Behavior and viability of spontaneous oxidative stress-resistant Lactococcus lactis mutants in experimental fermented milk processing.

Previously, we isolated two strains of spontaneous oxidative (SpOx2 and SpOx3) stress mutants of Lactococcus lactis subsp cremoris. Herein, we compared these mutants to a parental wild-type strain (J60011) and a commercial starter in experimental fermented milk production. Total solid contents of milk and fermentation temperature both affected the acidification profile of the spontaneous oxidative stress-resistant L. lactis mutants during fermented milk production. Fermentation times to pH 4.7 ranged from 6.40 h (J60011) to 9.36 h (SpOx2); V(max) values were inversely proportional to fermentation time. Bacterial counts increased to above 8.50 log(10) cfu/mL. The counts of viable SpOx3 mutants were higher than those of the parental wild strain in all treatments. All fermented milk products showed post-fermentation acidification after 24 h of storage at 4 degrees C; they remained stable after one week of storage.

Oxidative stress in Lactococcus lactis

Lactococcus lactis, the most extensively characterized lactic acid bacterium, is a mesophilic- and microaerophilic-fermenting microorganism widely used for the production of fermented food products. During industrial processes, L. lactis is often exposed to multiple environmental stresses (low and high temperature, low pH, high osmotic pressure, nutrient starvation and oxidation) that can cause loss or reduction of bacterial viability, reproducibility, as well as organoleptic and/or fermentative qualities. Among these stress factors, oxidation can be considered one of the most deleterious to the cell, causing cellular damage at both molecular and metabolic levels. During the last two decades, considerable efforts have been made to improve our knowledge of oxidative stress in L. lactis. Many genes involved with both oxidative stress resistance and control mechanisms have been identified; functionally they seem to overlap. The finding of new genes, and a better understanding of the molecular mechanisms of stress resistance in L. lactis and other lactic acid bacterium, will lead to the construction and isolation of stress-resistant strains. Such strains could be exploited for both traditional and probiotic uses