Effect of Lactose on Acid Tolerance of Yogurt Culture Bacteria

Lactose, the sugar naturally present in milk, provides energy to lactic acid bacteria used in fermented dairy foods. Increasing concentrations of lactose may improve survivability of lactic acid bacteria in the dairy foods and in human gut enhancing their probiotic benefits. Acid tolerance is an important probiotic characteristic. The objective was to determine the influence of lactose on acid tolerance of yogurt starter culture Streptococcus thermophiles ST-M5 and Lactobacillus bulgaricus LB-12. The M 17 broth was used for Streptococcus thermophiles ST-M5 and MRS broth was used for Lactobacillus bulgaricus LB-12. Lactose was added to both broths at 0% (control), 1%, 3%, and 5% (wt/vol). Both broths were acidified to pH 2.0. Upon sterilizing and tempering, both broths were inoculated. Acid tolerance was determined as viable counts in acidified broths after 120 minutes of incubations. In an incubation period of 2 hours, dilutions were plated every 30 minutes. Three replications were conducted. The highest acid tolerance for Streptococcus thermophiles ST-M5 and Lactobacillus bulgaricus LB-12, was observed in lactose concentration of 3% and 5% (wt/vol).

Acid tolerance of lactate-utilizing bacteria of the order Bacteroidales contributes to prevention of ruminal acidosis in goats adapted to a high-concentrate diet

The rapid accumulation of organic acids,particularly lactate,has been suggested as the main cause of ruminal acidosis(RA)for ruminants fed high-concentrate diets.Previous research has shown that a gradual shift from low-to high-concentrate diets within 4 to 5 weeks effectively reduces the risk for RA.However,the mechanisms remain unknown.In this study,20 goats were randomly allocated into four groups(n=5)and fed with a diet containing a weekly increasing concentrate portion of 20%,40%,60%,and 80%over 28 d.At d 7,14,21,and 28,one group(named C20,C40,C60,and C80 according to the last concentrate level that they received)was killed and the ruminal microbiome was collected.Ruminal acidosis was not detected in any of the goats during the experiment.Nonetheless,ruminal pH dropped sharply from 6.2 to 5.7(P<0.05)when dietary concentrate increased from 40%to 60%.A combined metagenome and metatranscriptome sequencing approach identified that this was linked to a sharp decrease in the abundance and expression of genes encoding nicotinamide adenine dinucleotide(NAD)-dependent lactate dehydrogenase(nLDH),catalyzing the enzymatic conversion of pyruvate to lactate(P<0.01),whereas the expression of two genes encoding NAD-independent lactate dehydrogenase(iLDH),catalyzing lactate oxidation to pyruvate,showed no significant concomitant change.Abundance and expression alterations for nLDH-and iLDH-encoding genes were attributable to bacteria from Clostridiales and Bacteroidales,respectively.By analyzing the gene profiles of 9 metagenome bins(MAG)with nLDH-encoding genes and 5 MAG with iLDH-encoding genes,we identified primary and secondary active transporters as being the major types of sugar transporter for lactate-producing bacteria(LPB)and lactate-utilizing bacteria(LUB),respectively.Furthermore,more adenosine triphosphate was required for the phosphorylation of sugars to initiate their catabolic pathways in LPB compared to LUB.Thus,the low dependence of sugar transport systems and catabolic pathways on primary energy sources supports the acid tolerance of LUB from Bacteroidales.It favors ruminal lactate utilization during the adaptation of goats to a high-concentrate diet.This finding has valuable implications for the development of measures to prevent RA.

Divergent responses of growth rate and antioxidative system of ten Bacillus strains to acid stresses

Response of growth rate and antioxidative system of ten Bacillus strains to acid stresses was assayed.Strong acid treatment significantly decreased the growth rate of the strains.Acid stresses increased the GPX activity and GSSG content of the tested strains.Divergent changes occurred in ROS and antioxidative system(SOD,CAT,GR,MDA and GSH).Environmental changes including soil acidification exert obvi-ous stresses on soil ecosystems and influence soil microor-ganisms.In this study,ten microbial strains were incubated under different acid treatments to investigate responses of microbial growth and antioxidative system to acid stress.All the strains belong to Bacillus genus,but exhibit distinct ecological functions.We observed that these microbial strains had obviously different pH tolerance threshold,in spite of the close phylogenetic classification among strains.Acid stresses exerted significant effects on microbial antiox-idative system,including superoxide dismutase(SOD),cata-lase(CAT)and glutathione transferring enzymes(GPX and GR)and reactants(GSH and GSSH),but the effects were strain specific.Furthermore,we found acid stress effects on total variances of the investigated microbial antioxidative system along the first two principal components(PCs).Activities of CAT and SOD contributed substantially to PC1 that reflected obvious acid effects on NC7 and ZC4,and closely related to intracellular malondialdehyde content.The GSSG activities and GSH/GSSG contributed greatly to PC2 that unveiled acid stress effects on most of the microbial strains.Our results highlight substantially heterogeneous responses of microbial strains to acid stress and support that phylogenetic closeness does not imply functional similarity of soil microorganisms under environmental changes.