Complete Genome Sequence of Lactococcus lactis subsp. lactis Strain 14B4, Which Inhibits the Growth of Salmonella enterica Serotype Poona In Vitro

The lactococcal ICE-ome encodes a repertoire of exchangeable traits with potential industrial relevance

Dairy industries apply selected lactococcal strains and mixed cultures to produce diverse fermented products with distinctive flavor and texture properties. Innovation of the starter culture functionality in cheese applications embraces natural biodiversity of the Lactococcus species to identify novel strains with alternative flavor or texture forming capacities and/or increased processing robustness and phage resistance. Mobile genetic elements (MGE), like integrative conjugative elements (ICEs) play an important role in shaping the biodiversity of bacteria. Besides the genes involved in the conjugation of ICEs from donor to recipient strains, these elements also harbor cargo genes that encode a wide range of functions. The definition of such cargo genes can only be achieved by accurate identification of the ICE boundaries (delimiting). Here, we delimited 25 ICEs in lactococcal genome sequences with low contig numbers using insertion-sites flanking single-copy core-genome genes as markers for each of the distinct ICE-integrases we identified previously within the conserved ICE-core genes. For ICEs in strains for which genome information with large numbers of contigs is available, we exemplify that CRISPR-Cas9 driven ICE-curing, followed by resequencing, allows accurate delimitation and cargo definition of ICEs. Finally, we compare and contrast the cargo gene repertoire of the 26 delimited lactococcal ICEs, identifying high plasticity among the cargo of lactococccal ICEs and a range of encoded functions that is of apparent industrial interest, including restriction modification, abortive infection, and stress adaptation genes.

Bacillus amyloliquefaciens ALB65 Inhibits the Growth of Listeria monocytogenes on Cantaloupe Melons

Listeria monocytogenes is a foodborne pathogen that causes high rates of hospitalization and mortality in people infected. Contamination of fresh, ready to eat produce by this pathogen is especially troubling because of the ability of this bacterium to grow on produce under refrigeration temperatures. In this study, we created a library of over 8,000 plant phyllosphere-associated bacteria and screened them for the ability to inhibit the growth of L. monocytogenes in an in vitro fluorescence-based assay. One isolate, later identified as Bacillus amyloliquefaciens ALB65, was able to inhibit the fluorescence of L. monocytogenes by >30-fold in vitro. B. amyloliquefaciens ALB65 was also able to grow, persist, and reduce the growth of L. monocytogenes by >1.5 log CFU on cantaloupe melon rinds inoculated with 5 × 10³ CFU at 30°C and was able to completely inhibit its growth at temperatures below 8°C. DNA sequence analysis of the B. amyloliquefaciens ALB65 genome revealed six gene clusters that are predicted to encode genes for antibiotic production; however, no plant or human virulence factors were identified. These data suggest that B. amyloliquefaciens ALB65 is an effective and safe biological control agent for the reduction of L. monocytogenes growth on intact cantaloupe melons and possibly other types of produce.IMPORTANCE Listeria monocytogenes is estimated by the Centers for Disease Control and Prevention and the U.S. Food and Drug Administration to cause disease in approximately 1,600 to 2,500 people in the United States every year. The largest known outbreak of listeriosis in the United States was associated with intact cantaloupe melons in 2011, resulting in 147 hospitalizations and 33 deaths. In this study, we demonstrated that Bacillus amyloliquefaciens ALB65 is an effective biological control agent for the reduction of L. monocytogenes growth on intact cantaloupe melons under both pre- and postharvest conditions. Furthermore, we demonstrated that B. amyloliquefaciens ALB65 can completely inhibit the growth of L. monocytogenes during cold storage (<8°C).