Sensitivity tests with cephalosporins

The sensitivity of 127 strains of miscellaneous Gram-negative bacilli to various cephalosporins was determined by three methods. Disc tests with cephaloridine (CLOR), cephalothin (CTIN) and cephalexin (CLEX) made most strains of beta-lactamase-producing Escherichia coli appear sensitive. Eight cephalosporins were tested by the agar break-point method using concentrations of 16 and 4 mg/l for all except cefotaxime (CTAX), when 8 and 1 mg/l were used. In repeated tests there was considerable variation in the response of many strains to the lower concentration of CLOR, CTIN, CLEX and to a lesser extent, cefoxitin (CFOX) and cefuroxime (CROX), and beta-lactamase-producing E coli appeared as sensitive or moderately sensitive to all. Cephalosporins CLOR, CLEX, and CROX were then tested by inoculating 2 ml volumes of broth containing a 30 microgram disc; all the beta-lactamase-producing E coli grew in CLOR and CLEX but were inhibited by CROX.