[The "side-effects" of plasmids (R-plasmids and virulence)]

Many plasmids affect the host cells. Their effects cannot be explained only by the expression of the well-known genes coding for antibioticresistance, bacteriocinogeny and hemolysis or the analogous genes (side-effects). The side effects are not characteristic of all plasmids operating under similar conditions. Forecasting of the side-effects inducikility by any definite plasmid is impossible now. Sometimes the same functions exert the contrary effects on the bacterial cell. The connection between the presence of plasmids, especially R-plasmids and the complex cellular property, virulence, is of great interest. Often, bacteria become less virulent obtaining the plasmids. Two possible reasons causing such an effect are discussed. The first one is a direct effect of plasmids on cellular physiology. The second reason is connected with population shifts caused by the fact that the cells with initial low virulence possess the recipient ability predominantly. The decreased virulence of bacteria harbouring R-plasmids, in authors opinion, is quite a natural phenomenon based on plasmid host cells adaptation to the existence in "the realm of antimicrobial agents".