Studies on haemagglutination and serum resistance status of strains of Moraxella catarrhalis isolated from the elderly.

A total of 40 strains of Moraxella catarrhalis, isolated from the sputum of elderly persons with respiratory tract infections and from nasopharyngeal swabs from healthy elderly, were examined for haemagglutination of human red blood cells and resistance to bactericidal activity in normal human serum (NHS). 15 of 20 strains isolated from the infected elderly and 3 of 20 strains isolated from the healthy elderly showed haemagglutinating properties (p < 0.001). Similarly, 13 of 20 strains from the infected group and 2 of 20 strains from the healthy group were found to be resistant to the bactericidal activity of NHS (p < 0.001). Strains of M. catarrhalis which were associated with respiratory tract infections in the elderly, in contrast to strains colonizing the healthy elderly, were found to be predominantly haemagglutinating for human red blood cells and resistant to complement killing in NHS.