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Journal of Antimicrobial Chemotherapy (1986) 17, 139-146
© 1986 The British Society for Antimicrobial Chemotherapy

research-article

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

D. M. Livermore, F. Moosdeen, M. A. Lindridge*, P. Kho{dagger} and J. D. Williams

Department of Medical Microbiology, London Hospital Medical College Turner Street, London E1, England

accepted 22 August 1985

Escherichia coli isolates which synthesised the extremely common ‘TEM-1’ plasmid mediated ß-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 (<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>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 ß-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.


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