G.A. Dykes

Bacteriocins: ecological and evolutionary significance

Bacteriocins are compounds that are produced by bacteria and are antagonistic to other bacteria. Although they have been known for many years, recent interest in these compounds has increased because of their potential use as natural food preservatives. Although most of this research has been directed at the molecular level, a clearer picture of the ecological role played by bacteriocins in natural environments is beginning to emerge. In addition, the importance and practical implications of evolutionary aspects of bacteriocins and bacteriocin resistance are now being assessed.

Sequence and structural relationships of leucocins A-, B- and C-TA33a from Leuconostoc mesenteroides TA33a

Amino acid sequences of two of the three bacteriocins from Leuconostoc mesenteroides TA33a were determined and their sequence-structure relationships investigated. Leucocin B-TA33a consists of 31 amino acid residues, with a molecular mass of 3466 Da. Leucocin B-TA33a does not belong to the pediocin family of bacteriocins, but shares 62% homology with mesenterocin 52B. A partial sequence of 36 amino acids of leucocin C-TA33a (4598 Da) was determined. Leucocin C-TA33a belongs to the class II bacteriocins having the consensus YGNGV motif. The third bacteriocin, leucocin A-TA33a, is identical to leucocin A-UAL 187. Circular dichroism spectra of the leucocins in aqueous solution and micellar SDS indicated that they undergo a structural transition when in a membrane-mimicking environment. Theoretical predictions from circular dichroism data suggest that leucocins A-, B- and C-TA33a adopt a beta-structure (48%) in membrane-mimicking environments. Sequence alignments and secondary structure predictions for the N-terminus of leucocins A- and C-TA33a predicted that Cys-9 and Cys-14 are connected by a disulfide bridge and form two beta-strands.

Biogenic amine formation by poultry‐associated spoilage and pathogenic bacteria

The production of biogenic amines by 50 poultry‐associated bacterial strains (25 Pseudomonas, 13 Salmonella and 12 Listeria) was investigated on amine agar plates containing lysine, histidine, ornithine, phenylalanine, tryptophan and tyrosine. Seventy‐four per cent of all the strains produced cadaverine and putrescine, while phenylethylamine, histamine, tyramine and tryptamine were produced by 72, 56, 34 and 24% of strains, respectively. Different patterns of biogenic amine production amongst the three bacterial genera tested were apparent as well as amongst strains of the same genus. This study highlighted a high incidence of biogenic amine‐producing bacterial strains associated with poultry.

Quantification of microbial populations associated with the manufacture of vacuum-packaged, smoked vienna sausages.

Sources of contamination of vacuum-packaged vienna sausages by spoilage microorganisms were examined in a meat-processing plant. Microbial numbers present in the environment, on working surfaces and workers hands and aprons were quantified by plate counting on selective media. Product line samples were taken at critical control points in the manufacturing process and analysed before and after preliminary incubation of vacuum-packaged samples at 25 degrees C for 24 h. In all samples the numbers of aerobic bacteria, Enterobacteriaceae, lactic acid bacteria and yeasts were determined by standard procedures. Contamination of sausage surfaces by lactic acid bacteria occurred as a result of the manufacturing and handling processes. The environment and specifically packers hands, as well as working surfaces contributed to microbiological contamination of various types after removal of the peel from individual sausages. The preliminary incubation procedure allowed detection of low numbers of spoilage microorganisms.

Taxonomy of lactic acid bacteria associated with vacuum-packaged processed meat spoilage by multivariate analysis of cellular fatty acids

The taxonomy of lactic acid bacteria from vacuum-packaged processed meats is problematic, and atypical members of the leuconostocs and the Lactobacillus sake / curvatus group are often encountered. In order to resolve this problem the cellular fatty acid (CFA) content of 61 isolates from vacuum-packaged vienna sausages and 18 reference strains was determined by gas chromatography. The relationship between strains was derived by principal component analysis of data. The CFA profiles were highly reproducible. Although no relationships could be derived using only one or two differentiating CFAs, plots of the first two principal components based on only the six most variable CFAs allowed grouping of strains. The two genera (Leuconostoc and Lactobacillus) could not be clearly separated when analysed together, but differentiation of species within each of the genera was achieved when they were analysed independently. Examination of plots for the reference strains confirmed previously established relationships between these strains. From the plot of the Lactobacillus sake / Lactobacillus curvatus component of the study it was found that most atypical Lactobacillus sake / curvatus strains were closely related to the typical Lactobacillus sake isolates and reference strain, while the Lactobacillus curvatus strains formed an independent grouping. A small cluster of atypical strains, however, indicated that this relationship may not be true for all these strains. Among the leuconostocs only isolates of Leuconostoc mesenteroides could be clearly differentiated.

Strain typing in the genus Lactobacillus.

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Yeasts associated with Vienna sausage packaging

A total of 123 representative yeast isolates from a previous study of a Vienna sausage processing plant were identified according to conventional methods and long-chain fatty acid analyses. The most prevalent isolates belonged to the genera Candida and Debaryomyces. Other genera encountered were Rhodotorula, Yarrowia, Pichia, Galactomyces, Cryptococcus, Trichosporon and Torulaspora.

Taxonomic status of atypicalLactobacillus saké andLactobacillus curvatus strains associated with vacuum-packaged meat spoilage

The taxonomic status of nine typical and atypicalLactobacillus saké andLactobacillus curvatus strains associated with vacuum-packaged meat spoilage was investigated by DNA-DNA homology and compared with four alternative identification methods. Phenotype-based identification methods as well as 23S rRNA targeted probes produced ambiguous results in the case of atypical strains. DNA-DNA hybridization indicated homologies of 63–78% between typical strains and the corresponding type strain, whereas values ranged from 38% to 54% for atypical strains. This apparent spectrum of relatedness observed was ascribed to the recent phylogenetic divergence of the two species. Atypical strains should, therefore, be designated as such and not assigned to particular species based on results of identification methods other than DNA-DNA hybridization.

Advantages of sucrose‐dependent extracellular polysaccharide production to lactic acid bacteria in sucrose‐rich environments

G.A. DYKES, I. GEORNARAS AND A. VON HOLY. 1995. Some possible advantages of sucrosedependent extracellular polysaccharide production to Lactobacillus L191 from bakers yeast were investigated. A ca log 1 plaque‐forming units ml‐1decrease in attachment of bacteriophage AB1 to cells of Lactobacillus L191 grown in the presence of sucrose as compared to cells grown in the absence of sucrose was noted. On the other hand, a ca log 1 colony‐forming units cm‐2increase in attachment of Lactobacillus L191 to stainless steel surfaces when grown in the presence of sucrose compared to the absence of sucrose was observed. It was concluded that sucrose‐dependent extracellular polysaccharide may provide a series of individually small but jointly synergistic selective advantages to strains producing it.