Metal and antibiotic resistance in psychrotrophic bacteria associated with the Antarctic sponge Hemigellius pilosus (Kirkpatrick, 1907)

High concentrations of heavy metals have been previously detected in Antarctic sponge tissues, but their effect on the associated bacterial assemblages has been never investigated. Metal tolerance is often linked to antibiotic resistance and can also affect biochemical activities within microbial populations. In the present work, the response to heavy metals and antibiotics, as well as the enzymatic profile, of bacteria associated with the sponge Hemigellius pilosus, was analyzed. Tolerance to mercury, cadmium and zinc (at concentrations between 10 and 10,000 ppm) was tested by the plate diffusion method. Almost all isolates completely tolerated zinc and cadmium up to 1,000 and 2,500 ppm, respectively, whereas complete tolerance to mercury was generally observed at concentrations between 10 and 500 ppm. As bacteria can develop resistance in the growing presence of toxic compounds in the environment, this finding could be related to the concentrations of metals in the sponge tissues. The susceptibility assay to 11 antibiotics revealed that multiple antibiotic resistance was generally exhibited, with gentamicin that inhibited all Antarctic isolates. The comparison of the heavy metal and antibiotic resistance patterns at phylogenetic level revealed some distinctive features, suggesting that the dissemination of heavy metal tolerance and antibiotic resistance may possess great relevance for the population dynamics. Additionally, growth patterns often highly differed among strains in the same species, thus appearing to be more likely strain specific rather than species specific. The enzyme expression by the isolates was not really affected by the heavy metal tolerance they showed, as variation in the enzymatic profiles was observed in strains within the same genus that showed different/similar heavy metal tolerance patterns.