Growth assessment of mixed cultures of probiotics and common pathogens

In this work, an isothermal microcalorimeter was applied to investigate the antipathogenic activity of three probiotics (Lactobacillus acidophilus, Bifidobacterium lactis and Bifidobacterium bifidum) against Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli using the probiotics in mixed culture with the pathogenic microorganisms.

A microcalorimeter was used to monitor the growth of the microorganisms as pure cultures and as co-cultures at 37 °C. Relative growths of the probiotics and pathogenic species were determined after microcalorimetric measurements by serial dilution and plate incubation. Relative growth of mixed cultures of E. coli with L. acidophilus or B. lactis was also determined by traditional plate growth assay for 5.5 h.

The results showed growth profiles of the microorganisms that were characteristic and showed different lag and peak times for the species. The pathogenic species grew faster than the probiotic species. In the co-cultures, the growth profile of both pathogenic species and probiotics could be identified with the microcalorimeter. Although the pathogenic species grew faster, at the end of the assay, the results showed that the pathogenic species were inhibited in growth by the probiotics as no viable growth of the pathogenic species was detected whereas 10⁷-10⁸ CFU/mL of the probiotics were enumerated after the microcalorimetric assay. Using the traditional plate assay, the data confirmed co-growth of the probiotics and E. coli although cell numbers of E. coli were higher than the probiotics during 5.5 hours of co-culture incubation when both were inoculated at 10⁶ CFU/mL.

The results demonstrate the antipathogenic effects of probiotics and highlights the potential of microcalorimetry in live mixed culture assays and its limitation.