beta-Lactamases: quantity and resistance

Rates of enzyme-mediated catalysis are proportional to enzyme quantity, so increasing the amount of beta-lactamase should increase resistance. Kinetic considerations support this argument for both Gram-positive and Gram-negative bacteria. Direct relationships between resistance and enzyme quantity are most obvious with constitutive beta-lactamases, e.g. the TEM types from Gram-negative bacteria. As the level of TEM enzyme rises, so do the MICs of substrates and the concentrations of inhibitors required to potentiate these substrates. The position is more complex for inducible beta-lactamases, e.g. the AmpC types of Enterobacter spp., Citrobacter freundii, Serratia spp. and Pseudomonas aeruginosa. Third-generation cephalosporins are labile to these enzymes, but are only weak inducers, so the beta-lactamase-inducible strains appear susceptible. Once, however, the enzyme is derepressed, resistance is apparent. The behavior of inhibitor combinations against inducible beta-lactamases is complicated by the propensity of some inhibitors to induce further enzyme synthesis. As the inoculum is raised in laboratory tests, the amount of beta-lactamase and MICs also rise. This is notorious for staphylococci but is seen also for some Gram-negative organisms, notably klebsiellae with extended-spectrum beta-lactamases. A beta-lactamase-related inoculum effect generally predicts clinical failure.