Predictive microbiology and transcriptomic analysis of heat-resistant and spoilage-associated traits in Microbacterium species isolated from dairy products.

Microbacterium spp. are frequently isolated from dairy products and are implicated in spoilage. In this research, we studied the growth and thermal resistance of two strains, Microbacterium oxydans and Microbacterium laevaniformans. Their growth behaviors were analyzed at 4, 10, 15, and 28 °C using the Baranyi & Roberts and Ratkowsky models. All temperatures supported growth, with the time for a 1 log increase ranging from 9.17 h to 71.42 h, indicating these spoilage-associated strains can proliferate across a broad temperature range. Furthermore, both strains were confirmed as protease-producing and biofilm-forming at 28 °C, highlighting their potential to cause product degradation and persist in dairy processing environment. Thermal resistance was evaluated by exposing high inoculum (6-7 log CFU/mL) and low inoculum (4-5 log CFU/mL) to 63 °C for 1 h, simulating pasteurization temperature, and the survival data were fitted to Weibull model. The tested strains demonstrated thermal resistance, with M. oxydans showing a higher δ-value and a longer inactivation time than M. laevaniformans at the same inoculum, suggesting pasteurization may not sufficiently eliminate Microbacterium. To further clarify the mechanism of Microbacterium spp. during heat treatment, RNA-seq revealed that the heat shock response was characterized by upregulation of genes encoding chaperone proteins, response regulators, and sigma factors, with enrichment in valine, leucine and isoleucine biosynthesis, suggesting a link between amino acid metabolism and stress adaptation that may support their thermal tolerance. These findings provide a scientific basis for industrial people to understand Microbacterium spp. as a potential spoilage risk and to reassess thermal processing controls.