Exploring suppression subtractive hybridization (SSH) for discriminating Lactococcus lactis ssp. cremoris SK11 and ATCC 19257 in mixed culture based on the expression of strain-specific genes.

AIM

An approach based on quantitative reverse transcriptase PCR (RT-qPCR) was developed for monitoring two strains of lactococci in co-culture in milk by measuring the expression of specific genes identified by suppression subtractive hybridization (SSH).

METHODS AND RESULTS

SSH was used to identify strain-specific genes of Lactococcus lactis ssp. cremoris SK11 and ATCC 19257. RT-qPCR was then employed to validate gene specificity and compare the expression of selected specific genes (glycosyltransferase and amidase genes for L. lactis ssp. cremoris ATCC 19257 and a hypothetical protein for SK11) identified by SSH. The time profile of changes in gene expression relative to ldh transcription differed between pure and mixed cultures as well as between media. At the stationary phase, gene expression of mixed cultures in GM17 attained the highest proportion of ldh transcription while mixed cultures in milk peaked at the postexponential phase. Strain ratios expressed as RNA proportion appear to favour SK11 in GM17 medium, while ATCC 19257 dominated in milk co-cultures.

CONCLUSIONS

This approach was useful to determine the contribution of strain SK11 in relation to strain ATCC 19257 during co-culture in milk compared to rich medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

The ability to track the metabolic contribution of each lactococcal strain during fermentation of milk or cheese ripening will extend our understanding of the impact of process parameters on the production performance of strains.