Deciphering the role of cyanobacteria in water resistome: Hypothesis justifying the antibiotic resistance (phenotype and genotype) in Planktothrix genus

The importance of environmental microorganisms in the emergence and dissemination of antibiotic resistance is an undeniable fact. However, cyanobacteria are not seen yet as putative players in the dynamic of environmental resistome, despite their ubiquity in water environments, where they are exposed to antibiotic pollution and in straight contact with native and pathogenic bacteria harboring antibiotic resistance genes (ARGs). In this work we evaluated the susceptibility of 8 strains of Planktothrix agardhii (from surface freshwaters reservoirs) and 8 strains of Planktothrix mougeotii (from a wastewater treatment plant) to several classes of antibiotics, using a microplate dilution method previously described by us. We also search for ARGs in those strains by molecular methods. None of the 16 tested strains were susceptible to trimethoprim, nalidixic acid and norfloxacin, from 0.0015–1.6 mg/L, but all were susceptible to streptomycin, gentamicin, kanamycin, ceftazidime and ceftriaxone. The minimum inhibitory concentrations (MICs) ranged between 0.05–0.8 mg/L for the aminoglycosides and 0.4–1.6 mg/L for the two β‑lactams. Major differences were found in the susceptibility to amoxicillin and tetracycline, with P. agardhii being susceptible (MIC of 0.05 mg/L and 0.4 mg/L, respectively) and P. mougeotii not susceptible. These distinct responses might be due to differences between species. However, the lower susceptibility of wastewater strains suggests that antibiotic resistance phenotype of cyanobacteria is related with their habitat. The failure to detect acquired genes conferring resistance to trimethoprim/quinolones, strongly supports the hypothesis that cyanobacteria are intrinsically resistant to these antibiotics. Interestingly, we detected a class-1-type integron and a sul1 gene in 3 strains of both P. agardhii and P. mougeotii, which supports the possibility of cyanobacteria to acquire and transfer antibiotic resistance determinants. In conclusion, the identification of ARGs and related integrons, as well as the reduced susceptibility to some antibiotics, suggests that cyanobacteria may play a role on environmental resistome.