Lysogenic control of Bacillus subtilis morphology and fitness by Spbetavirus phi3T

In lysogenic conversion, temperate phages can profoundly alter the phenotypes of their bacterial hosts. When studying the effects of SPbeta-like viruses on a Bacillus subtilis soil isolate, we observed that phage phi3T induced a stable and heritable acquisition of a spherical cell morphology, departing from the typical rod shape of its ancestor. Although time-lapse imaging revealed that the lysogen retained cell wall integrity and symmetric division, it exhibited reduced fitness and increased susceptibility to cell wall-targeting antimicrobials. Remarkably, when SPβ, a homologous SPbeta-like virus, was present, the host retained its rod-shaped morphology despite phi3T superinfection. This study uncovers a novel example of lysogenic conversion in which phage integration into the bacterial genome induces heritable changes in host biology. Additionally, we revealed intriguing phage-phage interactions during polylysogeny that may benefit the host. These findings offer insight into the persistence and absence of specific Spbetavirus variants in natural B. subtilis populations.