Aeromonas species are ubiquitous inhabitants of freshwater environments, and are responsible for fish motile aeromonad septicemia (MAS). A. hydrophila is implicated as the primary etiologic agent of MAS. Here, we analysed MAS epidemiological data for cyprinid fish in southern China, and found that A. veronii infections dominated. Consistent with this observation, A. veronii isolates were generally more virulent than A. hydrophila isolates when infecting germ-free zebrafish larvae via continuous immersion challenge. Through in vivo screening of the transposon library of the A. veronii strain Hm091, aerolysin was identified as the key virulence factor. Further results indicated that A. veronii Hm091 aerolysin disrupts the intestinal barrier of zebrafish, enabling systematic invasion by not only A. veronii Hm091 in a mono-infection, but also A. hydrophila NJ-1 in a mixed infection. Moreover, the differences in aerolysin expression and activity were the major contributor to the observed differences between the A. veronii and A. hydrophila strains regarding invasion efficacy via intestine. Together, our results provide new insights into the aetiology and pathogenesis of Aeromonas infections, and highlight the importance of A. veronii-targeted treatments in future efforts against MAS.

AbstracrIn this study, we tested the distribution of 49 Lactobacillus strains in the mucus and mucosa of the intestine tissue of zebrafish. We observed a progressive change in the spatial distribution of Lactobacillus strains, and suggested a division of the strains into three classes: mucus type (>70% in mucus), mucosa type (>70% in mucosa) and hybrid type (others). The hybrid type strains were more efficient in protection of zebrafish against Aeromonas hydrophila infection. Three strains representing different distribution types (JCM1149, CGMCC1.2028, and JCM 20300) were selected. The mucosa type strain JCM1149 induced higher intestinal expression of inflammatory cytokines and Hsp70 than the other strains. Furthermore, we used L. rhamnosus GG and its mutant (PB22) lacking SpaCBA pili to investigate the influence of pili on spatial distribution. LGG showed a mucosa type distribution, while PB22 revealed a hybrid distribution and the disease protection was accordingly improved. The different protection ability between LGG and PB22 did not involve the intestinal microbiota, however, LGG induced injury to the mucosa of zebrafish. Collectively, the disease protection activity of Lactobacillus in zebrafish is correlated with their spatial distribution in the intestinal tissue, with strains showing a balanced distribution (hybrid type) more efficient in protection.

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