Changes in heterotrophic microbial communities induced by biocidal treatments in the Monastery of San Martino Pinario (Santiago de Compostela, NW Spain)

Previous studies have provided evidence that chemical biocides can condition the behaviour of microbial populations, thereby promoting recolonization by some groups of microorganisms. We conducted a year-long study to evaluate the extent to which fungal and bacterial communities are affected by the biocidal treatments Biotin R (R), Biotin T (R), Preventol RI80 (R), New Des 50 (R), benzalkonium chloride and an ethanol-water (1:1) mixture, used to remove Apatococcus lobatus from the granite walls of the San Martino Pinario Monastery in Santiago de Compostela (NW Spain). Bacterial, fungal, algal and cyanobacterial concentrations were enumerated via microscopic counts of colony-forming units on agar plates, one, six and twelve months after application of the biocidal treatments. Cultivable species were further characterized by morphological taxonomic analysis (algae and fungi) and DNA analysis by sequencing of 16S rDNA gene (bacteria) and the ITS region plus the D1/D2 region of the 28S rDNA gene (fungi) before and after one-year treatment. None of the treatments led to over-growth of bacteria or fungi, while benzalkonium chloride and water (used as control) treatments were the most effective in preventing bacterial recolonization and Biotin T (R) was the most effective in preventing fungal recolonization. Although the bacterial population did not increase in numbers after any of the treatments, the diversity was altered, with Bacillus halotolerans disappearing and Bacillus pumilus and Brachybacterium paraconglomeratum appearing in the areas treated with biocides. In fungi, Eutypa sp. was eliminated with all biocides except with Preventol RI80 (R) and no species was promoted.

Automated summary

The study found that benzalkonium chloride and water treatments were most effective in preventing bacterial recolonization, while Biotin T was most effective in preventing fungal recolonization after the application of biocidal treatments to remove Apatococcus lobatus from granite walls. Despite no increase in bacterial numbers after treatment, the diversity of bacterial populations was altered.