Behaviour of TEM-1 beta-lactamase as a resistance mechanism to ampicillin, mezlocillin and azlocillin in Escherichia coli.

Escherichia coli isolates which synthesised the extremely common 'TEM-1' plasmid mediated beta-lactamase were more resistant to the alpha-aminopenicillins, ampicillin, mezlocillin and azlocillin, than were strains which lacked this enzyme. However, many TEM-1+ isolates remained sensitive to therapeutic concentrations of mezlocillin (less than 64 mg/l), whereas virtually none was susceptible to such levels of ampicillin or azlocillin. Transconjugants of E. coli K12 into which we introduced various TEM-1 coding plasmids similarly acquired lower levels of resistance to mezlocillin than to ampicillin and azlocillin. Those which expressed relatively small amounts of enzyme remained sensitive to 64 mg/l of mezlocillin, whereas they were substantially resistant (MIC greater than 64 mg/l) to the other alpha-aminopenicillins. These data suggested that the enzyme afforded weaker protection against mezlocillin than against azlocillin and ampicillin and we attempted to relate this finding to its hydrolytic activity. Extracted TEM-1 beta-lactamase hydrolysed high concentrations of mezlocillin more rapidly than ampicillin and azlocillin; however, mezlocillin was calculated to be the weakest substrate at the low concentrations which are likely to be obtainable in the bacterial cell. These data may partly account for the residual activity of mezlocillin against enzyme producers, but target and permeability factors probably also contribute.