Detection of beta-lactamase in Haemophilus influenzae by immunofluorescence.

Beta-lactamase immunizing antigen was prepared from cells of an ampicillin-resistant strain of Haemophilus influenzae by cold osmotic shock followed by DEAE column fractionation. Nonspecific antibodies were removed by cross-absorption with cells of an ampicillin-sensitive strain of H. influenzae. An residual nonspecific antibodies remaining after cross-absorption were effectively eliminated by dilution of the anti-beta-lactamase serum 1:50. Twenty strains were tested for presence of beta-lactamase by the indirect fluorescent antibody technique. By this technique 91% of the strains in multiple smears were correctly identified as to the presence or absence of beta-lactamase in a blind study. The beta-lactamases of other gram-negative bacteria were not detectable by this technique.