Indirect Contact

This can occur when for instance medical equipment or OR personnel that are contaminated with S. aureus originating from the nose of the patient, come into contact with the surgical site. However, considering the high level of infection control and especially aseptic conditions during surgery, this transmission route is considered unlikely as a cause of S. aureus SSI.

Hematogenous Transmission

This can occur when S. aureus enters the bloodstream while the patient is undergoing surgery. For instance, S. aureus may enter the blood circulation following injury of the oropharyngeal mucosal surface during mechanical intubation. Incidental S. aureus bacteremia may cause secondary infection of the surgical site. While this is theoretically possible, it is common practice to treat patients perioperatively with antibiotic prophylaxis consisting of broad-spectrum antibiotics with good activity against S. aureus. ^{Reference Bratzler, Dellinger and Olsen14,Reference Righi, Mutters and Guirao15} Therefore, this is probably also not a major transmission route in regard to S. aureus SSI.

Airborne Transmission

This can occur when pathogens are carried to the surgical site by small airborne particles, such as droplet nuclei. These pathogen-loaded particles arise during procedures related to the surgery, for instance during inhalation anesthesia, are disseminated by air currents in the OR, and can remain airborne for extended periods of time. Earlier studies have shown that improper OR ventilation can lead to displacement of airborne particles carrying bacteria to the surgical site. ^{Reference Solberg16,Reference Popp, Alefelder and Bauer17} In addition, compared to S. aureus carriers with lower bacterial loads of colonizing S. aureus, patients who are more heavily colonized with S. aureus are known to spread more S. aureus to their surroundings. ^{Reference Solberg16} Therefore, airborne transmission could be an important source of infection in S. aureus SSI.

Considering the abovementioned arguments, we argue that airborne transmission is currently the most likely route of transmission of S. aureus from the nares to the surgical wound, even though we cannot exclude the other options. There is no question that the filtered air that is introduced into the OR is clean and does not contain S. aureus. However, there are many factors in the OR that can disturb the airflow during surgery, including OR lights, frequent door openings, patient warming devices, and people. These disturbances cause air turbulence, and this can subsequently lead to contaminated particles being transported to the incision site. ^{Reference Perez, Holloway and Ehrenfeld18} As an example, this online video shows how particles, that are produced near the head of a patient, are transported to the incision site (ref: https://youtu.be/yq-hVBjgZEk). There are also experimental data showing that direct ventilation of the operative site with a mobile laminar airflow unit, which is not influenced by lights etc., does significantly decrease the bacterial contamination of the surgical site. ^{Reference Sossai, Dagnino, Sanguineti and Franchin19}

However, the literature is still ambiguous with regard to the role of airborne transmission in the etiology of SSI. The studies conducted to date assessing the relationship between different air flow types in the OR and SSIs have yielded contradicting results. In addition, a recent meta-analysis found no evidence that laminar flow (which is considered the standard of care in modern surgery) is associated with a lower occurrence of SSI compared to conventional turbulent airflow, ^{Reference Bischoff, Kubilay, Allegranzi, Egger and Gastmeier20} which is consistent with results from earlier reviews. ^{Reference Gastmeier, Breier and Brandt21,Reference James, Khan, Nannaparaju, Bhamra and Morgan-Jones22} Whilst this is true, it is important to note that the majority of these studies were conducted with national surveillance and registry data, which often lack comprehensive confounder data. In addition, these studies were not conducted with S. aureus carriers specifically, which may have diluted the effect.

Based on the above, we conclude that the current evidence does not completely rule out airborne transmission as a major player in the etiology of S. aureus SSI in S. aureus carriers. Current guidelines, including the WHO guidelines, recommend the use of perioperative decolonization in combination with skin disinfection prior to surgery in nasal carriers of S. aureus undergoing high-risk surgical procedures. ^{Reference Allegranzi, Bischoff and de Jonge23} While this approach is proven effective and targets the reservoir of the infection, it does not take into consideration the route of transmission. As a matter of fact, current guidelines do not account for airborne transmission at all as a possible transmission route in the etiology of SSI. Therefore, we believe that more research is needed to elucidate the role of airborne transmission in the development of S. aureus SSI. For instance, modeling studies assessing the airflow dynamics in the OR under different conditions, or trials including S. aureus carriers specifically and assessing the association between different air flow modalities in the OR and the occurrence of S. aureus SSI, could inform us about the importance of airborne transmission in the etiology of S. aureus SSI. With this knowledge, we will hopefully be better suited to develop more effective antistaphylococcal preventive interventions.