Electrophoretic and antigenic characterisation of Dermatophilus congolensis extracellular products

Dermatophilus congolensis is the causative agent of bovine dermatophilosis and lumpy wool in sheep. Two field isolates of D. congolensis, one each from a cow in Ghana and a sheep in Scotland, were cultured for 24-72 h in a synthetic medium based on RPMI-1640. Culture filtrates were examined by SDS-PAGE and considered to contain extracellular products released by growing hyphae and filaments. Electrophoretic profiles of culture filtrates of the two isolates contained common bands and bands that were unique to each isolate. The composition of extracellular products altered with increasing culture periods indicating that specific products were released at different stages of growth. Culture filtrate prepared in the presence of serine protease and metalloprotease inhibitors contained more and better defined bands than that prepared without protease inhibitors indicating the presence of proteases in culture filtrates. Western blot analysis of extracellular products using a panel of sera showed that the two isolates from different host species and distant geographical locations contained cross-reactive antigens. Natural and experimental infections stimulated antibody responses to antigens in culture filtrates, sera from animals that were disease free but in-contact with dermatophilosis-infected animals also contained antibodies to extracellular antigens. The antigens recognised by most sera had molecular weights of 200 kDa in the bovine isolate, 170 kDa in the ovine isolate and 67, 27 and 52-55 kDa in both isolates. The number of antigenic bands of both isolates was positively correlated with the intensity of challenge and the severity of infection: antibodies in sera from disease-free cattle in Ghana recognised more antigens than sera from disease-free sheep in Scotland and more antigens were recognised by sera from chronically-infected Ghanaian cattle than by sera from experimentally-infected calves and sheep. The latter developed antibodies to antigens of 27 and 24 kDa during the course of infection. The electrophoretic profiles of extracellular products of D. congolensis are less complex than those of other structures of the bacterium yet they exhibit differences between the two isolates. Extracellular products contain antigens recognised by sera from naturally exposed and experimentally-infected animals that may be involved in immunity to D. congolensis or immunopathogenesis of dermatophilosis.