Abstract
Lactococcus lactis cheese starter cultures typically contain a mix of many strains and may include variants that produce and/or tolerate the antimicrobial bacteriocin nisin. Nisin is well-established as an effective agent against several undesirable Gram-positive bacteria in cheese and various other foods. In the current study, we have examined the effect of nisin on 710 individual L. lactis strains during milk fermentations. Changes in milk acidification profiles with and without nisin exposure, ranging from unaltered acidification to loss of acidification, could be largely explained by the type(s) and variants of nisin immunity and nisin degradation genes present, but surprisingly, also by genotypic lineage (L. lactis ssp. cremoris vs. ssp. lactis). Importantly, we identify that nisin degradation by NSR is frequent among L. lactis and therefore likely the main mechanism by which dairy-associated L. lactis strains tolerate nisin. Insights from this study on the strain-specific effect of nisin tolerance and degradation during milk acidification is expected to aid in the design of nisin-compatible cheese starter cultures.
Highlights
Lactococcus lactis is a lactic acid bacterium widely used in the dairy industry for milk fermentation
A total of 710 L. lactis strains were monitored, in a timeresolved fashion, to evaluate their ability to acidify milk in the presence of nisin (CFS collected from the nisin producing L. lactis strain ATCC 11454, CFSnisin) or in absence of nisin (CFS collected from the non-nisin producing L. lactis strain Wg2, CFScontrol)
Nisin-producing strains are added to milk fermentations for cheese production, but how genetic variation in L. lactis strains contributes to differences regarding milk acidification remains largely unexplored
Summary
Lactococcus lactis is a lactic acid bacterium widely used in the dairy industry for milk fermentation. Nisin in powder format was first introduced to the market in the 1950’s as a natural product to contribute to shelf-life extensions of cheese by preventing growth of food spoilage organisms including Clostridia, Propionibacteria and Listeria (Hirsch, 1951; Delves-Broughton, 1996; Molloy et al, 2011). Nisin-producing strains can be added or are naturally present in cheese starter cultures (Delves-Broughton, 1996; de Arauz et al, 2009; Ávila et al, 2014). Starter cultures for semi-hard cheeses like Gouda or Edam are typically composed of a mixture of lactococcal strains, including Lactococcus lactis ssp.
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