1:1. Combination with cephradine was synergistic on sixteen occasions and amoxycillin on six. Synergy, which might have clinical relevance, was demonstrated only when the test organism was either fully or moderately sensitive to one of the two antibiotics combined but not when resistant to both.