- Volume 145, Issue 9, 1999

The strongly hydrophobic bacteriocin amylovorin L471 from Lactobacillus amylovorus DCE 471 was isolated and purified to homogeneity from complex culture broth by a novel, rapid and simple three-step protocol including (i) ammonium sulphate precipitation, (ii) chloroform/methanol extraction/precipitation and (iii) reversed-phase HPLC, the only chromatographic step involved. The molecular mass of the peptide was determined to be 4876·9 Da by electrospray mass spectrometric analysis. N-terminal amino acid sequencing identified 35 amino acid residues as being identical to the N-terminal sequence of lactobin A, a bacteriocin from another L. amylovorus strain. These non-identical strains produce bacteriocins that display small differences in molecular mass and inhibitory spectrum. The amino acid sequence of amylovorin L471 shared significant homology with lactacin X, one of the two bactericidal peptides produced by Lactobacillus johnsonii VPI11088. A purified amylovorin L471 preparation permitted confirmation of the inhibitory spectrum previously established with a crude extract. It displayed a bactericidal mode of action on lactobacilli after an extremely rapid adsorption to the target cells. Two Listeria spp. were only weakly sensitive. Amylovorin L471 appears to be produced constitutively. Ethanol not only stimulated specific bacteriocin production but also prevented adsorption of the bacteriocin molecules to the producer cells upon prolonged fermentation. The latter result supports the hypothesis that the apparent inactivation of bacteriocin observed during the stationary phase of batch fermentations is due to adsorption.

The external pH appeared to be the main factor governing oxalic acid production by Aspergillus niger. A glucose-oxidase-negative mutant produced substantial amounts of oxalic acid as long as the pH of the culture was 3 or higher. When pH was decreased below 2, no oxalic acid was formed. The activity of oxaloacetate acetylhydrolase (OAH), the enzyme believed to be responsible for oxalate formation in A. niger, correlated with oxalate production. OAH was purified from A. niger and characterized. OAH cleaves oxaloacetate to oxalate and acetate, but A. niger never accumulated any acetate in the culture broth. Since an A. niger acuA mutant, which lacks acetyl-CoA synthase, did produce some acetate, wild-type A. niger is apparently able to catabolize acetate sufficiently fast to prevent its production. An A. niger mutant, prtF28, previously isolated in a screen for strains deficient in extracellular protease expression, was shown here to be oxalate non-producing. The prtF28 mutant lacked OAH, implying that OAH is the only enzyme involved in oxalate production in A. niger. In a traditional citric acid fermentation low pH and absence of Mn2+ are prerequisites. Remarkably, a strain lacking both glucose oxidase (goxC) and OAH (prtF) produced citric acid from sugar substrates in a regular synthetic medium at pH 5 and under these conditions production was completely insensitive to Mn2+.

A comparison of 42 yeast species with respect to growth in the presence of high NaCl concentration and characteristics of glycerol uptake is presented. The yeast species were classified into four classes on the basis of their ability to grow in the presence of 1, 2, 3 or 4 M NaCl. Considering that two different types of active-transport systems for glycerol uptake have been described, Na+/glycerol and H+/glycerol symports, glycerol transport was investigated by testing for proton uptake upon glycerol addition in cells incubated in the absence and in the presence of NaCl. Only strains belonging to the two higher classes of salt tolerance showed constitutive active glycerol uptake, and could accumulate glycerol internally against a concentration gradient. Five of these strains exhibited a H+/glycerol symport. All the other strains showed evidence of the activity of a salt-dependent glycerol uptake similar to that described in the literature for Debraryomyces hansenii. The strains within the two lower classes of salt tolerance showed, to varying degrees, glycerol active uptake only when glycerol was used as the carbon and energy source, suggesting that this uptake system is involved in glycerol catabolism. The results within this work suggest that active glycerol uptake provides a basis for high halotolerance, helping to maintain a favourable intracellular concentration of glycerol. The relation between the constitutive expression of such carriers and a higher level of salt-stress resistance suggests that this may be an evolutionary advantage for growth under such conditions.