Study outcome questions antimicrobial effect of dilute bleach bath

Bleach diluted to the strength clinically recommended to atopic dermatitis patients for baths does not kill the bacteria Staphylococcus aureus or S. epidermidis, an in vitro study has found. This suggests that any benefits that the bleach baths may have for treating atopic dermatitis are not due to antimicrobial effects, the researchers say.¹

The benefits of bleach baths for patients with atopic dermatitis have long been controversial. While some suggest they are helpful, others have found that they do not appear to be any more effective than water baths alone.

It has been assumed that dilute beach baths may be helpful in atopic dermatitis because they reduce the bacterial burden on the skin.² However, some studies have found no effect on S. aureus and suggested that bleach baths may benefit patients in other ways perhaps through an anti-inflammatory effect.³

To try and clarify whether any potential mode of action could be antimicrobial, researchers tested whether diluted bleach solution (sodium hypochlorite, NaOCl 0.005%) at the concentrations recommended for bleach baths is an effective antibacterial agent.⁴

They grew three laboratory strains of S. aureus (USA300, Newman, and Sanger 252) on agar plates, which were then submerged in various concentrations of bleach for 15 minutes at 37 °C to simulate a bleach bath. Twenty four hours after this treatment, direct colony counting showed that there was no significant difference in the survival of S. aureus exposed to between 0% and 0.01% NaOCl in water. The findings, reported in Journal of Allergy and Clinical Immunology, show that bactericidal effect of dilute bleach against these strains of S. aureus was only evident at concentrations above 0.03% - a concentration of bleach that is cytotoxic to human cells much greater than should be used clinically.

The researchers then repeated the experiment using two S. epidermidis strains (1475 and ATCC12228), representing another abundant bacterial species found on the skin, and a biofilm forming strain (1457) and a non-biofilm former (ATCC12228). These S. epidermidis strains were also not killed by clinically relevant concentrations of NaOCl in water.

As agar and  nutrient-rich broth do not accurately model conditions on the skin epidermis, which has a complex three-dimensional structure composed of skin folds and skin appendages such as sebaceous glands, eccrine glands, and hair follicles, S. aureus USA300 was applied to pigskin for 15 minutes at room temperature, and then the skin was submerged in a range of NaOCl concentrations for 15 minutes. Again 0.005% NaOCl had no significant bactericidal effect on S. aureus compared with water alone.

Finally, the researchers tested whether NaOCl might have a beneficial therapeutic effect against S. aureus by influencing expression of virulence functions rather than directly killing them. The agr quorum-sensing system plays a central role in the regulation of S. aureus virulence by controlling the expression of toxins that can cause epidermal damage and skin inflammation, so an agr-YFP reporter strain of S. aureus was examined during exposure to bleach for 24 hrs in TSB at 37°C. A bleach bath solution of 0.005% showed no significant effect on agr activity compared with water.

“It has been a common assumption that the recommendation of inclusion of 1/4 to 1/2 cup of 6% household bleach in a bathtub full of water (40 gallons) is an effective method to reduce bacterial load on the skin, and that clinical effects may related to this presumed antibacterial activity,” says Richard Gallo, M.D., department of dermatology, University of California, San Diego. “However, our study clearly demonstrates that bleach baths have no direct bactericidal activity against S. aureus or S. epidermidis. To obtain an antibacterial effect as previously reported, bleach concentrations must be at least 0.03%, higher than is safe for the skin or that has been proposed for clinical use. Therefore, a benefit from use of bleach bath cannot be attributed to the direct killing or inhibition of S. aureus.

“These observations are not intended to directly support or refute the potential benefits of bleach bath therapy, only to dispel the false conclusion that it is a form of antimicrobial therapy,” he adds.

References:

1. Chopra R, Vakharia PP, Sacotte R, Silverberg JI. Efficacy of bleach baths in reducing severity of atopic dermatitis: A systematic review and meta-analysis. Ann Allergy Asthma Immunol. 2017;119(5):435-440.
2. Huang JT, Abrams M, Tlougan B, Rademaker A, Paller AS. Treatment of Staphylococcus aureus colonization in atopic dermatitis decreases disease severity. Pediatrics. 2009;123(5):e808-e814.
3. Leung TH, Zhang LF, Wang J, Ning S, Knox SJ, Kim SK. Topical hypochlorite ameliorates NF-κB-mediated skin diseases in mice. J Clin Invest. 2013;123(12):5361-5370.
4. Sawada Y, Tong Y, Barangi M, et al. Dilute bleach baths used for treatment of atopic dermatitis are not antimicrobial in vitro. J Allergy Clin Immunol. 2019;143(5):1946-1948.