Isolation of an Autochthonous Bacillus pumilus with Broad-Spectrum In Vitro Antagonistic Activity Against Bacteria Associated with Aulacomya atra Larval Mortality

Intensive larviculture of the South American ribbed mussel (Aulacomya atra) is constrained by recurrent mortality during pre-metamorphic development. This study characterized cultivable bacteria associated with an acute 79% mortality event on day 12 post-fertilization and screened autochthonous bacterial isolates for in vitro antagonistic activity. Two independently collected and homogenized samples per larval condition were examined; serial dilutions and plate observations were treated as technical observations and summarized within the sample. Cultivable heterotrophic abundance on Plate Count Agar was 5.54 ± 4.75 × 107 CFU g−1 in surviving larvae and 1.24 ± 0.05 × 108 CFU g−1 in moribund larvae. Because only two independent samples were available per condition, these counts were interpreted descriptively. Presumptive Vibrio spp. were not detected in either surviving-larvae sample but were detected in one of two moribund-larvae samples. Sequencing of the 16S rRNA gene assigned representative isolates recovered from moribund larvae to Staphylococcus saprophyticus (P2 and P4), Staphylococcus warneri (P20), and Vibrio sp. (P22). In a blocked analysis of 12 target-by-plate combinations, inhibition differed among the four candidate isolates and the negative control (Friedman χ2 = 16.275, df = 4, p = 0.0027; Kendall’s W = 0.339). After Holm adjustment of four planned paired comparisons with the control, only Bacillus pumilus B3 differed significantly from the control (adjusted p = 0.025). Its mean inhibition varied by target, from 3.58 ± 0.55 mm against P2 to 0.58 ± 1.01 mm against P22. These findings identify B3 as a promising candidate with target-dependent in vitro antagonistic activity. Controlled in vivo challenge and safety studies are required before probiotic efficacy or practical biological-control value can be established.