Isolation of Shewanella algae from blue crabs (Callinectes sapidus) in Northwestern Adriatic Sea

Abstract Blue crab (Callinectes sapidus) is one of the last reported invasive species that have colonized the Italian coasts, in particular the lagoons of the northern Adriatic Sea. Their massive presence is compromising shellfish farming, causing a serious economic crisis among the valley populations. To limit the economic damage, blue crabs have been introduced as human food in the Italian markets. This study aimed to evaluate the microbiological contamination of blue crab as potential risk to human health. From July 2023 to April 2024, 72 blue crabs were collected in coastal lagoons and 5-6 nm from the coast of the Emilia Romagna Region. Microbiological analysis revealed Shewanella algae as one of the most commonly identified microorganisms (18/72, 25%). Shewanella was isolated on Chromagar Vibrio™ and TCBS after an enrichment step in Alkaline Saline Peptone Water. Colony morphology, Gram stain, and biochemical tests (oxidase, catalase) sustained Shewanella identification, later confirmed by MALDI-TOF MS (MALDI Biotyper® Sirius, Bruker Daltonics). S. algae is a Gram-negative, rod-shaped, aerobic microorganism widely distributed in seawater, freshwater, and soil. In humans, it is considered an opportunistic pathogen, especially in people with impaired immune systems. Shewanellosis has been strongly associated with seawater exposure and typically arises in hot months. Clinical manifestations include skin and soft tissue infections, otitis media, keratitis, biliary tract infections, vertebral discitis, and bloodstream infections. Notably, in the last years, S. algae infections have increased globally, even in healthy young people. The present study suggests blue crabs as a potential source of S. algae throughout the year. The increasing identification of S. algae in marine waters worldwide, as well as in marine-derived foods like blue crabs, underscores the growing importance of S. algae as an emerging pathogen and its impact on public health. Key messages • Shewanella algae is an opportunistic pathogen, increasingly isolated in marine water and seafood. • Blue crab is an invasive species that has recently conolonised the Italian coasts.