Effect of Lactococcus lactis UQ2 and its bacteriocin on Listeria monocytogenes biofilms

Listeria monocytogenes is a foodborne pathogen of major concern in food processing due to its wide environmental distribution and ability to form biofilms. Lactococcus lactis UQ2 is a bacteriocinogenic strain isolated from Mexican-style cheese, genetically identified as nisin A. It was grown on supplemented whey medium, representing an alternative, and probably more economical way to obtain this bacteriocin. A spray-dried crude bacteriocin fermentate (CBF) of L. lactis UQ2, or L. lactis UQ2 cells were used to test their antimicrobial activity against planktonic and sessile cells of L. monocytogenes Scott A on stainless steel chips. L. lactis UQ2 reduced >5 log cycles of this pathogen biofilms per chip, in competitive-exclusion tests at 37 °C. A new approach was developed to evaluate the antagonic effect of L. lactis UQ2 which outnumbered Listeria cells, by using fluorescent in situ hybridization utilizing labeled specific probes. An in vitro study showed that the combination of the two main antimicrobials produced by L. lactis UQ2 (lactic acid and nisin A) can be more efficient to suppress L. monocytogenes growth than each one on its own. This study may provide information to develop strategies to suppress or control L. monocytogenes biofilms on stainless steel surfaces, one of the most common materials used for food processing equipment.