Growth in the presence of inhibitory concentrations of copper enhances the tendency of yeast to flocculate. Many yeasts will not flocculate unless calcium is included in the growth medium and Guinness strain 522 used in the present work required a relatively large amount. Single cell cultures may undergo variation during subculture, resulting in the production of a large number of variants (Chester 1963). The cells of these variants differ considerably in their ability to adhere together. Flocculation variants of strain 522 differed among themselves in the amount of calcium necessary for flocculation, the most flocculent variants requiring least calcium. Washed cells of the more flocculent yeasts removed more calcium from a calcium chloride solution than did those with lesser powers of adhesion. In a copper medium containing calcium the more flocculent variants replaced the less flocculent. Calcium protected cells from copper and the more flocculent variants enjoyed most protection. All variants acquired resistance to copper during growth in the copper medium. Despite the selection of the more flocculent yeasts during growth in liquid medium, their copper resistance was less than that of the less flocculent yeasts. When calcium was added to the liquid copper medium, cultures developed less resistance. It is concluded that the less flocculent cells, having less protection by calcium, were exposed to what was effectively a greater concentration of copper and therefore became more resistant. This greater resistance did not enable these cells to compete with the flocculent cells in liquid cultures.
Rotavirus Persistence in Cell Cultures: Selection of Resistant Cells in the Presence of Foetal Calf Serum