Inhibition of enterobacteria and Listeria growth by lactic, acetic and formic acids.

Minimum inhibitory concentrations (MIC) of undissociated lactic, acetic and formic acids were evaluated for 23 strains of enterobacteria and two of Listeria monocytogenes. The evaluation was performed aerobically and anaerobically in a liquid test system at pH intervals of between 4.2 and 5.4. Growth of the enterobacteria was inhibited at 2-11 mmol l-1, 0.5-14 mmol l-1 and 0.1-1.5 mmol l-1 of undissociated lactic, acetic and formic acids, respectively. The MIC value was slightly lower with anaerobic conditions compared with aerobic conditions. The influence of protons on the inhibition was observed for acetic acid at the low pH values. Undissociated lactic acid was 2 to 5 times more efficient in inhibiting L. monocytogenes than enterobacteria. Acetic acid had a similar inhibitory action on L. monocytogenes compared with enterobacteria. Inorganic acid (HCl) inhibited most enterobacteria at pH 4.0; some strains, however, were able to initiate growth to pH 3.8. The results indicate that the values of undissociated acid which occur in a silage of pH 4.1-4.5 are about 10-100 times higher than required in order to protect the forage from the growth of enterobacteria and L. monocytogenes.

Binding of Lactobacillus reuteri to fibronectin immobilized on glass beads.

Human fibronectin was immobilized on glass beads. The beads were used to evaluate binding of Lactobacillus reuteri to fibronectin. Organisms bound to the glass beads were detected using fluorescence microscopy after treatment with acridine orange. This binding was confirmed and quantified with the use of [3H]-labelled organisms. Three strains of Lactobacillus reuteri, three strains of Lactobacillus acidophilus and one strain of Lactobacillus fermentum were tested for binding capacity. L. reuteri strain 1063 exhibited a strong binding to the immobilized fibronectin, and L. acidophilus 1754 showed a slight binding. The binding of L. reuteri to the fibronectin was mediated by a protein as judged by the absence of binding after treatment of the bacteria with proteolytic enzymes. Treatment of the bacteria with urea, SDS and heat (80 degrees C) also reduced binding. Treatment of the bacterial cells prior to the assay with fibronectin interfered with binding. Albumin did not show this interaction.

Antagonistic activities of lactic acid bacteria in food and feed fermentations.

Many factors contribute to a successful natural fermentation of carbohydrate-rich food and feed products. Metabolic activities of lactic acid bacteria (LAB) play a leading role. Their ability to rapidly produce copious amounts of acidic end products with a concomitant pH reduction is the major factor in these fermentations. Although their specific effects are difficult to quantitate, other LAB metabolic products such as hydrogen peroxide and diacetyl can also contribute to the overall antibiosis and preservative potential of these products. The contribution of bacteriocins is also difficult to evaluate. It is suggested that they may play a role in selecting the microflora which initiates the fermentation. Bacteriocins are believed to be important in the ability of LAB to compete in non-fermentative ecosystems such as the gastrointestinal tract. During the past few decades interest has arisen in the use of the varied antagonistic activities of LAB to extend the shelf-life of protein-rich products such as meats and fish. Recent findings indicate that the newly discovered Lactobacillus reuteri reuterin system may be used for this purpose.