SUMMARY: Passaging of the K88-positive strain CN6913 through synthetic medium containing immune colostrum gave rise to large numbers of K88-negative CN6913 variants. These K88-negative variants had all lost a single large plasmid known to encode the K88 genetic determinant. Four other large plasmids harboured by this strain were unaffected. Viable K88-positive and K88-negative variants of CN6913 accumulated at a similar rate in synthetic medium and in medium containing non-immune colostrum. In the presence of immune colostrum, viable cells of the K88-negative variant accumulated faster and to a greater extent in cultures than the K88-positive variant if incubated at 37 °C, which favours the phenotypic expression of K88. However, when similar cultures were incubated at 18 °C, a temperature known to inhibit phenotypic expression of K88, the accumulation of viable cells of the two variants was strictly comparable in all media and no loss of plasmid or increase in K88-negative variants was observed. Cells containing a pBR322-based K88-encoding recombinant plasmid were also eliminated by immune colostrum whereas cells containing pBR322 were not. Plasmids encoding the K99 antigen were not readily eliminated from strains passaged through medium containing immune colostrum. K99-negative variants that were detected still harboured the K99-encoding plasmid.


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