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The spread of plasmid-mediated colistin resistance has posed a serious threat to public health owing to its effects on the emergence of pandrug-resistant bacteria. In this study, we investigated the prevalence and characteristics of mcr-1-positive Escherichia coli isolated from retail meat samples in Korea. In total, 1,205 E. coli strains were isolated from 3,234 retail meat samples in Korea. All E. coli strains were subjected to antimicrobial susceptibility testing and were examined for the presence of mcr-1 gene. All mcr-1-positive E. coli (n = 10, 0.8%) from retail meat were subjected to pulse-field gel electrophoresis (PFGE) and whole-genome sequencing (WGS). The transferability of mcr-1 gene was determined by conjugation assays. The mcr-1-positive strains exhibited diverse clonal types. Our mcr-1 genes were located in plasmids belonged to the IncI2 (n = 1) and IncX4 (n = 8) types, which were reported to be prevalent in Asia and worldwide, respectively. Most mcr-1 genes from mcr-1-positive strains (9/10) were transferable to the recipient strain and the transfer frequencies ranged from 2.4 × 10-3 to 9.8 × 10-6. Our data suggest that the specific types of plasmid may play an important role in spreading plasmid-mediated colistin resistance in Korea. Furthermore, our findings suggest that the retail meat may be an important tool for disseminating plasmid-mediated colistin resistance.

While the detrimental effect of bacteriophages on lactic acid bacterial fermentation is well documented, the importance of Bacillus subtilis phages in soybean-based fermented foods is not. In this study, we show for the first time that 100% of Korean soybean-based fermented foods (Doenjang, Gochujang, and Cheonggukjang) and 70% of raw materials (Meju and rice straw) were contaminated with B. subtilis-infecting phages (as high as 3.7 × 104 PFU g−1). Among 15 isolated B. subtilis-infecting phages, BSP18 was selected for further studies due to its specificity to and relatively broad host infectivity (34%) against B. subtilis. This Myoviridae family phage, BSP18 could infect all of the tested wild-type and commercially-used strains for soybean-based fermented food preparation. Furthermore, artificial contamination of as low as 102 PFU g−1 of BSP18 significantly inhibited B. subtilis growth during Cheonggukjang fermentation. Moreover, phage-treated samples contained considerably more degraded γ-PGA which could negatively affect the functional property of Cheonggukjang. We also present the data, strongly suggesting BSP18-encoded, not bacterial, γ-PGA hydrolase was responsible for γ-PGA degradation. In conclusion, B. subtilis phages are widespread in Korean soybean-based fermented foods and it should be of great concern as phages may hamper the bacterial growth during fermentation and yield poor quality products.

Previous studies on Bacillus subtilis-producing bacteriocins (or bacteriocin-like inhibitory substance, BLIS) against Clostridium perfringens provided with limited information to show their potentials. In this study, B. subtilis BSC35 was newly-isolated and its BLIS-containing cell-free supernatant exhibited a strong antimicrobial activity (maximum 24 mm of lysis zone in standard well diffusion assay) against all the tested C. perfringens strains (22 farm isolates and 2 references). The culture condition was optimized (30 °C, 240 rpm, TSB at pH 7.5) to maximize BLIS BSC35 production and stability of BLIS BSC35 was tested. After zymogram study following partial purification, BLIS BSC35 was measured approximately 4 kDa in size. Finally, when cell-free-supernatant or B. subtilis BSC35 culture was used to control C. perfringens in liquid medium and a food model (Cheonggukjang), as low as 0.1% and 104 CFU/g addition, respectively, was enough to strongly inhibit the growth of C. perfringens with a rapid bactericidal mode of action, implying high anti-C. perfringens potentials and wide range of applicabilities.

While the harmful effects of lactic acid bacterial bacteriophages in the dairy industry are well-established, the importance of Bacillus subtilis-infecting bacteriophages on soybean fermentation is poorly-studied. In this study, we isolated a B. subtilis-infecting bacteriophage BSP10 from Meju (a brick of dried fermented soybean) and further characterized it. This Myoviridae family bacteriophage exhibited a narrow host range against B. subtilis strains (17/52, 32.7%). The genome of bacteriophage BSP10 is 153,767 bp long with 236 open reading frames and 5 tRNAs. Comparative genomics (using dot plot, progressiveMauve alignment, heat-plot, and BLASTN) and phylogenetic analysis strongly suggest its incorporation as a new species in the Nit1virus genus. Furthermore, bacteriophage BSP10 was efficient in the growth inhibition of B. subtilis ATCC 15245 in liquid culture and in Cheonggukjang (a soybean fermented food) fermentation. Artificial contamination of as low as 102 PFU/g of bacteriophage BSP10 during Cheonggukjang fermentation significantly reduced bacterial numbers by up to 112 fold in comparison to the control (no bacteriophage). Moreover, for the first time, we experimentally proved that B. subtilis-infecting bacteriophage greatly enhanced poly-γ-glutamic acid degradation during soybean fermentation, which is likely to negatively affect the functionalities of Cheonggukjang.