# Management of Scalp Biofilm in Local Dermatosis: From In Vivo Visualization to Optimal Treatments
## 1 Introduction  
Biofilms are widely observed when fungi and bacteria adhere to surfaces, secreting a sticky substance in environments with high moisture levels. These biofilms can also be detected on human tissues in the form of slimy layers on teeth, referred to as plaque, which can lead to tooth decay [1]. The scalp provides a distinct microenvironment compared to other areas of the skin, characterized by unique physiological conditions, such as sebum content, moisture, pH, and topography [2]. It is hypothesized that the scalp exhibits unique features related to biofilm formation for two reasons: the high moisture condition created by hair fibers and the high sebum production that promotes the growth of _Malassezia_ and _Cutibacterium_[3]. Dandruff is a common scalp condition that many people experience at some point in their lives. It is characterized by the presence of flakes on the scalp and hair, and it can cause itching [4]. While dandruff is widely recognized as a symptomatic problem, there has been limited research on its pathophysiology [5]. One popular theory is that dandruff is caused by the overgrowth of a fungus called _Malassezia_, which thrives on the lipids present in sebum [6]. When sebum is degraded by _Malassezia_, this leads to inflammation, irritation, and the flaking of the scalp. This process is associated with an increase in pro-inflammatory mediators like interleukin-8 [7]. Evidencesupporting the role of _Malassezia_ in dandruff includes the fact that antifungal agents like ketoconazole can alleviate symptoms, and inoculating animals with _Malassezia_ can induce dandruff-like symptoms [8; 9]. In natural environments, biofilms commonly consist of multiple species or poly-microbial biofilms, coexisting as part of a larger community structure [10]. Similar to single-species biofilms, the formation of poly-microbial biofilms is affected by various factors, such as the physiochemical properties of the surface environment, host receptors, nutrient availability, aggregation patterns, and local immune system responses [11]. The presence of different species in a particular habitat can involve different forms of communication between species, including quorum sensing [12]. Biofilms can also form within human organs, such as the middle ear and upper respiratory tract, oral cavity, cardiovascular system, lung, stomach, colon, urogenital system, bone, and soft tissue wounds [13]. Among them, the most extensively studied biofilms are oral biofilms, mainly due to the ease of sampling accessibility and the ubiquity of dental plaque, which is a type of multispecies biofilm. The oral bacterial diversity in the human spans hundreds of species across nine phyla. Different compositions of these communities have been associated with healthy and diseased states, such as periodontitis. In particular, erythrosine is widely used for detecting biofilms in the mouth owing to its sensitive cell viability and non-toxicity with respect to humans. Previously, we took advantage of erythrosine and developed a quick and convenient method for staining biofilms in the scalp [14]. Herein, we investigated the microbial properties of the scalp using a biofilm staining method. There are two distinctive groups: the staining-positive and staining-negative groups. Various properties of the scalp were measured, including TEWL, sebum production, and IL-8 levels. Among them, we identified sebum production as the dominant factor controlling biofilm formation in the scalp owing to its strong correlation with fungi and bacteria growth. To determine the effect of fungi and bacteria on biofilm formation in the human scalp, the composition and distribution of _Ctibacterium_, _Staphylococcus_, and _Malassezia_ were analyzed. Based on our results, we suggest that climbazole and hexamidine distiethonate can effectively disturb biofilm formation as antimicrobial agents.