Exploring the impact of intraspecific interaction between sibling colonies of Bacillus cereus MSM-S1 in the absence and presence of antibiotics

Bacillus cereus is a spore-forming pathogen and food contaminant that can survive in hostile environments through different survival strategies. As part of survival, B. cereus may also engage in intra- or interspecific interactions that shape its community structure and stress responses, enabling its persistence under unfavourable conditions. The present study explored intraspecific interactions, at the cell and molecular levels, between two sibling colonies of Bacillus cereus MSM-S1 grown through swarming adjacently on nutrient agar in the absence and presence of gentamicin. While growing alone, gentamicin altered the symmetrical colony morphology of B. cereus MSM-S1, which was associated with cell elongation, decreased flagellation and increased expression of the toxin gene nheA. During interaction, both in the absence and presence of gentamicin, the motility of sibling colonies towards each other was restricted, forming a distinct line/zone of demarcation between the colonies. Microscopic analysis at different time points (day 3, 5 and 7) revealed progressive changes in the swirling arrangement of cells at the interaction zone. In the absence of gentamicin, cells became jammed at the interacting front by day 7, whereas this effect was not observed in the presence of gentamicin on day 7. Cells at the interacting zone underwent gradual sporulation; notably, sporulation was less in the presence of gentamicin. Further analysis suggested that nutrient limitation was the major contributory factor for the formation of the zone of no growth between the interacting colonies. Moreover, the expression of quorum-sensing genes was down-regulated in cells at the interacting zone without gentamicin, but notably, upregulated in the presence of gentamicin. The expression of chemotaxis regulator gene, cheY, was also upregulated in interacting zone cells both in the absence and presence of gentamicin. In addition, increased expression of the global transcriptional regulator codY in the interacting zone under both conditions suggested its involvement in metabolic regulation during sibling colony interaction. The findings highlight modulated quorum sensing and sporulation as adaptive mechanisms in Bacillus cereus MSM-S1 to cope with external stress imposed by nutrient limitations caused by the growth of sibling colonies, whereas, cell elongation and toxin production together is a preferred survival strategy for a growing MSM-S1 colony when stress is imposed by gentamicin as antibiotic but sufficient nutrient is available.