Resistance to bacteriophage φ6 by HB10Y occurred at a frequency of about 1 in 120000 cells. Statistical analysis by the fluctuation and re-spreading tests indicated that the majority of φ6 bacteria resulted from random mutation rather than from contact with the phage. However, exposing the bacteria to heated φ6 (50 °C for 1 min) or φ6 nucleocapsid (virus minus the lipid envelope) prior to testing for resistance increased the frequency two- to threefold; φ6 dsRNA, unheated φ6, or heated φ91 did not have this effect. Nine resistant variants were selected for further characterization. When DNA isolated from the variants was subjected to agarose gel electrophoresis, seven of the nine strains had two plasmid DNA bands typical of HB10Y; the remaining two strains had three plasmid bands. φ6 attached to the pili of six of the nine variants; four of the variants actually had higher adsorption rate constants than HB10Y. Culture supernatants from the φ6 strains did not inactivate φ6. One of the resistant strains, D5, produced large amounts of infectious φ6 particles during its exponential phase of growth without a significant effect on its growth rate. Treatment of D5 with φ6-antiserum resulted in the loss of φ6 production; however, unlike typical carrier-state cells, D5 retained its resistance to φ6.

The results indicate that resistance to φ6, although occurring at a relatively high frequency, is not due to one common event, such as loss of a plasmid and/or attachment sites.


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