The role of hygrodynamic resistance compared to biofilm formation in helping pathogenic bacteria dominate air-conditioning units recovered from odour problems

We previsouly found that installing filters in odourous air-conditioning units (ACUs) to block the entry of skin squames could well tackle the odour problems. In this study, we revisited and sampled the ACUs installed with filters earlier to study the bacterial communities inside the ACUs using 16S amplicon sequencing. We identified 26 genera and found that the skin bacteria isolated in the previous work were absent in this study. Two pathogenic bacteria, Methylobacterium and Sphingomonas, dominated ACUs instead. Afterwards, these two bacteria were identified to species level (Methylobacterium organophilum and Sphingomonas paucimobilis, respectively), and examined in terms of their biofilm formation ability and resistance to changing moisture conditions together with another prevalent species isolated in our previous study, namely Micrococcus luteus, in order to understand the mechanisms of the survival of bacteria in ACUs. In general, M. organophilum and M. luteus showed good biofilm formation ability at all tested temperature levels, but S. paucimobilis only displayed limited biofilm formation. Whereas, all these three bacteria well maintained their survival after wet-dry cycles. These results suggest that compared to biofilm formation, ability to survive under hygrodynamics tends to play a more important role in helping bacteria dominate ACUs. Further, this study implies that the absence of odour problem does not guarantee a healthy environment, more attentions should be given to limit the abundance of hydrodynamic-resistant pathogenic bacteria.