| Summary: | Withering syndrome (WS), an infectious disease caused by intracellular bacteria <i>Candidatus</i> Xenohaliotis californiensis, has provoked significant economic losses in abalone aquaculture. The pathogen infects gastroenteric epithelia, including digestive gland, disrupting the digestive system and causing a progressive wilting in abalone. Nonetheless, our knowledge about WS implications in digestive gland microbiota, and its role in diseases progress remains largely unknown. This study aims to determine whether digestive gland-associated microbiota differs between healthy red abalone (<i>Haliotis rufescens</i>) and red abalone affected with WS. Using high-throughput sequencing of the V4 region of the 16S rRNA gene, our results revealed differences in microbiota between groups. Bacterial genera, including <i>Mycoplasma</i>, <i>Lactobacillus</i>, <i>Cocleimonas</i> and <i>Tateyamaria</i> were significantly more abundant in healthy abalones, whilst <i>Candidatus</i> Xenohaliotis californiensis and <i>Marinomonas</i> were more abundant in WS-affected abalones. Whilst <i>Mycoplasma</i> was the dominant genus in the healthy group, <i>Candidatus</i> Xenohaliotis californiensis was dominant in the WS group. However, <i>Candidatus</i> Xenohaliotis californiensis was present in two healthy specimens, and thus the <i>Mycoplasma</i>/<i>Candidatus</i> Xenohaliotis californiensis ratio appears to be more determinant in specimens affected with WS. Further research to elucidate the role of digestive gland microbiota ecology in WS pathogenesis is required.
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