In-vitro antagonism by N-formimidoyl thienamycin and cefoxitin of second and third generation cephalosporins in Aeromonas hydrophila and Serratia marcescens.

Antagonism of beta-lactam antibiotics by second and third generation cephalosporins was studied in 28 clinical strains of Serratia marcescens and Aeromonas hydrophila. Of the antibiotics tested, both cefoxitin and N-formimidoyl thienamycin (N-f-thienamycin) were capable of antagonism in Ser. marcescens (17 of 20) but in Aerom. hydrophila (7 of 8), only cefoxitin induced antagonism. Antagonism was observed to the greatest degree with cefotaxime and cefoperazone; intermediate with cefamandole and latamoxef (moxalactam) and essentially none with N-f-thienamycin. Inducible beta-lactamase was found in all strains of both genera which exhibited antagonism. The induced beta-lactamase showed enhanced inactivation for other antibiotics in only four strains with none occurring in Aeromonas.