Summary: β-Lactamases (EC. can be directly compared by analytical isoelectric focusing. Using this technique, 242 strains from five Gram-positive and 16 Gram-negative genera were examined. A preparation of each strain focused as a single group of bands which did not match the pattern of any R factor-associated β-lactamase. None of the strains was known to carry an R factor and resistance transfer experiments were unsuccessful. The enzymes studied were therefore thought to be chromosomally mediated. The isoelectric points ranged from 3.9 to 8.7 and were not related to the substrate profiles or other biochemical properties. The chromosomal β-lactamases appeared to be specific for genus, species and sub-species, and strains that produced identical β-lactamases had identical bacterial characteristics. Correlation of bacteriological differences with differences in β-lactamase patterns is discussed with particular reference to strains of and spp. Since β-lactamases may be universally produced by bacteria, separation of the enzymes by analytical isoelectric focusing could be used in bacterial taxonomy.


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