Wiley

TÓM TẮT: Giới thiệu một môi trường nuôi cấy cải tiến cho vi khuẩn lactobacilli. Môi trường này hỗ trợ sự phát triển tốt của lactobacilli nói chung, và đặc biệt hữu ích cho một số chủng nhạy cảm chỉ phát triển kém trong các môi trường chung khác. Ngoài ra, trong môi trường này không cần sử dụng nước ép cà chua, một nguyên liệu có sự biến đổi cao. Khi được điều chỉnh một chút, môi trường này cũng có thể được sử dụng như môi trường cơ bản cho các thử nghiệm lên men.

E.A. SMITH AND G.T. MACFARLANE. 1996. Concentrations of phenolic compounds in human gut contents were more than fourfold higher in the distal colon (6.2 mmol kg^‐1) compared to the proximal bowel (1.4 mmol kg^‐1). Tryptophan metabolites were never found in more than trace amounts in large intestinal contents and phenol substituted fatty acids were the major products of aromatic amino acid fermentation that accumulated in the proximal colon, whereas phenol and p‐cresol were more important in the distal gut, accounting for 70% of all products of dissimilatory aromatic amino acid metabolism. In vitro incubations of colonic material showed that phenol was produced most rapidly (1.0 μmol g^‐1 h^‐1), whereas indole was formed comparatively slowly (0.06 μmol g^‐1 h^‐1). Most probable number (MPN) estimations demonstrated that large populations of phenol and indole producing bacteria occur in the large intestine (range log₁₀ 9.8–11.5 (g dry wt faeces)^‐1, mean 10.6, N=7). With respect to phenolic compounds, phenylacetate and phenylpropionate producers predominated, while indoleacetate‐forming bacteria were the major tryptophan‐utilizing organisms. Quantitation of products of dissimilatory aromatic amino acid metabolism in MPN tubes showed that phenol and phenylpropionate mainly accumulated at low sample dilutions, whereas phenylacetate, p‐cresol, indoleacetate and indolepropionate were formed in greatest amounts at high sample dilutions. The significance of pH and carbohydrate availability with respect to aromatic amino acid metabolism was shown in batch culture fermentation studies, where net production of phenolic compounds by mixed populations of intestinal bacteria was reduced by approximately 33% during growth at pH 5.5 compared to pH 6.8, and by 60% in the presence of a fermentable carbohydrate. Experiments with 16 species of intestinal bacteria belonging to six different genera showed that environmental factors such as low pH and high carbohydrate availability markedly reduced dissimilatory aromatic amino acid metabolism in some organisms, but stimulated this process in others. A three‐stage continuous culture model of the colon was used to investigate the effect of system retention time (27.1 or 66.7 h) on aromatic amino acid fermentation. Qualitative and quantitative increases in phenol production occurred from vessel 1 to vessel 3 in this model. Concentrations of phenolic compounds in vessel 3 were three times greater at R=66.7 h compared to R=27.1 h. Phenol and p‐cresol were not detected in vessel 1, though formation of these metabolites increased from vessel 2 to vessel 3, in a pattern similar to that observed in the distal colon.

An antimicrobial peptide designated pediocin AcH was isolated from Pediococcus acidilactici strain H. The pediocin AcH was purified by ion exchange chromatography. The molecular weight of pediocin AcH was determined by SDS‐PAGE to be about 2700 daltons. Pediocin AcH was sensitive to proteolytic enzymes, resistant to heat and organic solvents, and active over a wide range of pH. Pediocin AcH exhibited inhibition against several food spoilage bacteria and foodborne pathogens including Staphylococcus aureus, Clostridium perfringens and Listeria monocytogenes. It was bactericidal to sensitive cells and acted very rapidly. The bactericidal effect was not produced by either cell lysis or apparent loss of membrane permeability.

Factorially designed experiments have been used to study the growth and survival of Listeria monocytogenes in different combinations of pH and salt concentrations at ambient and chill temperatures. Survival at low pH and high salt concentration was strongly temperature dependent. The minimum pH values that allowed survival after 4 weeks from an initial 10⁴ cells were 4·66 at 30†C, 4·36 at 10†C and 4·19 at 5†C. These limits were salt dependent, low (4–6%) salt concentrations improved and higher concentrations reduced survival at limiting pH values. The lowest pH that allowed a 100‐fold increase in cell numbers within 60 d was 4·66 at 30†C but this was increased to 4·83 at 10†C. At 5†C growth occurred at pH 7·0 but not at pH 5·13. Simple predictive models describing the effect of hydrogen‐ion and salt concentration on the time for at least a 100‐fold increase in numbers at 10†C and 30†C were constructed after analysis of the results for a least squares fit to a quadratic model. The interactions between salt and hydrogen‐ion concentration on growth were found to be purely additive.

The antimicrobial properties of aqueous solutions of peracetic acid and hydrogen peroxide have been compared. Peracetic acid exhibited excellent antimicrobial properties, especially under acidic conditions. Reductions by a factor of 10⁶ in the numbers of vegetative bacteria are obtained within 1 min at 25°C using a solution containing 1.3 mmol/l of peracetic acid. Rapid activity against bacterial spores and yeasts also occurs. Hydrogen peroxide is more effective as a sporicide than as a bactericide, with sporicidal action being obtained using a solution containing 0.88 mol/l. Bactericidal action is poor but hydrogen peroxide was bacteriostatic at concentrations above 0.15 mmol/l.

Colony characteristics, growth in litmus milk, precipitation in calcium glycerophosphate medium and utilization of carbon sources of the root‐nodule bacteria isolated from the tropical legumes Leucaena, Mimosa, Acacia, Sesbania and Lablab were similar to fast‐growing rhizobia of temperate legumes, particularly Rhizobium meliloti. In agglutination tests, isolates from each host shared antigens with one or more of five Rhizobium strains from Leucaena. Infective characteristics of the fast‐growing rhizobia were studied in modified Leonard jars and in agar culture. Cross‐infections by rhizobia between these plants were common and the association often effective. Lablab was effectively nodulated by its own fast‐growing isolate but only formed root swellings, possibly ineffective pseudonodules, with the other isolates. Slow‐growing rhizobia which were able to nodulate Macroptilium atropurpureus were unable to form nodules on these legumes except Lablab which was considered more akin to the cowpea group. All fast‐growing isolates nodulated, often effectively, Vigna unguiculata and V. unguiculata ssp. sesquipedalis. The isolate from Lablab also effectively nodulated a number of other tropical legumes which have previously only been reported to nodulate with slow‐growing nodule bacteria and it also produced ineffective nodulation on Medicago sativa. This is the first record of an effective fast‐growing isolate from Lablab.

Protease activities in human ileal effluent were approximately 20‐fold greater than in normal faeces. Comparative studies with faeces from a person who did not have a pancreas suggested that a substantial proportion of the proteolytic activity in normal faeces was of bacterial origin. Thimerosal, iodoacetate, EDTA and cysteine significantly inhibited proteolysis in faeces, but not in small intestinal contents, showing that cysteine and metalloproteases were produced by bacteria in the large gut. These results, together with results from studies using p‐nitroanilide substrates, demonstrated that faecal proteolysis was both qualitatively and quantitatively different from that in the small intestine. Studies with pure cultures of proteolytic gut bacteria indicated that the cell‐bound proteases of Bacteroides fragilis‐type organisms were likely to contribute significantly towards proteolytic activity associated with the washed cell fraction and washed particulate fraction of faeces. Extracellular proteases were formed by Streptococcus faecalis ST6, Propionibacterium acnes P6, Clostridium perfringens C16, Cl. bifermentans C21 and Cl. sporogenes C25. Inhibition results suggested that these bacteria, and similar organisms, may be partly responsible for the extracellular proteolytic activity found in the cell‐free supernatant fraction of faeces.