Browsing by Author "Ahonen, Susanna"

Bacteriocins are ribosomally synthesized antimicrobial proteins or peptides that inhibit growth of other bacteria. Bacteriocins can be divided into three classes based on their characteristics. Bacteriocins from the genus Lactococcus have only been found from the classes I and II. These two classes include small heat resistant peptides. Class III includes larger heat sensitive proteins. Bacteriocins have different ways to inhibit bacteria. They can either kill the bacteria or prevent them from multiplying. Due to their safe properties, bacteriocins can be used in food production to prevent the growth of harmful bacteria. In this Master’s thesis, antimicrobial compound produced by Lactococcus lactis LAC460 isolated from idli batter, was studied. Based on inhibition tests it was found that the LAC460 prevented the growth of some lactic acid bacteria by forming an inhibition zone on indicator plate. Uncommon to bacteriocins, the zone expanded during several weeks of incubation. This indicates that the antimicrobial compound can also kill inactive non-growing target cells. Based on the enzymatic tests the antimicrobial compound turned out to be a protein, probably a bacteriocin. The LAC460 bacteriocin lost its activity already at 53 °C, as well as both in low and high pH. This indicates that the compound was not a peptide but rather a bigger protein. The strain LAC460 produced more bacteriocin when the growth media was strongly buffered at least to pH 7. The bacteriocin was produced only at later growth phase. Based on the results from the ribosomal 16S rDNA sequencing, API-test and whole genome sequencing, LAC460 turned out to be a mixed culture of two bacteria, L. lactis and Enterococcus faecium. These bacteria were separated from each other with selective media and it was found out that Lactococcus was the actual producer of the bacteriocin. No obvious bacteriocin gene was found from the genome sequence, but the results suggest that the LAC460 bacteriocin belongs to class III bacteriolysins. As Lactococcus strains have not been found to produce class III bacteriocins, the antimicrobial protein characterized in this study seems to be a new bacteriocin.