Antibiofilm Activity of Small Molecules Produced by Staphylococcus epidermidis against Staphylococcus aureus

LETTER

We read the article by Glatthardt et al. (1) with great interest. The skin microbiome is indeed considered a promising source of antipathogen entities produced by resident members of the normal microflora, including bacteriocins and antimicrobial peptides (2). The current study reported the presence of small molecules in the culture supernatant (CS) of Staphylococcus epidermidis, which displayed antibiofilm activity against methicillin-sensitive and methicillin-resistant Staphylococcus aureus, including all the four agr types and agr-defective strains. The active components were found to impact S. aureus biofilm formation and preestablished biofilms without affecting the cell viability, were resistant to heat and proteases, and exhibited a molecular weight of 3 to 10 kDa, although their precise identity remains to be elucidated by the authors (1). In this context, we would like to propose a plausible role of phenol-soluble modulins (PSMs) in the observed results.

PSMs are secreted by staphylococci during the stationary growth phase and play diverse functions in staphylococcal growth and pathogenesis. These peptides have an α-helical amphipathic nature, with sizes in the range of 20 to 25 amino acids to 40 to 44 amino acids, depending upon the type of PSM (3). Few of these PSMs have been shown to exhibit antimicrobial activity against other bacteria (4, 5) and even fungi, as reported by us recently (6). However, most of the PSMs exhibit a surfactant-type action, wherein they mediate biofilm restructuring as well as the dispersal of biofilm cells (7–10) without affecting the cell viability (5, 6). This feature plays an important role for the producing staphylococcal species itself (7–10) and is likely to impact the biofilms of other microbes, as observed for many biosurfactants, including rhamnolipids produced by Pseudomonas aeruginosa (11, 12). PSMs are known to form high-molecular-weight aggregates (4, 13). Many of the properties of these multimeric aggregates differ from those of their monomeric counterparts. For instance, the biofilm disassembly activity is limited only to the monomeric, soluble forms of PSMs (<3 kDa) (14, 15). Further, the impact of PSMs on biofilm detachment is independent of the biofilm matrix composition (7). These peptides are also hydrophobic and heat resistant (16–18). Thus, the physiochemical properties of the active components reported by Glatthardt et al. (1), as well as the observed nature of their antibiofilm action, correlate well with those of the PSMs. However, PSMs are sensitive to proteinase K and trypsin, whereas the antibiofilm activity detected in S. epidermidis CS by Glatthardt et al. (1) was not influenced by these proteases. This could be attributed to differences in the experimental setups, e.g., time period used for protease treatment (16) and/or the presence of both monomeric and multimeric PSM forms in S. epidermidis CS (6), which may impact their response to protease action. Correlating the physiochemical properties of various molecular-weight fractions after serial size-dependent fractionation of the CS with the antibiofilm activity may shed light on this aspect. Alternatively, the antagonistic activity may be multifactorial, with PSMs being one of the components. The possibility of the S. epidermidis CS acting through multiple pathways has also been suggested by Glatthardt et al. (1), although they presumed PSMs to be unlikely candidates for the detected bioactivity. It would be imperative to evaluate the antibiofilm activity of the CS from PSM-negative (agr-negative) strains of S. epidermidis. At present, correlation of the RNA sequencing data of Glatthardt et al. (1) with the role of PSMs is not feasible, as the impact of PSMs on gene expression is not well understood, and only limited literature is available on their gene regulatory functions (19, 20). Nevertheless, the involvement of PSMs in the observed antibiofilm activity of S. epidermidis CS against S. aureus needs to be explored more definitively.