Mating due to loss of surface exclusion as a cause for thermosensitive growth of bacteria containing the Rtsl plasmid

Summary

At 25° C, Rtsl⁺ bacteria grow to about 5x10⁹ bacterial/ml before leveling off, whereas at 42° C they grow from 2.6x10⁸ bacteria/ml for only 2.3 generations after temperature shift before the growth is inhibited with a zig-zag pattern at the plateau. When diluted, Rtsl⁺ bacteria grow rapidly at 42° C, until the concentration reaches as high as the undiluted 42° C culture when growth measured by colony counts stops and the zig-zag pattern again appears. This density-dependent growth inhibition is not due to the presence of stable growth inhibitor(s). Mating experiments show that at 42° C, Rtsl⁺ bacteria retain good donor ability; at the same time, they become good recipients in mating with Hfr (Rtsl) bacteria. SDS-PAGE reveals that membranes are altered at 42° C. Examination of DNA synthesis indicates that chromosomal DNA is synthesized at both 25° C and 42° C at high bacterial concentration, but that of the Rtsl plasmid is slowed down at 42° C. The labeling experiments suggest that in 2 h there are 2 rounds of plasmid replication at 25° C, 3.5 rounds at 42° C when bacteria are diluted, and 0.6 rounds at 42° C when bacteria are not diluted. These results suggest that the growth inhibition of Rtsl⁺ bacteria at 42° C is probably the consequence of mating initiated due to loss of surface exclusion.