Monitoring Growth Compatibility and Bacteriocin Gene Transcription of Adjunct and Starter Lactic Acid Bacterial Strains in Milk

ABSTRACT

When developing protective starter cultures for application in cheese technologies, monitoring growth interactions between starter and adjunct lactic acid bacterial (LAB) species and in situ expression of bacteriocin genes in the mixtures is crucial. This study first aimed to monitor the growth of mixed LAB strain populations during milk model fermentations by microbial counts and real-time quantitative PCR. The primary starter strains, Streptococcus thermophilus ST1 and costarter Lactococcus lactis subsp. cremoris M78, served as the basic starter composite coinoculated in all milk treatments. Adjunct bacteriocinogenic Enterococcus faecium strains KE82 and GL31 and the ripening Lactiplantibacillus plantarum H25 strain were added separately to the starter composite, resulting in four LAB combination treatments. The second aim was to quantify gene transcripts of nisin and enterocins B and A synthesized by strains M78, KE82, and GL31, respectively, by reverse transcription-real-time quantitative PCR and to detect the in situ antilisterial effects of the cocultures. Adjunct LAB strains showed growth compatibility with the starter, since all of them exhibited 2- to 3-log-unit increases in their population levels compared to their initial inoculation levels, with ST1 prevailing in all treatments. KE82 grew more competitively than GL31, whereas cocultures with KE82 displayed the strongest in situ antilisterial activity. Nisin gene expression levels were higher at the exponential phase of microbial growth in all treatments. Finally, the expression levels of nisin and enterocin A and B genes were interrelated, indicating an antagonistic activity.

HIGHLIGHTS

How bugs kill bugs: progress and challenges in bacteriocin research

A Biochemical Society Focused Meeting on bacteriocins was held at the University of Nottingham on 16-18 July 2012 to mark the retirement of Professor Richard James and honour a scientific career of more than 30 years devoted to an understanding of the biology of colicins, bacteriocins produced by Escherichia coli. This meeting was the third leg of a triumvirate of symposia that included meetings at the Île de Bendor, France, in 1991 and the University of East Anglia, Norwich, U.K., in 1998, focused on bringing together leading experts in basic and applied bacteriocin research. The symposium which attracted 70 attendees consisted of 18 invited speakers and 22 selected oral communications spread over four themes: (i) Role of bacteriocins in bacterial ecology, (ii) Mode of action of bacteriocins, (ii) Mechanisms of bacteriocin import across the cell envelope, and (iv) Biotechnological and biomedical applications of bacteriocins. Speakers and poster presenters travelled from around the world, including the U.S.A., Japan, Asia and Europe, to showcase the latest developments in their scientific research.