A comparison of serum bactericidal activity and phenotypic characteristics of bacteremic, pneumonia-causing strains, and colonizing strains of Branhamella catarrhalis.

Four blood isolates, 12 pneumonia isolates, and seven colonizing isolates of Branhamella catarrhalis were compared with respect to their ability to grow in normal human serum and in convalescent serum of a patient with B. catarrhalis bacteremia. Disease-causing isolates showed seven of 16 serum-resistant strains (43 percent) compared with one of seven (13 percent) colonizing strains. Bacteremic strains were not more serum-resistant than pneumonia-causing strains. Trypsin zones of inhibition were higher for disease-causing strains. There was no correlation between source of isolation and colistin sensitivity or ability to hemagglutinate red blood cells.