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Predicting the resistance of beta-lactamase producing isolates to hydrolysis-susceptible beta-lactam antimicrobials
Various bacteria produce a class of enzymes called beta-lactamases, which may be mediated by genes on plasmids or chromosomes. Production of beta-lactamase may be constitutive or induced by exposure to antimicrobials. Beta-lactamases hydrolyze (and thereby inactivate) the beta-lactam rings of a variety of susceptible penicillins and cephalosporins. Beta-lactamases are classified by their preferred antimicrobial substrate and the effect of various inhibitors (such as clavulanic acid) on them.
Some antimicrobials, such as cefazolin and cloxacillin are resistant to such hydrolysis (at least for staphylococcal beta-lactamases).
Beta-lactamase producing strains of the following are resistant to many types of penicillin: Staphylococcus species, Hemophilus influenzae, Neisseria gonorrhoeae, Bacteroides species, Enterococcus species, and Moraxella catarrhalis.
The above organisms, when isolated from critical specimens such as blood or spinal fluid, should always be tested for beta-lactamase production.
Addition of a beta-lactamase inhibitor to a beta-lactam (such as sulbactam plus ampicillin) restores the activity of the antimicrobials.
Negative (reported as positive or negative)
A positive test indicates production of beta-lactamase.
Some beta-lactamase tests (iodometric and acidometric methods) may not detect certain beta-lactamases.
Many bacteria that do not produce beta-lactamase will be resistant to beta-lactams by other mechanisms.
This test should not be used to detect extended-spectrum beta-lactamases.
Livermore DM, Williams JD: Beta-lactams: mode of action and mechanisms of bacterial resistance. In Antibiotics in Laboratory Medicine. Fourth edition. Edited by V Lorian. Baltimore, MD, Williams and Wilkins, 1996, pp 502-578