Triclosan‐induced alteration of gut microbiome and aggravation of asthmatic airway response in aeroallergen‐sensitized mice

To the Editor, Triclosan (2,4,4′‐trichloro‐2′‐hydroxydiphenyl ether) is a synthetic, broad‐spectrum antimicrobial agent used extensively in various healthcare products and household consumer goods such as soap, detergents, toothpastes, and other products. Triclosan (TCS) was first introduced in the 1960s for the use in clinical applications. Reports have suggested that TCS is absorbed in human skin and oral mucosa; and it has been detected in various human tissues and fluids. In addition, toxicological studies have shown that TCS exposure induces physiological perturbations in animals and in humans. Recently, a risk assessment undertaken by the US Food and Drug Administration (FDA) revealed that TCS was present in the urine samples of 75% of the US general population, and, in 2016, the FDA banned its use in soap products. Over the last 6 years, a number of studies have identified a positive correlation between the urinary TCS level and sensitization to food, aeroallergens, and perturbations in asthmatic patients. In addition, an experimental study has shown that the topical application of TCS enhances allergic response in ovalbumin (OVA)‐sensitized mice, suggesting that TCS might play an adjuvant role in the allergic response to environmental allergens. However, there has still been insufficient evidence to support a relationship between perturbation of the gut microbiome and allergic manifestations in relation to TCS exposure. The aim of this study was to evaluate the in vivo TCS‐induced alteration of the gut microbiome and aggravation of asthmatic airway response in aeroallergen‐ sensitized mice. We performed two independent experiments. In the first experiment, wild type BALB/cJ mice (n = 36) were randomly divided into 6 groups of six animals, including normal mice group (normal), Dermatophagoides farinae group (wild‐type + Der f), Der f and diesel exhaust particle (DEP)‐exposed group (asthma), vehicle‐ treated Der f‐sensitized mice group (wild type + Der f), 5 mg/kg TCS‐treated mice group (wild type + Der f + 5 mg/kg triclosan), 50 mg/kg TCS‐treated mice group (wild type + Der f + 50 mg/kg triclosan), and 500 mg/kg TCS‐treated mice group (wild type + Der f + 500 mg/kg triclosan). In the second experiment, knockout TLR‐ 2 (−/−) mice (n = 12) were randomly divided into two groups of six animals that received 4 μg of Der f intratracheally 2 days per week for a total of 4 weeks. Both groups were treated either with vehicle (TLR‐2 (−/−) mice + Der f group) or triclosan 500 mg/ kg (TLR‐2 (−/−) mice + Der f + triclosan group) through oral gavage 5 days per week for a total of 4 weeks. On the other hand, airway hyperresponsiveness (AHR) to intravenous acetylcholine (ACh) was performed as previously described, whereas bronchoconstriction was measured according to the overflow method using a bronchospasm transducer connected to the tracheal cannula. Changes in respiratory overflow volume were measured using an increasing dose of ACh. The area under the curve (AUC), calculated from the dose‐response curves for ACh, was used to express the magnitude of AHR (ImageJ software, National Institutes of Health, Bethesda, Maryland, USA). On day 23, bronchoalveolar lavage (BAL) was performed and BAL fluid (BALF) was recovered for eosinophils enumeration and also IL‐4 and IL‐13 level measurements by ELISA. Blood samples and lung specimens were collected to measure the serum level of anti‐Der f IgE and perform the histopathological examination, respectively. Furthermore, mouse fecal samples were collected to isolate microbial genomic DNA using a NucleoSpin DNA Stool Kit, and 16S rRNA gene sequencing was performed following procedures described by Shen et al. Data are presented as the mean ± SEM, and statistical comparisons among the treatment groups were performed with the use of one‐way ANOVA test followed by the nonparametric Tukey test, whereas the abundance of bacterial genes was compared using Student's t test (SPSS software package). Detailed methodology is provided in Supporting informations. As results, simultaneous exposure of wild‐type mice to TCS and Der f induced: