Comparison of Penicillinases Produced by R Factors Isolated from Ampicillin‐resistant Gram‐negative Bacteria

Among 9365 Shigella strains and 1435 other gram‐negative bacteria of clinical origin, 88 (0.9%) and 242 (16.9%) strains were resistant to aminobenzyl‐penicillin (AB‐PC). Of these 330 AB‐PC resistant strains, 138 (57%) transferred their AB‐PC resistance by conjugation. More than 50% of AB‐PC resistant Escherichia coli or Klebsiella strains carried R factors capable of conferring AB‐PC resistance. These R factors were transferred to E. coli K‐12 strains and after were examined for the levels of resistance to cephaloridine (CER) and 5 penicillins; (AB‐PC, penicillin‐G (PC‐G), phenethicillin, 6‐aminopenicillanic acid (6‐APA), and cloxacillin). Concerning PC‐G, AB‐PC or CER resistance, there were found 3 levels of drug‐resistance, labelled by us as low, middle and high. From the patterns of the resistance levels to 5 penicillins and CER, R + strains were classified into 4 major groups. Group I strains were highly resistant to all of the penicillins tested and group II strains showed high resistance only to 6‐APA. Penicillinase (PCase) produced by group II strains was similar to that reported previously, and group I PCase showed almost the same substrate specificity but has 20 times higher specific activity as that of the group II PCase. In group III or IV, the resistance levels to some penicillins were low. Group IV PCase was different from that which has been thus far reported.