Presence of pathogenic bacteria, bacteria resistant to antibiotics and microplastics in sea cucumbers under different levels of anthropogenization at Gran Canaria Island (Spain)

Abstract

The aim of this study was to assess the presence of pathogenic bacteria and bacteria resistant to antibiotics from the intestinal tract of the sea cucumber Holothuria sanctori (Delle Chiaje, 1823), a conspicuous marine invertebrate in coastal waters of the Canary Islands. A total of 79 specimens were collected from three sites at the island of Gran Canaria, Canary Islands, Spain. One site is directly influenced by continuous discharges of organic sewage through two coastal underwater sewage outlets, while the other two sites where at control sites with no direct sources of organic pollution. Individuals were dissected longitudinally through the ventral area and the intestinal tract removed. With the help of a swab, a small faecal sample was removed from the luminal epithelium of the anterior intestine. The swabs with faecal material were then seeded in different media (McConkey Agar (MC), McConkey Agar with Cefotaxime (MC+CTX), Mannitol Salt Agar (MSA) and Selenite Broth). Gram stain, catalase and coagulase tests were done to suspected Staphylococcus growing in MSA. Oxidase tests, Kligler Iron Agar (KIA) and biochemical identification were used for identification of Gram-negative bacteria. Susceptibility to several antimicrobial agents was determined. In addition, we compared the amount (number and weight) of microplastics in the intestinal contents of sea cucumbers in different sites. No presence of Staphylococcus aureus and Salmonella spp. was registered, but there is presence of others pathogenic bacteria (Enterobacteria and NFB) at all sites. The low concentrations of bacteria could be explained because sea cucumbers could have the capacity to phagocytize bacteria. In terms of resistance, almost all bacteria were resistant to ampicillin (>50%), while all of them were susceptible to imipenem and gentamicin (100%). Also, there was a greater resistance to antibiotics in polluted waters than in clean waters for Enterobacteria but not for NFB; still, our sample size was too low to support robust conclusions. A very small amount of microplastics were found on control sites, whereas sea cucumbers from the polluted site contained large amounts of these. As a result, Holothuria sanctori seems an ideal candidate for monitoring pollution impacts on nearshore waters of the Canary Islands.