Competition in A Multi-Species Probiotic Liquid Suspension

There is little information about the stability of multi-species probiotic products. A commercially available probiotic aqueous-based suspension, P1 which contains Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus acidophilus and Enterococcus faecium was tested using an isothermal microcalorimeter, an instrument which can monitor real-time metabolism of microorganisms.

The goal of the study was to detect the relative growth of the species of product P1 to determine whether inhibition occurs amongst the species.

The component probiotic species of P1 and the product were each inoculated into Brain Heart Infusion (BHI) broth or MRS broth (Oxoid, Basingstoke, UK) supplemented with 0.05% w/v L-cysteine hydrochloride (BHIc; MRSc) (in 3 mL calorimetric glass ampoules) to give pure cultures of densities of 10⁶ CFU/mL and monitored in the microcalorimeter. The species were also inoculated into the medium to give a mixed culture of the individual species at concentrations of 10⁶ CFU/mL of each. Cell free supernatant (CFS) produced by each species was also tested against the producing species and the other species by inoculating 10⁶ CFU/mL of the species in broth with the CFS, monitoring in the microcalorimeter and enumerating the species at the end of incubation.  

The growth profiles in the microcalorimeter were characteristic and unique to each species while the growth profile of P1 was most similar to that of L. plantarum, suggesting this is the dominant organism in mixed-culture. The species showed mutual inhibition however L. plantarum was found to be the most effective species at inhibiting L. rhamnosus. Conversely, L. rhamnosus was the most effective at limiting the growth of L. plantarum. Both L. plantarum and L. rhamnosus were inhibitory toward L. acidophilus and E. faecium. E. faecium was the least inhibitory towards all the other species.

The study shows that some probiotic mixtures may not be stable as a product.