The MICs of 21 P-lactam antibiotics were measured against 126 clinically significant and epidemiologically unrelated Yersinia enterocoEltica isolates. The most active antimicrobial agents tested (geometric means of MICs) were ceftriaxone, cefotaxime, ceftizoxime, and cefmenoxime (0.06 to 0.08 ,ug/ml). Mezlocillin (1.36 ig/ml) and piperacillin (1.57 ,ug/ml) were the most active penicillins. Aztreonam (5, 8-11, 17, 21, 30, 31), and the entire species was considered to be resistant to these 1-lactam antibiotics (23). Different patterns of ,B-lactam resistance were reported according to serotype (1, 5, 9, 10, 28).Several 1-lactamases including 1-lactamases A and B were found to be produced by Y. enterocolitica strains (3-5) and associated with serotypes (3, 14). Production of 3-lactamase is a resistance mechanism which could explain the clinical inefficiency of 1-lactamase-hydrolyzable antibiotics such as ampicillin and cephalothin against Y. enterocolitica infections (15,20,25).The recent availability of newer ,B-lactam antibiotics not actually hydrolyzed by ,B-lactamases gave rise to a renewed interest for these antibiotics. Marked in vitro activity of some newer 1-lactam antibiotics against Y. enterocolitica isolates has been reported (16,24,26,29).The purposes of the present study were to (i) determine comparatively countries) and the serotype (22 0 serotypes) of the clinical isolates are given in Table 1. The isolates were stored in semisolid conservation agar (catalog no. 63683; Institut Pasteur Production, Marnes-la-Coquette, France) at room temperature in the dark. All strains were susceptible by the agar dilution method to streptomycin, kanamycin, gentamicin, tetracycline, chloramphenicol, colistin, nalidixic acid, rifampin, trimethoprim (except three strains), and sulfonamides (except four strains).Antibiotics. The 21 antibiotics of known potency used in this study were kindly supplied by the manufacturing companies (or their subsidiary agents). For the purpose of generalization, these antibiotics were classified as follows: (i) Penicillins, including the old penicillins ampicillin and carbenicillin (Beecham, Paris, France) and the newer penicillins temocillin (Beecham), piperacillin (Lederle, Oullins, France), azlocillin and mezlocillin (Bayer, Puteaux, France), and apalcillin (Laboratoire Francais de Thdrapeutique, Bordeaux, France); (ii) aztreonam (Squibb, Neuilly s/Seine, France); a monobactam antibiotic; (iii) imipenem (formerly imipemide or N-formimidoyl thienamycin), a carbapenem antibiotic (Merck, Paris, France); (iv) cephalosporins, including the first-generation cephalosporins cephalothin (Lilly, Saint Cloud, France) and cephaloridine and cephalexin (Glaxo, Paris, France), the second-generation cephalosporins cefoxitin (Merck) and cefamandole (Lilly), and the third-generation cephalosporins cefotaxime (Roussel,
