[Characterization of Haemophilus influenzae's resistance to ampicillin]. / Caracterización de la resistencia de Haemophilus influenzae a la ampicilina.

ABSTRACT

BACKGROUND: The resistance of Haemophilus influenzae to ampicillin is nearly always dependent on plasmid-mediated betalactamase production. The betalactamase type most frequently identified was TEM-1; type ROB has occasionally been identified. The presence of plasmid-mediated betalactamase has been studied in H. influenzae strains moderately susceptible or resistant to ampicillin. METHODS: The test used in the detection of the presence of betalactamase was nitrocefin hydrolysis; the enzyme type was identified by analytic isoelectric focusing; the codifying gene of this betalactamase was identified by hybridisation with a TEM-1 probe labeled with digoxigenine. Strains studied were 110, obtained during the period 1987 to 1989 and isolated from four Hospitals of the Vallés area of Barcelona. RESULTS: Enzyme production was detected in 105 of 110 strains studied; TEM-1 type betalactamase was identified in 108 and positive hybridisation was obtained in these strains with the TEM-1 probe. CONCLUSIONS: Nitrocefin hydrolysis is a reliable test for the detection of betalactamase production, although it may be unable to detect it (falses negatives) when the production is low. The only betalactamase type identified was TEM-1. The frequency of the strains with non-enzymatic resistance was 1.8%.