Impact of pH on growth of Staphylococcus epidermidis and Staphylococcus aureus in vitro.

The pH of skin is critical for skin health and resilience and plays a key role in controlling the skin microbiome. It has been well reported that under dysbiotic conditions such as atopic dermatitis (AD), eczema, etc. there are significant aberrations of skin pH, along with a higher level of Staphylococcus aureus compared to the commensal Staphylococcus epidermidis on skin. To understand the effect of pH on the relative growth of S. epidermidis and S. aureus, we carried out simple in vitro growth kinetic studies of the individual microbes under varying pH conditions. We demonstrated that the growth kinetics of S. epidermidis is relatively insensitive to pH within the range of 5-7, while S. aureus shows a stronger pH dependence in that range. Gompertz's model was used to fit the pH dependence of the growth kinetics of the two bacteria and showed that the equilibrium bacterial count of S. aureus was the more sensitive parameter. The switch in growth rate happens at a pH of 6.5-7. Our studies are in line with the general hypothesis that keeping the skin pH within an acidic range is advantageous in terms of keeping the skin microbiome in balance and maintaining healthy skin.

Niacinamide leave-on formulation provides long-lasting protection against bacteria in vivo.

Antimicrobial peptides (AMPs) form a part of the skin's innate immune system. Their primary activity is to provide antimicrobial benefits and hence protect from infections. AMPs that are present on human skin include psoriasin (S100A7), RNase 7, lysozyme, LL-37 and defensins. Niacinamide is a well-known cosmetic ingredient that has been used traditionally for multiple skin benefits. Recent data indicate that niacinamide treatment can boost AMPs in human gut epithelial cells and in neutrophils. Treatment with niacinamide in mice also provided protection from skin infections by enhancing AMPs. In this article, we find that treatment with niacinamide formulation provides long-lasting protection against bacteria, potentially through the activation of an AMP response.