No abstract

Bacteriocins are a large group of antimicrobial compounds that are synthesized by representatives of the genus Bacillus and lactic acid bacteria. They are used extensively in the food industry as biopreservatives. Incorporated in the composition of edible coatings, bacteriocins can reduce microbial growth and decay incidence in perishable fruits, thus improving product shelf-life and commercial appearance. The present study aims to investigate the effect of edible coatings of 0.5 % carboxymethyl cellulose (CMC) enriched with a purified bacteriocin from Bacillus methylotrophicus BM47 on the shelf-life extension of fresh strawberries. During storage at 4 °C and 75 % relative humidity for 16 days, the measurements of mass loss, decay percentage, total soluble solids (TSS), titratable acidity (TA), pH, organic acids, total phenolic and anthocyanin contents and antioxidant activity were made. The results demonstrate that the application of edible coatings with 0.5 % CMC and 0.5 % CMC with bacteriocin (CMC+B) led to a significant decrease of mass loss in the treated strawberries compared to the uncoated fruit. After the 8th day of storage, significant reductions in decay percentage along with the absence of fungal growth in CMC+B-coated fruit were observed in comparison with the CMC-coated and control strawberries. During the second half of the storage period, CMC and CMC+B treatments reduced TSS amount in the coated fruit compared to the control, but did not affect the increase of TA and decrease of pH values that are normally associated with postharvest changes. The CMC and CMC+B coatings did not prevent the decrease of ascorbic acid, and total phenolic and anthocyanin contents during cold storage. The application of CMC and CMC+B coatings had a significant inhibitory effect on decreasing the antioxidant activity throughout the storage period and maintained the antioxidant levels in both treatments close to the initial value of 76.8 mmol Trolox equivalents per 100 g of fresh mass.

SUMMARY Members of the bacterial genus Bacillus are known as producers of a broad spectrum of antibiotic compounds of proteinaceous nature that possess inhibitory activity against different saprophytic and pathogenic microorganisms. In the current research, a peptide synthesized by Bacillus methylotrophicus strain BM47, previously isolated from a natural thermal spring in Bulgaria, was identified and characterized as a bacteriocin. In vitro antimicrobial screening of the crude bacteriocin substance of B. methylotrophicus BM47 showed activity against the plant pathogenic fungi Fusarium moniliforme , Aspergillus awamori , Penicillium sp., Aspergillus niger and Gram-negative bacterium Pseudomonas aeruginosa . The antimicrobial activity of the crude bacteriocin substance was partially inhibited by the enzymes trypsin, Alcalase®, Savinase®, proteinase K, papain and Esperase®, while catalase was not effective. The crude bacteriocin substance was relatively pH resistant, but sensitive to the action of heat and most organic solvents and detergents tested. To obtain the active protein fractions, crude bacteriocin substance was purified by fast protein liquid chromatography (FPLC) using a strong anion exchange column. Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated that the purified bacteriocin had molecular mass of 19 578 Da. The amino acid analysis performed by high-performance liquid chromatography (HPLC) revealed that the isolated bacteriocin consisted of 17 types of amino acids, with the highest mol fraction expressed as percent of serine (29.3), valine (10.3), alanine (9.8) and tyrosine (7.1).

Bacteriocins are biologically active compounds of proteinaceous nature synthesized by a large number of microorganisms, including members of bacterial genus Bacillus and lactic acid bacteria (LAB). The broad antimicrobial spectrum of bacteriocins against various spoilage and pathogenic microorganisms stimulated the research efforts for their investigation and potential application in different branches of food industry as natural preservatives. The promising antimicrobial activity of bacteriocins makes them suitable for application as biopreservatives and alternatives of chemical preservatives in dairy industry for production of fermented or non-fermented milk products. Therefore, the aim of the present study was to evaluate the antifungal effect of a bacteriocin isolated from Bacillus methylotrophicus strain BM47 and the possibilities for its application as a potential biopreservative of traditional Bulgarian yogurt. The results demonstrated that the addition of a purified bacteriocin in dose of 1 AU/mL of milk led to significant reduction of the fungal spores and mycelial growth of the indicator microorganism Penicillium sp. in the yogurt, without a change of its organoleptic properties, and biochemical and microbiological parameters during the 4-week period of storage.

After eight years of cultivation of Sea daffodil (Pancratium maritimum L.) shoot cultures at in vitro conditions, bacterial contamination on the plant tissues and medium surface appeared. The contamination was due to endophytic bacterium later isolated as a bacterial strain PM_YT. The colonial characteristics and cell morphology of the isolate were determined after cultivation on YEB-agar medium and coloring of microscopic preparations. The strain identification was implemented by biochemical methods (oxidase and catalase standard strip tests) and nucleotide sequencing of the 16S rRNA gene. The results from the morphological and cultural identification showed that the isolate was Gram-positive bacterium presented from small, short rods, forming small (size of 1-3 mm), round-shaped, yellow- to slightly orange-colored colonies. The results from biochemical identification showed that the strain was oxidase-negative and catalase-positive. The comparative 16S rRNA gene sequence-based phylogenetic analysis revealed 99% pairwise similarity of the strain PM_YT to the bacterial species Curtobacterium flaccumfaciens. Antibiotic susceptibility was implemented by Bauer-Kirby disc diffusion method and the results demonstrated that the isolate was most sensitive to the antibiotics belonging to inhibitors of protein synthesis group, especially to tetracycline, doxycycline, amikacin, rifampin, lincomycin, chloramphenicol, erythromycin, streptomycin and ciprofloxacin.