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Abstract An investigation was conducted with the aim to illucidate the presence of toxic substance, produced by various strains of some species of Condida. 195 specimens were collected from human cases of candidiasis in Assiut Governorate, and 55 strGins were isolated. Of these, 41 strains could be identified as such; 3 c. pseudotropicalis, 11 C. tropicalis, 10 Q. ~tilis, 4 S krusei, 7 C. albicans, 2 2:... parapsilosis, 3 ~ guilliermondii and one strain...s lipolytica. The pathogenicity to mice of the identified strains was determined by the intraperitoneal injection of a suspension containing 108 viable organisms. The most pathogenic strain of each of 6 species of Candida (~ pseudotropicalis, C. ropicalis, C. utilis, C. krusei, C. a.lbicans and C. parapsilosis) was selected for further studies together with a pathogenic strain of C. gtdlliermondii isolated in a previous occasion. Each strnin was cultured in fluid Sabouraud’s medium in a special pattern, the grow~th was centrifuged and tested for toxigenioity to mice. The cell-free supernatent was non-toxic while the deposit was toxic after being subjected to crushing and precipitation by alcohol. from certain evidences given it appears that the pathogenicity of Candida species depends on its power to produce a toxic substance more related to endotoxin rather than to exotoxin. The alcohol precipitate of each of the toxigenic strains was purified by using a cationic surface active agent. The LD50 of each purified substance was determined and no much diff2rence was noticed between the seven extracts. The histopathological examinations of the internal organs of the dead mj.ce were performed and the main changes were noticed in the kidneys, liver and lungs while the heart was affected iD ~ase of C. guilliermondii. The physico-chemical characters of the purified toxic substances were recorded, no much difference was obtained except for the thermolability of the extracts of C. albicans and C. parapsilosis. The chemical analysis of the 5 thermostable purified substances revealed the presence of carbon, hydrogen, nitrogen and oxygen, and the absence of sulphur in all of them. The purified materials of ~ pseudotropicalis, ~ utilis and C. krusei were found to contain a measurable percentage of ilalogen in the form of chlorine. According to the percentage of the chemical elements, the five extracts can be divided into 3 groups, each possesses a sim~ilar percentage. One group includes s... pseudotropicalis and ~ utilis, the second group includes C. tropicalis and S guilliermondii and the third group includes C. krusei. id The five thermostable purified substances were found to be devoid of nucleic acid, phosphoric acid and purine base and to contain a polypeptide. The extracts of ~ tropicalis and C. krusei contained a polysaccharide. The presenoe of a polypeptide indicates the protein nature of these extracts combined with a polysaccharide in two of them. The absence of nucleic acid is attributed to its loss during the process of purification. The I.R. spectrum of the 5 thermostable purified substances showed a similarity between ~ pseudootropicalis and C. utilis also Q.. tropicalis was similar to some extent to C. guilliermondii, C. krusei had a peculiar intermediate position between the above two groups. The results of the cross reactions between the five thernlostable purified extracts of Candida species ffild their ro1tisera prepared in rabbits showed the presence of 4 antigens. C. pseudotropicalis possesses antigens (1) and (4); C. tropic8,lis antigens (2) and (3); ~ utilis antigen (4), S lcrusci antigens (1) and (2); S. guilliermondii antigen (3). The antigenic relationship between the five Candida species \ would be as such: one related group is comprised of C. -- pseudotropicalis ro1d C. utilis, another related group is composed of c. tropicalis ffild C. guilliermondii and the last group is comprised of C. krusei which is related to only one of the two members of either the first or second group. It |