Contribution of natural amino acid substitutions in SHV extended-spectrum beta-lactamases to resistance against various beta-lactams

SHV extended-spectrum beta-lactamases (ESBLs) arise through single amino ac id substitutions in the parental enzyme, SHV-1, In order to evaluate the ef fect of genetic dissimilarities around the structural gene on MICs, we had previously devised an isogenic system of strains. Here, we present an exten ded version of the system that now allows assessment of all major types of SHV P-lactamases as well as of two types of promoters of various strengths. Moreover, we devised a novel vector, pCCR9, to eliminate interference of t he selection marker. A substitution within the signal sequence, I8F found i n SHV-7, slightly increased MICs, suggesting more efficient transfer of enz yme precursor into the periplasmic space. We also noted that combination of G238S and E240K yielded higher resistance than G238S alone. However, the i nfluence of the additional E240K change was more pronounced with ceftazidim e and aztreonam than with cefotaxime and ceftriaxone. The SHV enzymes chara cterized by the single change, D179N, such as SHV-8, turned out to be the w eakest SHV ESBLs, Only resistance to ceftazidime was moderately increased c ompared to SHV-1.