Microbial colonization on biocide-treated and untreated façades: The impact of surface orientation

Biocides such as Octylisothiazolinone (OIT) and Terbutryn (TER) are commonly used in building materials to prevent microbial infestation. Their release and antimicrobial efficiency are influenced by wind-driven rain (WDR), which is directly related to façade orientation. This study investigates the bacterial and fungal community composition on biocide-free (BFF) and biocide-containing (BCF) façades with N/W (WDR) or S/E orientation, representing contrasting microclimatic conditions in terms of solar radiation, drying rates, and moisture availability. Sampling was carreid out after one year of outdoor exposure using a cultivation-independent approach. Bacterial and fungal community composition significantly differed between BCF and BFF. Furthermore, the fungal and bacterial community composition was significantly affected by façade orientation. Façades with BCFs S/E showed significantly higher bacterial richness than other façades. Moreover, members of the genera Nesterenkonia, Paracoccus, Microbacteriaceae, Pseudomonas, and Dioszegia dominated BFFs N/W and BFFs S/E, whereas Klenkia, Massilia, Capnodiales, and Filobasidium, were mostly present on BCFs N/W. Only the fungal genus Vishniacozyma was present in every treatment. Bacterial and fungal genera affiliated to biodeterioration, discoloration, and biodegradation, such as Nocardioides, Sphingomonas, Kineosporiaceae, and Arthrobacter were mainly found on BCFs N/W, Arthrobacter was additionally observed on BFFs N/W, and fungal genera Cladosporium and Alternaria were frequently present on BFFs S/E and BFFs N/W. These results demonstrate selective pressure on the microbial colonization patterns by both the presence of biocides and façade orientation. Therefore, these findings provide a broader perspective on the façade microbiome and suggest the need to adjust biocide usage to redefine methods, including considerations of weathering side and biocide application.