Diversity of naturally occurring bacteriophage of Bacillus subtilis and their interactions with their hosts.

Abstract

Twenty-nine wild bacteriophages of Bacillus subtilis were isolated from each of two Sonoran Desert locations based on their ability to attack 29 wild strains of B. subtilis isolated from one of the two sites. Phages were isolated by their ability to attack one of the 29 strains and then tested for their ability to infect the others. Phages ranged from those able to attack only a few of the strains to those able to attack all 29 test strains. All 58 phages had different attack patterns across the 29 bacteria. Phages from the location from which the bacteria were collected attacked significantly fewer strains than those isolated from the other site. The bacteria exhibited a range of susceptibilities to phage attack, and there was a significant correlation between bacterial susceptibility to attack from phages from one location and susceptibility to attack from phages from the other site. The hypothesis that bacteriophage persistence during hostile conditions can be facilitated by encapsulation within bacterial spores was tested in the laboratory using soil microcosms. Asporogenic bacteria were used to examine the effect of sporulation on the interaction between host and phage. Use of a phage mutant incapable of temperate reproduction eliminated the possibility that phage persistence was due to lysogenic association with the bacteria. Under conditions of low host density in combination with soil conditions hostile to free phages, periodic entrapment in, and release from, bacterial spores facilitated phage persistence. Encapsulation within host spores provides phages with a dormancy mechanism that has the advantage of terminating automatically when conditions favorable for bacterial germination and growth occur. Transmission electron microscopy of the bacteriophages isolated from the two sites was used to examine the morphological diversity of this ecologically diverse collection of viruses, and to determine if correlations between phage ecology and phage morphology were present. No pairs of phages with identical morphologies/dimensions were found. There was no difference between sites in either the proportion of different viral families found or the range of bacteriophage sizes. Phage host range breadth was not found to be predicted by either bacteriophage family or bacteriophage size.