Identification and characterization of gut-associated lactic acid bacteria isolated from the bean bug, Riptortus pedestris (Hemiptera: Alydidae).

Lactic acid bacteria (LAB) are beneficial bacteria for humans and animals. However, the characteristics and functions of LAB in insects remain unclear. Here, we isolated LAB from the gut of Riptortus pedestris, a pest that is a significant problem in soybean cultivation in Korea, and identified two Lactococcus lactis and one Enterococcus faecalis using matrix-associated laser desorption/ionization-time of flight and 16S rRNA analyses. All three LAB strains survived at pH 8, and L. lactis B103 and E. faecalis B105 survived at pH 9 for 24 h. In addition, these strains survived well in simulated gastric juice of humans containing pepsin and exhibited high resistance to bile salts. Two strains of L. lactis and one of E. faecalis maintained constant density (> 104 colony-forming units [CFU]/mL) at pH 2.5, but viability at pH 2.2 was strain-dependent. The three LAB were reinoculated into second-instar nymphs of R. pedestris and colonized well, reaching a constant density (> 105 CFU/gut) in the adult insect gut. Interestingly, feeding of these LAB increased the survival rate of insects compared to the negative control, with the largest increase seen for L. lactis B103. However, the LAB did not increase the weight or length of adult insects. These results indicate that insect-derived LAB possess the traits required for survival under gastrointestinal conditions and have beneficial effects on insect hosts. The LAB infection frequency of the wild bean bug populations was 89% (n = 18) in Gyeongsangnam-do, South Korea. These LAB can be utilized as a novel probiotic in the cultivation of beneficial insects. This study provides fundamental information about the symbiosis between insects and LAB, and a novel concept for pest control.

Inhibition of Salmonella enterica growth by competitive exclusion during early alfalfa sprout development using a seed-dwelling Erwinia persicina strain EUS78.

Salmonella enterica outbreaks in sprouts originate from contaminated seeds; conventional prevention technologies have been reported from many research institutes. In this study, we applied a biological control approach to inhibit S. enterica growth using the seed-dwelling non-antagonistic bacteria. We isolated non-antibacterial seed-dwelling bacteria from vegetable sprouts. A total of 206 bacteria exhibiting non-antibacterial activity against S. enterica were subjected to alfalfa sprout development tests. Eight isolates exhibiting no deleterious effect on the growth of alfalfa sprouts were tested for S. enterica growth inhibition on alfalfa seeds and sprouts, and an isolate EUS78 was finally selected for further investigation. Based on 16S rRNA, gyrB, and rpoB gene sequence analyses, strain EUS78 was identified as Erwinia persicina. In population competition, the S. enterica population increased by >3 log CFU/g after 6 days of alfalfa sprout growth, whereas S. enterica growth was significantly inhibited by treatment with EUS78 (P < .05). This effect of S. enterica growth inhibition by EUS78 was sustained until the end of the alfalfa sprout harvest. Overall, bacterial strain EUS78 significantly reduced S. enterica growth on alfalfa sprouts in a manner consistent with competitive exclusion. These findings led us to monitor EUS78 behavior on seeds during early sprout development using fluorescence and scanning electron microscopy. Strain EUS78 initially colonized alfalfa sprout seed coat edges, cotyledons, and finally root surfaces during early sprout germination. As alfalfa sprouts grew, EUS78 bacterial cells established colonies on newly emerged plant tissues such as root tips. The results of this study suggest that strain EUS78 has potential as a biological control agent to inhibit S. enterica contamination in the sprout food industry.