Phenol-soluble modulins (PSMs) are a family of small

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

, that carry out a variety of functions, including acting as

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

s, assisting in

biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...

formation, and colony spreading. PSMs are produced by ''

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

'' bacteria including Methicillin-resistant ''Staphylococcus aureus'' (MRSA)'','' and ''

Staphylococcus epidermidis ''Staphylococcus epidermidis'' is a Gram-positive bacterium, and one of over 40 species belonging to the genus '' Staphylococcus''. It is part of the normal human microbiota, typically the skin microbiota, and less commonly the mucosal microbio ...

.'' Many PSMs are encoded within the core genome and can play an important virulence factor. PSMs were first discovered in ''S. epidermidis'' by Seymour Klebanoff and via hot-

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...

extraction and were described as a pro-inflammatory complex of three peptides.1. Cheung GYC, Joo HS, Chatterjee SS, Otto M. Phenol-soluble modulins - critical determinants of staphylococcal virulence. FEMS Microbiology Reviews. Blackwell Publishing Ltd; 2014. pp. 698–719. doi:10.1111/1574-6976.12057 Since their initial discovery, numerous roles of PSMs have been identified. However, due in part to the small size of many PSMs, they have largely gone unnoticed until recent years. Although PSMs are present in every ''Staphylococcal'' species, there is still diversity. ''Staphylococcus aureus'' encodes eight different PSMs, PSMα 1-4, PSMβ 1-2, PSMγ (Also known as δ-toxin in ''S. aureus''), and PSM-mec.Li S, Huang H, Rao X, Chen W, Wang Z, Hu X. Phenol-soluble modulins: novel virulence-associated peptides of staphylococci REVIEW. Futur Microbiol. 2014;9: 203–216. doi:10.2217/FMB.13.153 While ''

'' encodes one PSMα, PSMβ 1-2, PSMγ, and PSM-mec. In addition ''S. epidermidis'' encodes two unique PSMs, PSMδ and PSMε. PSM-mec is one of the most widely encoded PSM among ''Staphylococcal'' species. Which may be in part due to PSM-mec being encoded on the ''mec''I mobile genetic element.

Structure and location

As the PSM classes are closely related there are many conserved aspects. However each PSM class plays a different role, as such there are some distinctive features for each. Generally PSMs are encoded on the core genome of ''staphylococcal'' species however some, such as PSM-mec, are encoded on mobile genetic elements. PSMs are generally separated into one of two classes α-type PSMs and β-type PSMs, which are based upon characteristics of the two most well studied PSMs PSMα and PSMβ.

PSMα

PSMα forms an amphipathic α-helix structure that composes the entire length of the peptide. These peptides are relatively short, being composed of only 20-25

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

s. With regards to charge, α-type PSMs generally have a neutral charge, but may also be slightly positive.

PSMβ

PSMβ Are similar to PSMα in that they contain an amphipathic α-helix. However, the helix does not compose the entirety of the peptide, instead covering only the C-terminus of the peptide. PSMβ are generally larger than PSMα, being composed of 43-45 amino acids. Unlike α-type PSMs, β-type PSMs usually possess a negative charge.

PSMγ

PSMγ (also known as δ-Toxin) shares some homology with PSMα-3 encoded by ''S. aureus''.

PSMδ

PSMδ is encoded downstream of the PSMα gene in ''S. epidermidis.'' In addition PSMδ shares some homology with PSMγ.

PSMε

Little work has been done to determine the structure of PSMε. However it is believed to play a role in both biofilm formation as well as

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

PSM-mec

PSM-mec is encoded on the ''Staphylococcal'' Chromosomal Cassette

methicillin Methicillin (USAN), also known as meticillin (INN), is a narrow-spectrum β-lactam antibiotic of the penicillin class. Methicillin was discovered in 1960. Medical uses Compared to other penicillins that face antimicrobial resistance due ...

resistance island (SCC''mec)'' which encodes genes associated with methicillin resistance in different ''Staphylococcal'' species. Little work has been done to determine the exact structure of PSM-mec.

Regulation

PSM regulation in ''S. aureus'' is primarily controlled by the ''agr'' system. The exact mechanism of regulation differs from other ''agr'' controlled toxins, which are controlled by the ''agr'' effector molecule

RNAIII RNAIII is a stable 514 nt regulatory RNA transcribed by the P3 promoter of the '' Staphylococcus aureus'' quorum-sensing '' agr'' system ). It is the major effector of the ''agr'' regulon, which controls the expression of many '' S. aureus'' gene ...

. PSMs, however, are controlled by direct binding of AgrA to the promoter region. PSM-mec RNA has been implicated in the regulation of the agr system and as a result can influence the expression of other PSMs. In addition to the agr system, SarA as well as LuxS have both been implicated in PSM control, with mutations in either system showing decreased levels of PSM production. In addition the MgrA system has been shown to alter biofilm formation, via suppression of PSMs. The environment ''S. aureus'' is exposed to has been demonstrated to play a role in PSM expression. In intracellular environments it has been shown that PSM production is increased. In addition to being the subject of regulation, PSMs have been shown to regulate other toxins such as ''S. aureus'' alpha toxin.

Functions

Inflammation

PSMs were first described as a pro-inflammatory molecule. This role has been repeatedly demonstrated to be true. PSMs are able to induce the production of a variety of

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

s as well as induce neutrophils to migrate to sites of infection. PSMε in ''S. epidermidis'' is known to influence the production of IL-8. PSMα in ''S. aureus'' has been shown to influence IL-17 levels during infection. In addition to their pro-inflammatory properties, PSMs have been shown to be directly sensed by circulating leucocytes through the formyl peptide receptor FPR2, thus driving a rapid, pathogen-specific attraction of neutrophiles to the site of infection, via an EGR1-dependent signalling pathway.Nguyen TH, Cheung, GYC, Rigby KM, ''et al''. Rapid pathogen-specific recruitment of immune effector cells in the skin by secreted toxins. Nat Microbiol. 2022 Jan;7(1):62-72. doi:10.1038/s41564-021-01012-9

Infection

In addition to their role in attracting neutrophils to sites of infection, PSMs can also influence the function of neutrophils. It has been demonstrated that secreted PSMs are able to induce Neutrophil Extracellular Trap release. The PSMs also have been shown to decrease the number of

persister cells Persister cells are subpopulations of cells that resist treatment, and become antimicrobial tolerant by changing to a state of dormancy or quiescence. Persister cells in their dormancy do not divide. The tolerance shown in persister cells differs f ...

within a population of ''S. aureus''.Bojer MS, Lindemose S, Vestergaard M, Ingmer H. Quorum sensing-regulated phenol-soluble modulins limit persister cell populations in Staphylococcus aureus. Front. Microbiol. 2018;9. doi:10.3389/fmicb.2018.00255 MRSA production of PSMs is thought to be a possible cause of severe infections. PSM production is higher in community-acquired MRSA (CA-MRSA) than in healthcare-associated MRSA (HA-MRSA),{{Cite journal, last1=Rasigade, first1=Jean-Philippe, last2=Trouillet-Assant, first2=Sophie, last3=Ferry, first3=Tristan, last4=Diep, first4=Binh An, last5=Sapin, first5=Anaïs, last6=Lhoste, first6=Yannick, last7=Ranfaing, first7=Jérémy, last8=Badiou, first8=Cédric, last9=Benito, first9=Yvonne, date=2013-01-01, title=PSMs of hypervirulent Staphylococcus aureus act as intracellular toxins that kill infected osteoblasts, journal=PLOS ONE, volume=8, issue=5, pages=e63176, doi=10.1371/journal.pone.0063176, issn=1932-6203, pmc=3653922, pmid=23690994, doi-access=free and consequently CA-MRSA associated

osteomyelitis Osteomyelitis (OM) is an infection of bone. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The long bones of the arms and legs are most commonly involved in children e.g. the femur and humerus, while the ...

is more severe than HA-MRSA associated osteomyelitis.

Cell lysis

Many PSMs have cytolytic activity and play a major role in the nonspecific lysing of host cells, including Polymorphonuclear Neutrophils (PMNs). Lysis is carried out by integration of PSMs into membranes, in a nonspecific fashion, which results in disruption of the membrane. Different PSMs are able to lyse cells with different affinities. PSMα, in ''S. aureus'', and PSMδ, in ''S. epidermidis'', are the most potent cytolysins. WHile highly cytolytic PSMs, such as PSMα and PSMδ, are generally α-type PSMs the β-type PSMs tend to be less cytolytic.

Colony spreading

''S. aureus'' is a non-motile bacteria, and must rely on alternative forms of spreading. Phenol-Soluble Modulins have been implicated in assisting with colony spreading.Kizaki H, Omae Y, Tabuchi F, Saito Y, Sekimizu K, Kaito C. Cell-surface phenol soluble modulins regulate staphylococcus aureus colony spreading. PLoS One. 2016;11: 1–26. doi:10.1371/journal.pone.0164523 PSMα 1-4 have been shown to help ''S. aureus'' colonies spread on agar plates. However, δ-Toxin, which is another α-class PSM, does not play a role in colony spreading.

Biofilm

While the α-type PSMs are regarded as major cytolysins, the β-type PSMs are thought to play a role in biofilm formation. ''S. epidermidis'', which is a known opportunistic pathogen, has a high level of β-type PSM expression. In addition ''in vitro'' measurement of PSMβ expression in ''S. epidermidis'' has shown to be increased in biofilm as opposed to planktonic growth, further suggesting a link between PSMβ and biofilm formation. Altering the structure of PSMβ has been demonstrated to disrupt their ability to influence biofilm formation.

References

Staphylococcaceae Bacterial toxins Occupational safety and health