# _Legionella pneumophila_--Virulence Factors and the Possibility of Infection in Dental Practice
## 4 _Legionella pneumophila_ in Dental Practice  
Dental staff may be at high risk of _Legionella_ infection, and therefore, an occupational risk assessment is required. In addition to many dentists, other healthcare professionals in the dental clinic, such as dental assistants and hygients, are also exposed to the occupational risk of _Legionella_ infection. It is estimated that the occupational risk of _Legionella_ infection may affect 1 to 2 million healthcare professionals worldwide [104]. Based on research in dental offices conducted in 1986 in Austria, the presence of _Legionella pneumophila_ seogroup 1 was determined in 10% of water supply systems. The first death of a dentist due to Legionnaires' disease was in 1995, and _Legionella_ was discovered in the plumbing system of his office [104]. In 2012, an 83-year-old patient from Italy died of Legionnaires' disease, and the source of the infection was contaminated water in the dental office she visited [105]. In addition, in the same year, an elderly, immunocompromised man in Sweden died due to _Legionella_ in the cup filler outlet used for rinsing at the dental ward [106]. _Legionella pneumophila_ is fatal in many cases [20]. However, the cause of the fatal outcome related to the dental practice was determined only in the patients shown. According to a study by [PERSON] et al., antibodies to _Legionella_ were significantly higher in medical and dental professionals than in non-professionally exposed subjects [107]. The possibility of contamination in a dental unit water system with microorganisms has been discussed since the beginning of dental chair use. Due to the constant exposure of patients and staff in the dental team to the aerosol produced during operations, the microbial quality of the water is critical. Water supply systems might contain opportunistic and pathogenic  
Figure 2: The life cycle of _Legionella pneumophila_. _Legionella_ reproduces only inside other cells. Bacterial-feeding amoebae also live in the environment where _Legionella_ is found. After the _Legionella_ is eaten by the amoeba, it is encapsulated inside the amoeba, where it continues to grow and multiply. By releasing _Legionella_ bacteria from the amoeba, they can disperse into the environment and form a new biofilm with other bacteria, or humans can inhale them. In humans, this cycle is repeated, but in this case, the human lung cells are infected. bacteria, mostly Gram-negative species that pose a particular risk in immunocompromised individuals [108; 109]. The water systems of the dental unit can be initially contaminated through water coming into the system or by pulling and sucking saliva and other fluids from the patient's mouth. Dental chairs can receive water through a public water supply network or special tanks built into the chair to pour liquid. Other factors that might influence this are the work unit's model and the time of its construction, whether there is a built-in system against the return flow of patient fluids, infection prevention measures, used disinfection methods, etc. [109; 110]. The guidelines for the allowable number of bacteria in the dental unit's water are different and mostly coincide with the number of microorganisms allowed in the drinking water. In Europe, this number is up to 100 colony-forming units per milliliter of water (CFU/mL). The American Dental Association (ADA) set the allowable number of microorganisms in the water for dental supply at \(\leq\)200 CFU/mL, while the Centers for Disease Control and Prevention (CDC) recommended that the number be \(\leq\)500 CFU/mL [109; 110; 111; 112; 113; 114]. The risk of infection arises because most instruments that are necessary for work in dentistry, such as micromotors, turbines, sonic and ultrasonic scalers, water/air syringes, etc., produce an aerosol in the inhalation zone. The presence of _Legionella_ spp. in saliva and dental plaque biofilm has also been shown [115; 116].