To our knowledge, this is the first study confirming statistically significant changes in microflora found on dental practitioners’ foreheads after performing aerosol-producing dental treatments. In addition, our data demonstrate that the dental operator’s forehead is less likely to become contaminated during dental treatment than is the outer surface of the surgical mask, despite their distance from the patient being very similar. Our observations support previous assumptions that the skin possesses a natural protective mechanism which via various pathways either prevents microbial repopulation or eliminates transient bacteria that do not normally populate the site [19].
To our knowledge, the risk of bacterial contamination of the forehead skin after dental treatment has not been described so far, but the distribution of fluid splashes on a protective face shield has been studied [20]. However, these reports investigated only droplet distribution on the face shield and did not analyze the actual qualitative and quantitative bacterial colonization of the facial skin. The results showed the nose region and inner corner of the eye to be the main area of exposure. Another study found the highest level of contamination in the region of the operator’s right arm and the assistant’s left arm [21]. Currently, there are no existing recommendations for dental professionals as to how to clean or disinfect their facial skin to remove bacterial contamination acquired during treatment.
The contaminated forehead skin must also be considered as a host surface facilitating the transmission of microorganisms from the patients’ oral cavity, albeit with a lower likelihood of successful contamination than expected for the outer surface of the surgical mask. Further transmission of pathogens may occur manually if exposed individuals touch their foreheads after treatment. However, during the current COVID-19 pandemic, dental treatment has changed considerably in our observation. Complete protective equipment is worn during aerosol generating procedures. Thus, at least for the time being, the protection of the dentist's facial skin is guaranteed. The findings of the present study should, however, be taken into consideration when developing post-corona recommendations for future pandemics. The protection of the forehead by a face shield seems advantageous and should be considered a general recommendation.
The microbiological methodology used in this study had the advantage that the cultivation on agar only detects viable bacteria. The use of nucleic acid-based methods would probably have led to a larger number of detected species. This would have demonstrated the potential of aerosols to transport bacteria at all, regardless of their vitality. From the infectious disease perspective, however, only viable bacteria pose a potential risk to dental staff. The agar used usually serves to detect the majority of fast-growing bacteria. Slow-growing species may have been underestimated. However, it was to be expected that readily cultivable and robust bacteria, in particular, would play a role since the resident microflora would offer a certain degree of protection against invading bacteria. Additionally, bacteria that spread easily would also be at an advantage if further contamination were to occur from the forehead or mask onto surfaces, other regions of the body, or other individuals. The MALDI-TOF MS analysis we used was restricted to colonies that were identified as different phenotypes. This may potentially resulted in underestimation of the bacterial spectrum on both foreheads and surgical masks. In our study, surgical masks were worn for 60 minutes. Published data suggest, as shown by our own preliminary tests, that in the absence of aerosol-releasing treatments, surgical masks were completely free of detectable bacteria. Hence, this potential limitation is irrelevant to the conclusions of our study.
Forehead contamination rates were significantly lower than surgical mask contamination rates, with both sites exhibiting a similar spectrum of bacterial contaminants. The majority of the bacterial species detected in our study were typical members of the skin or oral microbiomes. The most prevalent species in this study was S. epidermidis, which was found on at least two thirds of the examined surgical masks and foreheads. Contamination with other Staphylococcus spp. was observed in one out of five masks and foreheads. M. luteus, R. dentocariosa, S. oralis, and various Bacillus spp. were each detected on more than ten masks and foreheads.
Certain bacterial contaminants observed in our study are of particular clinical significance. The coagulase-negative staphylococci, such as S. epidermidis or even S. aureus, are all potentially multi-resistant pathogens. The prevalence of S. aureus was low in this study, lower than previously reported by others [22]. Moreover, this pathogen was detected in only three surgical mask samples after exposure to dental aerosols and droplets. No additional pathogens were found on the study subjects’ foreheads after performing treatment. This may be due to patient selection as patients were only enrolled in this study if they reported not having any general disease. Moreover, the participating dentists and dental staff were informed of, and highly compliant with, the strict hygiene standards maintained at our dental center. In any event, S. aureus naturally represents a high-risk pathogen, for which this study has demonstrated a potential transmission path.
The most frequently isolated pathogen in this study was S. epidermidis. It was detected in fifty forehead swab and 32 surgical mask samples. This high detection rate is in line with results from other studies [2, 23]. S. epidermidis is the most common member of the coagulase-negative staphylococci found on human epithelial surfaces and must be considered an important nosocomial pathogen [24].
The other detected oral and dermal bacteria, such as S. capitis, S. oralis, M. luteus or R. dentocariosa, and others, are part of the commensal flora. These bacteria are not pathogenic to healthy individuals, but may cause disease in immunosuppressed or immunocompromised patients. [25–28] However, a patient’s health status and the risk factors causing a facultative pathogen to become pathogenic are not always clear. Therefore, it is reasonable and sensible to implement consistent compliance with regulations and recommendations for the prevention of nosocomial infections [29]. Important factors that determine infection and the clinical manifestation of disease in dental professionals include the frequency of exposure to a pathogen, and its virulence [1]. Consequently, consistent preventive behavior is of great importance since it is impossible in the dental practice to assess whether a patient is carrying an obligate or facultative pathogen that may be transferred at a dose high enough to harm a susceptible dental health care professional.
Our findings are also relevant in a wider military context. To members of the military, particularly those in the medical service, the interruption of infection chains and strict infection control are of particular significance. Overseas deployment inevitably entails contact with a different, unfamiliar spectrum of microbes. In addition, such missions often involve contact with military personnel from other nations. Furthermore, unaccustomed temperatures, stress and different climatic conditions can also be assumed to potentially affect the immune system and alter the body’s immune response [30]. Regardless of this, it should be noted that standards of oral and body hygiene are not always guaranteed to be the same during field deployment as in the home environment [31–33]. The transmission of microbes not belonging to the soldiers’ usual flora can be assumed to be increased in individuals whose immune defense is reduced due to the conditions in the mission.