Just days after the FDA ruled that antimicrobials are to be removed from liquid and bar soaps, yet more suspicion is falling on antibacterial ingredients, which are exempt from the upcoming ban, provided they occur in other products, such as paints, baby toys, bedding, and kitchen utensils. Antimicrobials have been found in indoor dust, along with antibiotic-resistance genes. Moreover, six significant positive associations have been observed between the concentration of an antimicrobial chemical and the relative abundance of an antibiotic-resistance gene.
These links were discovered by researchers based at the University of Oregon, who have published the first study to document the coexistence of antimicrobial chemicals–triclosan, triclocarban, and methyl-, ethyl-, propyl-, and butylparaben—and genes in indoor dust. The study, which appeared September 7 in the journal Environmental Science & Technology, was entitled, “Antimicrobial Chemicals Are Associated with Elevated Antibiotic Resistance Genes in the Indoor Dust Microbiome.”
When the study appeared, the University of Oregon issued a press release containing well-qualified statements provided by the study’s leader, Erica M. Hartmann, who was a postdoctoral fellow at the the University of Oregon when the study was conducted. She joined the faculty at Northwestern University this month.
"We might be tempted to think of the antimicrobial chemicals as being guilty by association," said Dr. Hartmann. "We don't really know how the genes or the chemicals got there. They may have arrived by completely different routes and their being found together is a coincidence.”
Nonetheless, Dr. Hartmann noted that the associations found in the study indicated that antimicrobials in household products could contribute to antibiotic resistance. This possibility, she continued, merits additional study.
“We know that antimicrobial chemicals can cause an increase in antibiotic resistance in other situations, so I think these results provide a good reason to take a closer look at what's going on in dust,” she explained. “We don't have solid proof that putting antimicrobials in these products makes them any healthier, but we do know that triclosan in the environment can be harmful.”
Triclosan has been linked with a gene that alters the ribosome in a way that makes bacteria antibiotic resistant. The research team identified several antibiotic-resistance genes, the most common of which conferred resistance to tetracycline antibiotics.
“Dust samples from a mixed-use athletic and educational facility were subjected to microbial and chemical analyses using a combination of 16S rRNA amplicon sequencing, shotgun metagenome sequencing, and liquid chromatography tandem mass spectrometry,” the authors of the current study wrote. “The dust resistome was characterized by identifying antibiotic resistance genes annotated in the Comprehensive Antibiotic Resistance Database (CARD) from the metagenomes of each sample using the Short, Better Representative Extract Data set (ShortBRED).”
The chemicals and genes came from 44 samples from 31 varied-use spaces, using vacuum-fitted collectors. The building, completed in 1921, has window ventilation as well as infiltration of outdoor air through cracks around doors and windows.
“The three most highly abundant antibiotic resistance genes were tet(W), blaSRT-1, and erm(B),” the authors continued. “The complete dust resistome was then compared against the measured concentrations of antimicrobial chemicals, which for triclosan ranged from 0.5 to 1970 ng/g dust.”
Despite the findings, Hartmann said, people don't need to be readily alarmed. Antibiotic-resistance genes in the environment, for example, are 10 to 100 times less abundant than in the human gut, she said. In infants, the genes occur naturally in the absence of antibiotics during initial microbial colonization.
"Antibiotic resistance is common in a lot of different places," she noted. "Just because we find it in a certain building doesn't mean that everyone who goes into that building is going to get a MRSA [methicillin-resistant Staphylococcus aureus] infection. The building is still as safe as it was before the study, but now we have a better idea of how many antibiotic-resistance genes there are, and we have reason to believe that the amount of antibiotic-resistance genes may be tied to the amount of antimicrobial chemicals."
Triclosan and antibiotic resistance have been found in other places and in the environment, Hartmann said, but finding them in indoor dust brings the threat closer to home. Median concentrations of triclosan found in the dust were much less than those found as the active ingredient in toothpaste, where it helps to reduce plaque and gum disease. The new FDA ban does not include toothpaste.
"The World Health Organization has said that we're underestimating community-acquired antibiotic-resistant infections," Dr. Harmann concluded. "We know that hospitals and other healthcare settings are burdened by antibiotic-resistant pathogens. Homes and other buildings also can contain antibiotic-resistance genes and the use of antimicrobial chemicals in these buildings may be a contributing factor."