| To further understand the role of lactic acid bacteria (LAB) in the piglet gut health, spatial and temporal changes of quantity of E. coli and Lactobacillus in the gut of piglets were investigated. Five strains of LAB were isolated and identified from the intestinal contents. The production of lactic acid and antagonistic property of the five LAB strains, and the effect of gastrointestinal factors on the survival of two strains were studied. This thesis includes four sections.In the first section, E. coli and Lactobacillus in the gastrointestinal tract of 12 conventionally raised piglets from the same litter was enumerated. The results showed that the number of Lactobacillus in jejunum and ileum increased as the piglets grew before weaning but decreased after weaning. The number of E. coli in jejunum and ileum increased from suckling into weaning time. There was no apparent change in the number of E. coli and Lactobacillus in caecum and rectum over the time period studied. These results suggested that weaning apparently affected the numbers of E. coli and Lactobacillus in small intestine, with E. coli fast proliferating while Lactobacillus decreasing. Weaning, however, had little effect on the numbers of E. coli and Lactobacillus in large intestine.In the second section, five strains of LAB were isolated from the intestinal contents of piglets. According to the features of Gram staining, and morphological and biochemical characteristics, the five strains, L1, L2, L3, L5 and L7, were preliminarily identified as Bacillus polymyxa, Lactobacillus casei, Lactococcus sp., Sporolactobacillus sp. and L. delbrueckii subsp. delbruecki, respectively. A strain of E. coli was isolated from the same intestinal contents of piglets.In the third section, lactic acid production and antagonistic property of five strains of LAB (L1, L2, L3, L5 and L7) isolated from piglet intestine were investigated. The results showed that among all strains L5 exhibited the most rapid production of lactic acid in the culture, with the highest production value. Accordingly, the pH in the culture of L5 showedthe fast decline, with the final value significantly lower than those of other cultures. Strain L1 showed the least production of lactic acid. The culture supernatants of the five strains showed different degrees of antagonistic effect against pathogenic E. coli K88, K99, 987P, O141, E1, and 5. aureus. The inhibitory effect was more significant when the pH in the medium was low. When subtracting the effect of the acid, the culture supernatants still showed 22%~53% inhibitory effect, suggesting that apart from lactic acid the bacteria produced other inhibitory substances. The inhibitory effect of the culture supernatant was above 92% after heat treatment and above 85% when treated with proteases.In the fourth section, effects of gastrointestinal factors (low pH, bile salt and pepsin) on the survival of strain L5 and L7, and the heat tolerance of strain L5 were studied. The results showed that there was no obvious effect on the survival of L5 while incubating at pH over 2.5 for 150 min; and L5 was still able to survival well while incubating at pH1.5 for 30 min. Strain L7 exhibited a lower acid tolerance than L5, it was only able to survival at pH3.5 and 4.5. The survival of two strains was both inhibited by bile. The survival rates of L5 and L7 were 21.80% and 29.06% respectively after incubating in the medium with 0.1% of bile for 4 hours. The survival rates of two strains were both below 0.18% while the concentration of bile was over 0.2%. No significant effect of pepsin was observed on the growth of two strains. Heating at 50 癈 showed no effect on survival of L5 but heating over 80 癈 affect the survival of L5 significantly. |
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