Extra-cellular proteins may play a significant role in the antimicrobial or immunological response against food spoilage microorganisms and pathogens invading the honey crop, but also aid the uptake of nutrients by enzymatic breakdown. It is well known that LAB produce bacteriocins which are ribosomally synthesized

antimicrobial peptides [24] that are classified into 3 main classes: I (lantibiotic), II (heat-stable non-modified), and III (heat-labile) [5, 25]. The fraction of predicted secreted proteins classified as bacteriocins average around 2% in other published Lactobacillus genomes but can be GW-572016 mw as high as 22% in a strain of Oenococcus oeni[21]. One of the identified proteins produced by Lactobacillus Bma5N (Gene No. RLTA01902 in Additional file 1: Table S5, [GenBank: KC776075]) when stressed with LPS and LA, showed homology GSK126 nmr (Max ID of 51%) to a known bacteriocin named Helveticin J when compared with other species in NCBI BLAST (Additional file 1). Helveticin J is a Class III bacteriocin that is quite large

in size (> 30 kDa) [26] and was described as a heat-sensitive bacteriocin that could inhibit the growth of other Lactobacillus species [27]. However, the homologue we found contained no conserved signal peptides when searched through InterProScan, indicating a putative novel bacteriocin. Remarkably, Lactobacillus Bma5N was previously shown by us to be one of the most active LAB against the bee pathogen P. larvae[18]. These earlier observations might have been caused by this putative novel bacteriocin. Most bacteriocins are encoded on plasmids, yet Helveticin J is found chromosomally, and in the case of our helveticin homologue, on the secondary chromosome, not forming part of an operon. Instead the gene is singly located, surrounded by an S-layer protein and a protein with unknown function

(Figure  2). There were secreted proteins detected in 7 of the LAB spp. that had no known function (Table  2). Their genes were located in close proximity to peptide efflux ABC transporter ORFs in the genomes, indicating putative novel bacteriocins or antimicrobial proteins. Bacteriocins and ABC-transporter coding genes are BYL719 ic50 commonly seen in close proximity to each other in the same operon [28]. However, we need more research in order Tolmetin to understand their actual function. The majority of extracellular proteins produced by each honeybee-specific LAB under stress were enzymes (Table  2). However, the enzymes produced are not the same from each strain. An enzyme produced in Lactobacillus Fhon13N, Hon2N, and L. kunkeei Fhon2N, and Bifidobacterium Hma3N when under LPS stress for 1 and 3 days, was N-acetyl muramidase, a hydrolase that acts as a lysozyme (Additional file 1). These extra-cellularly produced lysozymes had conserved signal peptide sequences suggesting there importance as extracellular proteins.