Mezlocillin: a new acyl ureidopenicillin. Antimicrobial activity and combination effects with four aminoglycoside antibiotics.

Mezlocillin (Baypen), carbenicillin and mezlocillin combined with gentamicin, sisomicin, tobramycin and amikacin, respectively, were tested in vitro against 325 fresh clinical isolates of pathogenic gram-negative bacteria using the tube dilution procedure. The new acyl ureidopenicillin has a greater antimicrobial activity than carbenicillin against nearly all species tested. The comparison of the effectiveness of the mezlocillin-aminoglycoside combinations demonstrates the superiority of mezlocillin-sisomicin against Escherichia coli, Klebsiella, Serratia and Proteus spp., whereas mezlocillin-tobramycin is most active against gentamicin-sensitive Pseudomonas aeruginosa, and mezlocillin-amikacin against Enterobacter and gentamicin-resistant P. aeruginosa. The combination effects achieved in the majority of strains were addition or synergism. The resistance rate and the bactericidal efficacy of mezlocillin were clearly improved when mezlocillin was combined with aminoglycoside antibiotics. Mezlocillin-amikacin showed the lowest resistance rate (10.2%) followed by mezlocillin-sisomicin (12.3%), mezlocillin-gentamicin (15.1%) and mezlocillin-tobramycin (18.5%). In most species, mezlocillin-sisomicin has proved to be the combination with the best bactericidal potency.