Defective phages as an antagonistic factor of closely related bacilli

The antagonistic effect produced by the defective phage PBSX during coculti vation of the mutant strain Bacillus subtilis 168, in which this phage is h eat-inducible, and strain B. subtilis NRS231, which also bears a defective phage, was investigated. As soon as in the first hours of cocultivation und er conditions of PBSX induction, the number of viable cells of strain NRS23 1 decreased by two orders of magnitude. The effect was not observed if the temperature of cocultivation was noninducing. The results confirm the suppo sition that defective phages may play a role in the competition between clo sely related bacilli.