**5. Skin surface of** *S. aureus* **in athletes**

The skin is the largest organ of the human body, representing more than 10% of the body mass [38]. In athletes who participate in contact and collision sports, the risk of transmission of SA has been shown to be particularly high [39–41]. It has been hypothesized that "skin-toskin contact" might be the main cause of SA transmission in athletes, with the physical contact inducing SA dissemination in these athletes [1]. The average area of the skin surface of a human adult is 2 m2 [42]. Although a dry, salty, low-pH skin surface discourages SA growth [43], the skin of an athlete is usually soaked in sweat, which provides a moist and nourishing environment that is suitable for SA growth. Therefore, skin sweat has been considered to be a key point of transmission during physical contact [44].

Recent evidence suggests that nasal SA has a high propensity to colonize the skin surface [45]. This idea is supported by the finding that colonization often simultaneously disappears from other body sites if an intranasal topical antibiotic is used to temporarily eliminate the SA nasal carriage [46]. Furthermore, cutaneous investigations that examined sweat glands, sebaceous glands, and hair follicles have reported that these areas are likely to be associated with their own unique microbiota [47]. Sebaceous glands secrete lipid-rich sebum, with this hydrophobic coating able to protect and lubricate the hair and skin. In general, sebum serves as an antibacterial coating and acts as a molecular defense mechanism [48]. However, the relationship between exercise-induced sweating and SA transmission in physical contact sports among athletes remains unclear.

It has been reported that the nasal cavity is the primary reservoir for SA and that these carriers are an established risk factor for transmission. Two factors may be involved in the SA transmission in an athletic setting. First, nasal carriers also carry the organism on their hands. Thus, not only are contaminated hands considered to be a likely source for causing the transmission, the hands actually serve in many cases as the primary vectors for transmitting the nasal SA. Second, SA can also live on the skin, which makes it easy to transmit from one person to another via sweat. This route is considered to be the major mode of transmission. The reason for the presence of a higher density of SA on the skin surface is due to the sweat that occurs during exercise in nasal carriers [11].

Even though SA is found on the skin, the nose appears to be the primary reservoir for its replication and transmission to other body sites. This hypothesis is supported by studies that have demonstrated that the use of an intranasal topical antibiotic will temporarily eliminate the transmission of the SA from the nasal carriage to the colonized body site [46]. Pulsed-field gel electrophoresis (PFGE) has also shown that nasal SA isolates are often identical to the strains that later cause clinical infections [49, 50]. Since 10% of the nasal SA carriers exhibited more than one genotype or phage type in their nose, this suggests that many of the infections might be of endogenous origin [32, 51].

Direct physical contact with bodily fluids is believed to be one source of SA dissemination [52]. Examples of direct contact in rugby occur in the scrum, when making tackles, shaking hands, or coming in contact with perspiration and skin lesions. To determine the factors behind SA transmission in physical contact sports, Suzuki and Tagami [11] examined the skin surface SA before and after exercise. The findings of this study showed that the density of the nasal SA was correlated with that of the skin surface SA in nasal carriers with perspiration on the skin surface after exercise, which indicates that perspiring during exercise promotes the appearance of SA in nasal carriers. Eda et al. also provided direct evidence of skin surface SA in healthy adult males after participating in high-intensity endurance exercises [53]. Perspiring during exercise appears to be a key part in the self-infection and transmission of SA in nasal carriers. Thus, the chances of team players transmitting SA to other team members would be increased during practices and while taking part in other exercises.
