Isolation, characterization, and mode of action of a class III bacteriocin produced by Lactobacillus helveticus 34.9

Traditionally fermented foods and beverages are still produced and consumed at a large scale in Romania. They are rich sources for novel lactic acid bacteria with functional properties and with potential application in food industry or health. Lactobacillus helveticus 34.9, isolated from a home-made fermented milk is able to inhibit the growth of other bacteria, such as other lactic acid bacteria, but also strains of Bacillus subtilis, Bacillus cereus, Staphylococcus aureus, and Halobacillus hunanensis, a halobacterium isolated from the degraded wall of a Romanian monastery. L. helveticus 34.9 produces a large bacteriocin (35 KDa), active in a wide pH range, but inactivated by heat and proteinase K treatment. It shares about 20% sequence coverage with helveticin J, as determined by LC-MS analysis. Bacteriocin production was enhanced under stress conditions, especially when combined stresses were applied. Its mode of action and degree of inhibition depended on the concentration and on the indicator strain that was used; L. delbrueckii subsp. bulgaricus LMG 6901^T cells from a suspension were killed, but the viability of H. hunanensis 5Hum cells was only reduced to 60%, within 8 h. However, the bacteriocin was able to prevent the bacterial growth of both indicator strains when added to the cultivation medium prior inoculation. Scanning electron microscopy images revealed morphological changes induced by the bacteriocin treatment in both sensitive strains, but more severe in the case of L. delbrueckii subsp. bulgaricus. Due to the broad antibacterial spectrum and its production under various stress conditions, the bacteriocin or the producing strain may find application in health, food and non-food related fields, including in the restoration of historical buildings.