Bacterial Metabolites from Intra- and Inter-Species Influencing Thermotolerance: The Case of Bacillus cereus and Geobacillus stearothermophilus

Introduction: Bacterial metabolites with communicative functions could provide protection against stress conditions to members of the same species. Yet, information remains limited about protection provided by metabolites in Bacillus cereus and inter-species.

Purpose: This study investigated the effect of extracellular compounds derived from heat shocked (HS) and non-HS cultures of B. cereus and Geobacillus stearothermophilus on the thermotolerance of non-HS vegetative and sporulating B. cereus.

Methods: Cultures of B. cereus and G. stearothermophilus were subjected to HS (42°C or 65°C, respectively, for 30 min) or non-HS treatments. Cells and supernatants were separated and mixed in a combined array, then exposed to 50°C for 60 min and viable cells determined. For spores, D-values (85 and 95°C) were evaluated after 120 h.

Results: In most cases, supernatants from HS B. cereus cultures added to non-HS B. cereus cells caused thermotolerance to increase (D₅₀: 12.2–51.9) in comparison to supernatants from non-HS cultures (D₅₀: 7.4–21.7). The addition of supernatants from HS and non-HS G. stearothermophilus cultures caused the thermotolerance of non-HS cells from B. cereus to decrease, initially (D₅₀: 3.7–7.1), and in most cases, to subsequently increase (D₅₀: 18–97.7). Most supernatants from sporulating G. stearothermophilus added to sporulating cells of B. cereus caused the thermotolerance of B. cereus 4810 spores to decline, whereas the thermotolerance of B. cereus 14579 increased.

Significance: This study clearly shows that metabolites in supernatants from either the same or different species (such as G. stearothermophilus) influence the thermotolerance of B. cereus.