Hong Kong Institute of Medical Laboratory Sciences Ltd. - Education Materials

Education Sheets 2001-B

For MM2116 - Haemophilus influenzae (b-lactamase-negative
ampicillin-resistant) in antimicrobial susceptibility testing

Resistance to b-lactam antibiotics of most Haemophilus influenzae strains is due to the production of plasmid-mediated TEM or ROB b-lactamase. However, another ampicillin resistance mechanism involves production of altered penicillin-binding proteins (PBPs) and/or reduction in permeability. These b-lactamase-negative ampicillin-resistant isolates (BLNAR) show extended spectrum resistance, including reduced susceptibility to first- and second-generation cephalosporins. The problems with both disk diffusion and the broth dilution methods most often noted are equivocal endpoints with BLNAR strains with several b-lactam antibiotics. NCCLS recommended that BLNAR strains (best detected by tests with ampicillin and b-lactamase production) should be considered resistant to amoxicillin-clavulanic acid, ampicillin-sulbactam, cefaclor, cefuroxime, cefetamet, cefprozil, and loracarbef. b-lactamase production is easily detected by chromogenic cephalosporin (e,g, nitrocefin disk), acidometric or iodometric methods.

In both 1997 and 1998, this strain was sent out for antimicrobial testing. For MM708 (1997), the percentage of all-correct for ampicillin was 55%. For MM812 (1998), the percentage of all-correct for ampicillin was 68% and a similar performance (61%) was obtained for MM2116 (2001).

Since BLNAR strains of H. influenzae become common, reporting of in vitro susceptibility testing should be aware because some technical staffs may assume b-lactamase-negative strains are susceptible to ampicillin.

References:

J.A. Hindler and J.M. Swenson. 1999. Susceptibility Testing of Fastidious Bacteria, P.1544-1562. In P.R. Murray, Manual of Clinical Microbiology, 7^th edition. American Society for Microbiology, Washington, D.C.

National Committee for Clinical Laboratory Standards. 2000. Performance standard for antimicrobial disk susceptibility tests - seventh edition. Approved standard M2-A7. NCCLS, Wayne, Pa.

National Committee for Clinical Laboratory Standards. 2000. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically - fifth edition. Approved standard M7-A5. NCCLS, Wayne, Pa.

Last updated on 5 March, 2002.

Prepared by HKIMLSQAP Medical Microbiology Panel.