Screening for antimicrobial agents against the fish pathogens, Streptococcus agalactiae and Yersinia ruckeri, in fermented fruit and vegetables

Antibiotics have been used for a long period of time both for human and animal health, as well as economical benefits. Because of an emerging antibiotic resistance in pathogen microbes, there is an increasing need for new antimicrobials to control outbreaks of disease caused by pathogenic bacteria. One of these antimicrobials could be the group of antimicrobial peptides (AMP) called bacteriocins. These are peptides produced by bacteria in order to protect its niche from other competing bacteria. The aim of this study was to search for bacteria able to produce bacteriocins inhibiting the two fish pathogens Streptococcus agalactiae and Yersinia ruckeri in samples made from fermented fruit and vegetables. This was done by screening, using a method based on separating the fish pathogen and the samples in different layers on an agar plate. Colonies from the samples that inhibited the growth of either of the fish pathogens were isolated and their bacteriocins were characterized by: spot-on-lawn inhibition assays, Sanger sequencing, REP-PCR, antimicrobial micro titer assays and mass spectrometry. 11 colonies were found producing bacteriocins against S. agalactiae. None bacteriocin producing bacteria were found against Y. ruckeri. All the 11 strains found when screening for S. agalactiae were shown to be L. lactis producing nisin Z. Purification of nisin Z produced by L .lactis in BHI and MRS resulted in more bacteriocins retrieved from a culture grown in BHI than in MRS. As a preliminary attempt for using L. lactis as a probiotic in aquaculture, we saw that our L. lactis was able to inhibit the fish pathogen when inoculated together in bottles of water