Single amino acid replacements at positions altered in naturally occurring extended-spectrum TEM beta-lactamases.

By directed mutagenesis, we constructed a set of seven TEM-1 derivatives containing single replacements in each one of the amino acids substituted in naturally occurring extended-spectrum TEM beta-lactamases. The exact contribution of each mutation to the resistance phenotype was determined. In addition, mutant enzyme production and stabilities were studied. Five of seven mutations determined to some extent variations in cephalosporin and/or monobactam activity. Dramatic changes in the hydrolysis of ceftazidime and aztreonam occurred when a serine was at position 164. Changes at positions 104, 238, and 240 showed more leaky variation in activity towards cephalosporins and aztreonam. Replacements at positions 237 and 265 caused no variation in susceptibility to cephalosporins. Interestingly, the change from Gln to Lys at position 39 found in TEM-2, classically considered a neutral change, slightly but consistently increased the MIC of ceftazidime and aztreonam. The in vitro construction of mutations appearing in naturally occurring TEM-beta-lactamases, studied in the same genetic context, may help to understand the evolution of extended-spectrum beta-lactamases.