SUMMARY: In order to describe morphologically the structures on the cell surface of bovine enterotoxigenic , variants of reference strain B41 (K99+F41+) either negative for K99 and positive for F41 antigens (variants B41A, B41C), or phenotypically negative for both antigens (variants B41B1, B41B2, B41CB), and a transconjugant harbouring the K99 plasmid and expressing the K99 adhesin [transconjugant B41 × H510a:H510(2)] were examined by transmission electron microscopy using negative staining. Several negative staining procedures were tested for strain B41 and variant B41A: direct harvesting of strains into ammonium molybdate (2%, w/v), with bacitracin (50 μg ml) as wetting agent, gave the best results. Three morphologically distinct structures on the cell surface could be identified in cultures grown on Minca medium. Firstly, thin, filamentous, flexible fibrillar structures, presenting a helical structure and a mean diameter of approximately 3 nm, were recognized as K99 fimbriae, since they were present on strain B41 and on transconjugant H510(2), but not on K99-negative variants nor on the recipient strain H510a. Secondly, coil-like structures with a diameter of about 17-20 nm were observed on strain B41 and on variants B41A and B41C. These structures appeared to consist of two or more curled filaments (diameter 3 nm) joined to coil on themselves into dense spirals. They were very rare in variants B41B1 and B41B2 and were absent on variant B41CB and on a transconjugant B41 × B41CB, which had re-acquired the K99 plasmid and which again exhibited K99 fimbriae. Strains B41 and variant B41A grown at 37 C for 24 h on sheep-blood agar exhibited coiled structures like those seen on Minca medium. In contrast, after growth at 18 C for 48 h (which inhibits the synthesis of F41 antigen), coiled structures were no longer expressed on the cell surface of strain B41 and of variants B41A and B41C. Thus the presence of coiled structures correlated with the expression of F41 antigen in strains and variants, which suggests that F41 had a coiled morphology. Finally, straight fimbriae (diameter 6.5-7 nm) were observed on the cell surface of every strain and variant. Their expression on the cell surface was enhanced by several subcultures in static broth, and it was inhibited by subculture on agar, but not by culture at 18 °C after serial subcultures in static broth. These facts indicated that the straight fimbriae could be common fimbriae, and excluded their being F41 structures.


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