Dilute bleach baths used for treatment of atopic dermatitis are not antimicrobial in vitro

To the Editor:

Colonization of the skin of patients by Staphylococcus aureus is considered a risk for skin infection and an exacerbating factor in atopic dermatitis.¹ Because of the negative effects associated with S aureus colonization, clinicians commonly seek methods to eradicate colonization by S aureus. A diluted bleach solution (sodium hypochlorite [NaOCl] 0.005%) is often used for this purpose, with conflicting reports of its efficacy to decrease the severity of inflammation or eradicate S aureus on the skin.^2-4 In this study we sought to directly test the assumption that NaOCl at concentrations recommended for bleach baths is an effective antibacterial agent.

Three laboratory strains of S aureus (USA300, Newman, and Sanger 252) were initially selected for this study. Bacterial colony growth was initiated by plating 2 × 10³ colony-forming units (CFUs) of each strain on 100-cm² Tryptic Soy Broth (TSB) agar plates and then submerging each plate in various concentrations of bleach for 15 minutes at 37°C to test survival on a nutrient-rich surface. Twenty-four hours after this treatment, direct colony counting showed no significant difference in the survival of S aureus exposed to between 0% and 0.01% NaOCl in water (Fig 1, A). A bactericidal effect of dilute bleach against these strains of S aureus was only evident at concentrations of greater than 0.03%, a concentration of bleach that is cytotoxic to human cells and much greater than should be used clinically.

FIG 1.

Bleach at 0.005% is not antimicrobial. A, CFUs of 3 strains of S aureus growing on agar when exposed to bleach. B, Survival of 2 strains of S epidermidis growing on agar after exposure to bleach. C, Survival of S aureus growing in TSB solution after exposure to different household bleach solutions (Pure Bright, Clorox, and Waxie). D, S aureus survival when growing at log phase or stationary phase during exposure to bleach. E, S aureus surviving on pig skin after exposure to bleach. F, S aureus agr reporter activity (RFU) and survival (CFU) after exposure to bleach. Results are means ± SDs. *P < .05, Student t test. All data are representative of one of 3 independent experiments. Data of Fig 1, A-C, were evaluated by using 3 measurements made in parallel in each experiment.

The unexpected observation that the concentrations of NaOCl used in bleach baths were not antibacterial against S aureus prompted us to further explore the role of other variables on bacterial survival in defined laboratory culture conditions. Two Staphylococcus epidermidis strains (1475 and ATCC12228) representing another abundant bacterial species found on the skin that were also biofilm-forming (1457) and non–biofilm-forming (ATCC12228) strains were next tested on agar, as done for S aureus. These S epidermidis strains were also not killed by clinically relevant concentrations of NaOCl in water (Fig 1, B).

Next, to test whether culture system or source of household bleach influenced these results, S aureus USA300 was grown in TSB broth at 37°C for 24 hours. The source of household bleach had no effect. Similar to growth on agar, bacterial survival was not inhibited at the clinically used concentration (0.005%) of NaOCl (Fig 1, C). Furthermore, because the bacterial growth phase can determine sensitivity to antibiotic agents, with bacteria in a growth phase (log-phase growth) often showing greater sensitivity than stationary phase bacteria,⁵ we also tested the sensitivity of S aureus USA300 in log-phase growth compared with bacteria at the stationary phase. No difference in sensitivity to bleach was observed under these conditions (Fig 1, D). Taken together, we conclude that the concentration of NaOCl recommended for clinical use in bleach baths does not inhibit the survival or growth of S aureus or S epidermidis under laboratory conditions.

S aureus growing on agar or in nutrient-rich broth does not accurately model conditions on the skin. The epidermis has a complex 3-dimensional structure composed of skin folds and skin appendages, such as sebaceous glands, eccrine glands, and hair follicles. The composition of the epidermal surface can also influence the capacity of bleach baths to act as antimicrobial agents. To examine this, 1 × 10⁶ CFUs of S aureus USA300 were applied to explants of pig skin for 15 minutes at room temperature, and the skin was then submerged in a range of NaOCl concentrations for 15 minutes to simulate immersion in a bleach bath. Immediately after this treatment, surviving CFUs were measured. Similar to the results in defined cultures, 0.005% NaOCl had no significant bactericidal effect on S aureus compared with water alone (Fig 1, E). Therefore these results suggest that a bleach bath has no antibacterial action against S aureus on skin.

In our final experiment, we assessed whether NaOCl might have a beneficial therapeutic effect against S aureus by influencing expression of virulence functions of bacteria rather than directly killing them. The accessory gene regulator (agr) quorum–sensing system plays a central role in regulation of S aureus virulence by controlling the expression of toxins that can cause epidermal damage and skin inflammation.^6,7 To test the action of NaOCl on agr activity, an agr–yellow fluorescent protein reporter strain of S aureus was examined during exposure to bleach for 24 hours in TSB at 37°C. A bleach bath solution of 0.005% showed no significant effect on agr activity compared with water (Fig 1, F). These results show that the S aureus agr quorum–sensing system is also not inhibited during bleach bath treatment.

Bleach baths have been reported by clinicians and patients to be associated with improvement of inflammation in patients with atopic dermatitis³ and reported to reduce colonization that could result in deep tissue infections.² It has been a common assumption that the recommendation of inclusion of one-quarter to one-half 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 might be related to this presumed antibacterial activity. 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,⁸ bleach concentrations must be at least 0.03%, which is greater than is safe for the skin or that has been proposed for clinical use. Therefore a benefit from use of a bleach bath cannot be attributed to direct killing or inhibition of S aureus.

These observations are not intended to directly support or refute the potential benefits of bleach bath therapy but only to dispel the false conclusion that it is a form of antimicrobial therapy. Indeed, because a recent meta-analysis concluded that water baths alone can significantly decrease the severity of atopic dermatitis,⁴ beneficial effects can be due to effects other than those acting directly on the skin microbiome. With improved disease, the bacterial burden might indirectly decrease because of enhanced innate antimicrobial functions. The laboratory strains tested here were selected as reflective of clinical S aureus strains on atopic dermatitis but might not represent the sensitivity of all strains. It is also conceivable that bleach baths might be directly anti-inflammatory on the skin. A previous study has reported that NaOCl can oxidize cysteine residues of the inhibitor of nuclear factor κB, inhibit nuclear factor κB activity, and improve skin disease in mice.⁹ However, the observations of our study show that a bleach bath does not directly inhibit S aureus. This information should be considered in the interpretation of future studies of topical bleach baths.