Lactococcus lactis RBT18: From the Rainbow Trout Farm to the Lab, the Tale of a Nisin Z Producer

Abstract

Infectious diseases, such as lactococcosis caused by Lactococcus garvieae, are portrayed as critical limiting factors in aquaculture. The antimicrobial properties of lactic acid bacteria (LAB), mainly the production of organic acids and bacteriocins (e.g., the lanthionine containing nisins A and Z; NisA and NisZ, respectively), led to propose LAB as probiotics to be used as an alternative and/or complementary strategy to vaccination and chemotherapy in aquaculture. L. lactis RBT18, isolated from cultured rainbow trout (Oncorhynchus mykiss, Walbaum), exerts strong, direct, and extracellular antimicrobial activity against L. garvieae and other ichthyopathogens, with the latter being heat-resistant (100 °C, 10 min), and thus suggesting the involvement of a thermostable antimicrobial compound (i.e., bacteriocin). Cross-immunity tests using the agar well diffusion test (ADT) and PCR assays suggested that NisA/Z is the bacteriocin responsible for the extracellular antimicrobial activity exerted by L. lactis RBT18. To demonstrate this hypothesis, the bacteriocin was purified to homogeneity by two multi-chromatographic procedures. MALDI TOF-MS analyses of purified samples after the last reverse-phase chromatography step identified the presence of NisZ (3330 Da), and its oxidized form (3346 Da), derived from the oxidation of a lanthionine ring. The oxidized NisZ showed a diminished antimicrobial activity that would increase the chances of bacterial pathogens to evade its antimicrobial activity. Further experiments are necessary to assess the in vitro and in vivo safety and efficiency of L. lactis RBT18 as a probiotic in aquaculture, but also to optimize the environmental conditions to reduce bacteriocin oxidation and thus bacterial pathogen resistance.

Probiotic, Paraprobiotic, and Postbiotic as an Alternative to Antibiotic Therapy for Lactococcosis in Aquaculture

“…In a recent work by Contente et al (2020), extracellular products of L. lactis RBT18 obtained from cultured rainbow trout gut exhibited antimicrobial activity against L. garvieae, suggesting the involvement of a thermostable antimicrobial compound (i.e. bacteriocin responsible for the extracellular antimicrobial activity exerted by L. lactis).…”

“…While Lb. rhamnosus isolated from the intestine of diseased fish exhibited no antagonistic activity against L. garvieae (Akayl et al, 2020), L. lactis RBT18, obtained from cultured rainbow trout exhibited antagonistic activity against L. garvieae (Contente et al, 2020). However, the disease resistance data for these LAB warranted future work to show a correlation between in vitro and in vivo results.…”

Isolation and Characterization of Lactobacillus crispatus, Lactococcus lactis, and Carnobacterium divergens as Potential Probiotic Bacteria from Fermented Black and Green Olives (Olea europaea): An Exploratory Study

“…Antimicrobial Activity. The antimicrobial activity of all isolates against indicator bacteria was determined by the agar-well diffusion method [48]. Pseudomonas geniculate, Microbacterium oxydans, Bacillus subtilis, Streptomyces laurentii, Klebiella pneumonia, Bacillus pumilus, Bacillus cereus, Acaligens facelis, Enterococcus faecium, and Enterococcus facelis were used as indicator bacteria (obtained from the Microbiology & Biotechnology Lab, FJWU) against which the antimicrobial activity of isolated strains was assayed.…”