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SUMMARY: The degree of piliation of 29 haemolytic and 4 non-haemolytic Australian strains of Moraxella bovis representing 7 different pilus antigen groups was determined. The infectivity and virulence for the eye was measured in steroid-treated mice and in cattle. Non-piliated strains failed to infect the murine eye. Most moderately or heavily piliated strains reproducibly produced the highest infectivity and virulence scores in mice when compared with lightly or very lightly piliated strains (p <0.05). Non-haemolytic, piliated strains were infective and in one instance virulent for mice. Almost similar levels of infectivity and virulence were observed for 7 representative haemolytic strains tested in both cattle and mice.

The relative molecular weight of pilin sub-units was compared using sodium dodecyl-sulphate polyacrylamide gel electrophoresis. Three classes of pili, α, β and γ of ascending sub-unit size were identified among the 7 pilus antigen serogroups. Pilin sub-unit size bore no relationship to the degree of piliation but most strains that were highly virulent in mice and cattle expressed a and γ sub-units. Some strains appeared capable of switching from α to β or form β to γ sub-unit production.