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Abstract
Resistance to bacteriophage ϕ6 by Pseudomonas phaseolicola 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 ϕ6r 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 ϕ6r 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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