A comparison between methods of identification and serotyping of encapsulated strains of Haemophilus influenzae

Seven methods of serotyping of Haemophilus influenzae were evaluated. Comparing slide agglutination, staphylococcal coagglutination, latex agglutination, counterimmunoelectrophoresis, immunofluorescence, capsular swelling, and cultivation on antiserum agar the commercial coagglutination test was most reliable, most rapid, and easiest to perform. To identify all six serotypes this coagglutination test had to be combined with slide agglutination. With most methods best results were achieved by using cultures incubated at 37 degrees C for 6 h. As nonencapsulated strains often agglutinated unspecifically, selection of probably typeable strains was useful. Differentiation with help of colonial morphology and opalescent growth was facilitated by cultivation on Brain Heart Infusion (BHI) Chocolate Agar and testing of growth factor requirements on translucent BHI Agar with strips containing the growth factors V, X, and VX, respectively. In broth turbid growth was a hint for encapsulation. Nigrosin staining, a negative capsule staining, proved to be useful if specific antisera are not available. From 252 clinical isolates of H. influenzae 216 were not typeable. 36 strains could be serotyped. 27 (75%) belonged to serotype b, 6 (16.6%) were serotype e, 3 (8.3%) were serotype f. Serotype e and f were most difficult to identify. Spectrum of patients and diseases were corresponding to the findings of other authors. Less well-known infections like cellulitis (erysipelas of the cheeks) and arthritis were observed, too. Rapid identification of at least H. influenzae type b could render treatment in some cases more effective by early application of a suitable antibiotic.