## 2 The Influence of Different Factors on Oral Microbiome
### Lifestyle Factors  
Lifestyle factors such as smoking, alcohol consumption, drug use, and specific diets profoundly affect the oral microbiome. These behaviors can lead to significant shifts in the composition and diversity of microbial communities in the oral cavity, often with detrimental effects on oral health. Smoking is one of the most well-documented lifestyle factors that negatively impacts the oral microbiome. Smokers typically exhibit a decrease in beneficial microbial populations and an increase in pathogenic species, such as _Porphyromonas gingivalis_ and _Tannerella forystthia_, which are associated with periodontal disease and have been implicated in oral carcinogenesis [99]. The carcinogenic compounds in tobacco smoke can cause direct DNA damage and promote a pro-inflammatory environment, facilitating malignant transformation of OPMD to OSCC [100; 101; 102; 103]. Alcohol consumption also significantly alters the oral microbiome and is linked to an elevated risk of OSCC. Regular alcohol intake can reduce microbial diversity and increase the abundance of harmful bacteria, such as _Streptococcus mutans_, which is linked to dental caries [104]. Alcohol can disrupt the balance of the oral microbiome by altering the pH and providing a nutrient-rich environment for acidogenic and aciduric bacteria. Additionally, alcohol can impair salivary flow, further exacerbating microbial imbalances and increasing the risk of oral diseases [105; 106]. Additionally, alcohol metabolism produces acetaldehyde, a known carcinogen, which can accumulate in the oral cavity, especially in individuals with poor oral hygiene or those who smoke, further increasing cancer risk [103]. Drug use, particularly the use of recreational drugs like methamphetamine and opioids, can have severe consequences for the oral microbiome. These substances often lead to poor oral hygiene, dry mouth (xerostonia), and changes in the oral environment that favor the growth of pathogenic bacteria. [PERSON] et al. [107] have demonstrated that drug users have alterations in the oral microbiome and are more likely to suffer from periodontal disease and dental caries. The resulting chronic inflammation and tissue damage may increase the susceptibility to OPMD and OSC [103]. Dietary choices play a crucial role in modulating the diversity and composition of the oral microbiome. Diets high in sugars and refined carbohydrates promote the growth of caricogenic bacteria such as _Streptococcus mutans_ and _Lactobacillus_ spp., which can lead to dental caries [108]. Conversely, diets rich in fruits, vegetables, and fiber support a more diverse and balanced microbial community. These diets can increase the presence of beneficial bacteria, such as _Streptococcus sanguitis_ and _Lactobacillus gasseri_, which are associated with good oral health [109]. The Mediterranean diet, which is high in fruits, vegetables, nuts, and whole grains, has been shown to positively influence the oral microbiome by enhancing microbial diversity and promoting the growth of beneficial species [110; 111]. Similarly, probiotic-rich foods like yogurt and fermented vegetables can help maintain a healthy balance of oral bacteria, potentially reducing the risk of oral diseases such as OPMD and OSCC.