Antibiotic susceptibility and heavy metal tolerance in bacterial isolates from uncontaminated Antarctic brines

A total of 151 bacterial strains were isolated from two pockets of liquid hypersaline brines (namely TF4-72 isolates and TF5-79 isolates) that were collected from the perennially frozen lake Tarn Flat (Antarctica)1. Based on results from the 16S rRNA gene sequencing, bacterial isolates were distributed across the phyla Proteobacteria, Actinobacteria and Firmicutes (62.9, 19.2 and 10.6%, respectively). The 7.3% of the isolates remained unidentified. Sequences affiliated to the Proteobacteria were divided in the sub-classes Gammaproteobacteria (49.0%) and Alphaproteobacteria (13.9%). Relative percentages of each phylogenetic group differed between TF4 and TF5 with Gammaproteobacteria (65.3%) that predominated in TF4, whereas they were equally represented with Alphaproteobacteria and Actinobacteria (12.5, 11.1 and 2.8%, respectively) in TF5. All isolates were tested for antibiotic resistance (AR) and heavy metal (HM) tolerance2,3. Resistant/tolerant isolates were among the genera Sporosarcina, Psychrobacter, Carnobacterium, Leifsonia, Planococcus, Maninobacter, Pseudomonas (only HM tolerant) and Rhodobacter. AS was assayed towards chloramphenicol (C), streptomycin (S), ampicillin (A), gentamycin (G), kanamycin (K), and polimixin B (P) at different concentrations (range 25-350ppm). Overall, AR decreased in the order A>P>S>G>K. Isolates from TF4 were generally more resistant than those from TF5. Psychrobacter and Leifsonia isolates tolerated more antibiotics. Isolates were first screened for tolerance towards five HM salts (i.e. mercury-HgCl2; cadmium-CdCl2; copper-CuCl2; cobalt-CoCl2; nickel-NiCl2) at two different concentrations (500 and 1000ppm). Tolerant isolates were then assayed up to 10000ppm of tolerated HM(s). HM resistance decreased in the order Ni>Cu>Co>Hg>Cd with this latter that was tolerated by any strain. Strains from TF4 resulted more tolerant than those from TF5. In TF4 Hg and Cu were tolerated up to 1000ppm, Co and Ni up to 500 and 5000ppm, respectively. In TF5, Hg and Cu were tolerated up to 500ppm, Co and Ni up to 1000 and 2500, respectively. Psychrobacter isolates generally tolerated almost all HMs.