The hygiene hypothesis—or more recently dubbed microbiome depletion theory—is a supposition proposed in the late 1980s from the observation that children in larger family households had fewer instances of hay fever since they were exposed to microbes by their older siblings. This observation led to further research making casual connections between the lack of early childhood exposure to infectious agents, symbiotic microorganisms, and parasites increases susceptibility to allergic diseases by suppressing the natural development of the immune system. While direct evidence for this hypothesis has been more difficult to come by, new research from investigators at the Environmental Biotechnology at Graz University of Technology (TU Graz) strengthens the theory by finds that show excessive hygiene promotes resistance to antibiotics.

Now, I know what you’re thinking, and no this doesn’t mean you shouldn’t wash your hands after using the bathroom. It has been well established that deficiencies in that type of hygiene can directly lead to illness. Just ask anyone who has been on a cruise ship during a norovirus outbreak. Moreover, the number of people who become ill and die from antibiotic-resistant germs is increasing worldwide. So much so, the World Health Organization thinks that understanding the spread of antibiotic resistance and developing countermeasures as one of the most important global challenges.

To address some of these concerns the TU Graz scientists compared the microbiome and the resistome—i.e., all existing microorganisms and antibiotic resistances—in the intensive care unit of the department of internal medicine at University Hospital Graz, with clean rooms subject to strong microbial control in the aerospace industry, and with public and private buildings which have hardly any microbial controls. Findings from the study were published recently in Nature Communications through an article titled “Man-made microbial resistances in built environments.”

“We compared the microbiota present on surfaces of clinical settings with other built environments. Using state-of-the-art metagenomics approaches and genome and plasmid reconstruction, we showed that increased confinement and cleaning are associated with a loss of microbial diversity and a shift from Gram-positive bacteria, such as Actinobacteria and Firmicutes, to Gram-negative such as Proteobacteria,” the authors wrote. “Moreover, the microbiome of highly maintained built environments has a different resistome when compared to other built environments, as well as a higher diversity in resistance genes.”

The analyses showed that microbial diversity decreases in areas with high levels of hygiene but that the diversity of resistance increases. “In environments with strong microbial control in the intensive care unit and industrially used clean rooms, there are increasing antibiotic resistances which show a high potential for combining with pathogens,” explained lead study investigator Alexander Mahnert, PhD, director of studies at the Institute of Environmental Biotechnology of TU Graz.

The results indicate that a stable microbial diversity in clinical areas counteracts the spread of resistance. “The microbial control of pathogens is already being successfully used in cultivated plants and in humans in the framework of stool transplantation. Our study provides an initial foundation to pursue such ideas in indoor areas in the future,” noted senior study investigator Gabriele Berg, PhD, who heads the Institute of Environmental Biotechnology at TU Graz.

The researchers surmise that regular airing, houseplants, the deliberate use of useful microorganisms, and the reduction of antibacterial cleaning agents could be the first strategies in maintaining or improving microbial diversity.

“Our results highlight that the loss of microbial diversity correlates with an increase in resistance and the need for implementing strategies to restore bacterial diversity in certain built environments,” the authors concluded.

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