Laboratory identification of Haemophilus influenzae: effects of basal media on the results of the satellitism test and evaluation of the RapID NH system

Abstract

The effects of four different basal media, tryptic soy agar, brain heart infusion agar, nutrient agar, and Mueller-Hinton agar, were investigated with respect to the identification of Haemophilus influenzae with a satellitism test in which X and V growth factors were supplied by factor-impregnated filter paper strips. A total of 187 recent clinical isolates of H. influenzae were examined. Of these, 179 strains (95.7%) were correctly identified with tryptic soy agar, 173 (92.5%) with brain heart infusion agar, 105 (56.1%) with nutrient agar, and 133 (71.1%) with Mueller-Hinton agar. Failure to obtain a correct identification was usually the result of satelliting growth around V factor-containing strips, possibly due to the presence of trace amounts of hemin in the basal media, or was because of an absence of growth due to nutritional deficiencies in the basal media. All 187 H. influenzae strains were also examined with a new biochemical and chromogenic substrate micromethod, the RapID NH system (Innovative Diagnostics Systems, Inc., Decatur, Ga.). A total of 168 (89.8%) strains were correctly identified with this system.