# _Legionella pneumophila_--Virulence Factors and the Possibility of Infection in Dental Practice
## 3 Virulence Factors
### Biofilm  
Nowadays, the increased use of drugs (especially antibiotics) has led to bacterial resistance, a significant problem in patient treatment [71]. The biofilm is one of the protective mechanisms that increases the resistance of bacteria to specific external agents [72; 73]. _Legionella_ spp. is characterized by the ability to survive in a biofilm and achieve high virulence and resistance, even after severe physical and chemical treatments [74]. In addition, many biological factors influence the persistence of the biofilm [75]. As such, the biofilmallows more excellent adhesion of specific bacterial species through various stages, from initiation to maturation of the biofilm and the formation of an extracellular matrix [74; 76; 77; 78]. Survival of _Legionella_ spp. under oligotrophic conditions with a lower content of organic matter and nutrients requires the incorporation of _Legionella_ into the microbial community with other bacteria [79; 80]. On this basis, it is possible to explain the increased numbers of _Legionella_ spp. in artificial habitats such as hot water systems [81]. Bacteria in these communities participate in interactions such as the food chain formation or the congregation continuation. The microorganisms in the biofilm may also repel other microbes that are unlikely to contribute to the community [58; 80].  
For _Legionella_ spp., biofilm formation is essential for survival [25]. Like many other microorganisms, it responds to environmental factors that significantly affect biofilm formation and/or colonization. Temperature is an influential agent affecting biofilm colonization and can affect the biofilm determinants produced by _Legionella_ spp. In vitro, monotypic biofilms at 37-42 \({}^{\circ}\)C consist of filamentous bacteria, but at 25 \({}^{\circ}\)C, they are thinner and dominated by rod-shaped bacteria. _Legionella_ cell length is associated with ppGpp signaling. A biofilm at 37 \({}^{\circ}\)C is much more solid than at 25 \({}^{\circ}\)C, and interestingly, at 25 \({}^{\circ}\)C, it is more prone to better adhesion potential [29; 82]. Specific genes may also regulate biofilm formation, such as the putative twin-arginine translocation pathway involving the _tatB_ and _tatC_ genes [83]. In the same case, expression of the _MIP_ gene has been found to promote biofilm creation at an early stage, when _Legionella_ spp. does not require a host for growth [83; 84].  
Quorum sensing (QS) in Gram-negative bacteria regulates the gene expression of various bacterial processes, including biofilm formation [85]. Bacteria that show QS signaling are usually found in artificial aquatic systems and may regulate biofilm production in the environment [86]. The QS autoinductor used by _Legionella_ pneumophila_ is LAI-1 (3-hydroxopentadecan-4-one), which produces and detects the Lqs system and contains the autoinductive synthase LqsA, homologous sensory kinase LqsS, and the LqsR response regulator [87]. The quorum sensing autoinducer (3-oxo-C12-HSL) possessing _P. aeruginosa_ inhibits _L. pneumophila_ biofilm formation [88]. This effect is associated with a decrease in LqsR. This suggests that QS could play a role in the dispersion of _L. pneumophila_ during the later stages of biofilm development [89]. Several biological factors such as microbial communities, temperature, and specific genes regulate _L. pneumophila_ biofilm production. These factors should suppress biofilm formation to reduce colonization in aqueous systems [74].