The effect of rhizobiophages on populations of Rhizobium trifolii in the root zone of clover plants

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Abstract
The effect of rhizobiophages CT3 and CT4 on R. trifolii SU91 and SU36, respectively, was examined. These phage-susceptible strains were used, either singly or paired with a competing phage-resistant type as inoculum for Trifolium subterraneum plants growing in vitro in agar medium. Addition of bacteriophage to single strain experiments produced no significant effects on total bacterial numbers, nodulation or plant dry weight. When phages were added to the paired inocula, significant decreases in counts of the susceptible strain were observed, frequently accompanied by a rise in numbers of the resistant type. An increase in the proportion of ineffective variants of the susceptible rhizobia was also observed, although plant dry weights and nodule numbers were not altered. These effects were dependent only upon a difference in phage susceptibility of the bacteria and were obtained even when the competing strain was only partially resistant and a relatively poor competitor.