Microbiological tools to explore LAB–Propionibacteria synergy in view of dairy by-product valorisation

Lactic acid bacteria (LAB) can enhance the growth rate and biomass production of Propionibacteria (PAB) (Piveteau et al., 1995). Additionally, adding lactate (1.5 g L⁻¹) to a lactose-containing medium shortens the lag phase by about 39% and increases the carrying capacity of selected PAB strains (Coronas et al., 2023). Based on this evidence, and within a project aiming to improve the fermentative performance of PAB in lactose-rich dairy by-products, this study focused on identifying LAB strains that positively interact with PAB; defining an inoculation strategy to optimise PAB growth; developing a differential medium for the selective enumeration of LAB and PAB; setting up a thermal treatment that preserves the fermentability of dairy by-products while avoiding undesired contamination. The results highlighted four LAB strains that enhanced the growth of selected PAB in sequential inoculation, while no appreciable effect was observed under co-inoculation. YELactose agar clearly differentiated LAB and PAB colonies, proving suitable for monitoring each bacterial group in mixed cultures. A 5-minute treatment at 110 °C proved effective in eliminating undesired microorganisms without detrimental effects on by-product fermentability. These findings provide practical tools for designing mixed starter cultures and establish a basis for further research on LAB–PAB synergistic interactions to improve the valorisation of dairy by-products through fermentation.