Characterization of a chromosomal gene encoding type B beta-lactamase in phage group II isolates of Staphylococcus aureus

In contrast to most Staphylococcus aureus isolates in which the gene for st aphylococcal beta-lactamase (blaZ) is plasmid borne, isolates typeable by g roup II bacteriophages frequently carry blaZ on the chromosome. Furthermore , the chromosomal gene encodes the type B variant of staphylococcal beta-la ctamase for which the nucleotide and deduced amino acid sequences have not yet been reported. To better understand beta-lactamase production among pha ge group II staphylococci and the nature of the type B beta-lactamase, we d etermined the type and amount of enzyme produced by 24 phage group II isola tes. Of these isolates, 1 did not produce beta-lactamase, 8 produced the ty pe B enzyme, and 15 produced the type C enzyme. In all eight type B beta-la ctamase-producing isolates, blaZ was located on the chromosome. This was in contrast to the type C beta-lactamase-producing isolates, in which blaZ wa s located on a 21-kb plasmid. The nucleotide sequence corresponding to the leader peptide and the N-terminal 85% of the mature exoenzyme form of type B S. aureus was determined. The deduced amino acid sequence revealed 3 resi dues in the leader peptide and 12 residues in the exoenzyme portion of the beta-lactamase that differ from the prototypic type A beta-lactamase sequen ce. These include the serine-to-asparagine change at residue 216 found in t he kinetically similar type C enzyme, a threonine-to-lysine change at resid ue 128 close to the SDN loop (residues 130 to 132), and several substitutio ns not found in any of the other staphylococcal beta-lactamases. In summary modern isolates of S. aureus typeable by group II phages produce type B or type C staphylococcal beta-lactamase. The type B gene resides on the chrom osome and has a sequence that, when compared to the sequences of the other staphylococcal beta-lactamases, corresponds well with its kinetic propertie s.