Evaluation of some methods for the laboratory identification of Haemophilus influenzae

Abstract

Five tests-satellitism, synthesis of porphyrins, acid production from sucrose, β-galactosidase activity (ONPG), and indole production-to differentiate between strains of Haemophilus influenzae and strains of V-dependent Haemophilus species were evaluated. Six per cent of strains of H infiuenzae were misidentified as Hparainfiuenzae by a test for satellitism using filter paper discs impregnated with X factor, V factor, or both, applied to Columbia Agar. None of seven nutrient agars tested grew Haemophilus species, and determined accurately the X factor requirement. Synthesis of porphyrins from 8-aminolaevulinic acid provided a reliable means of demonstrating that X factor was required. A test for the production of acid from sucrose discriminated successfully between strains of V-dependent Haemophilus species (positive) and H influenzae (negative). Most isolates were identified correctly by the ONPG test, but occasional V-dependent strains were negative and could be misidentified asH influenzae. The discriminative value of the indole test was unsatisfactorily low. The results of the tests are discussed in relation to the identification of H influenzae in the diagnostic laboratory.