Colonization of Legionella spp. In Dental Unit Waterlines
Legionella spp is a gram-negative bacterium that is ubiquitous in aquatic habitats, dental water lines are not exempt from their colonization. Legionella has been found in municipal water sources even after chlorination and filtration. This knowledge puts emphasis on the necessity to monitor dental unit waterlines (DUWL). In an article published in March 2018 in the Journal of Biological Regulators & Homeostatic Agents, the authors discuss the need to maintain and monitor DUWL.1
As mentioned, Legionella is found in municipal water sources, once it has reached a building’s water system it settles into a biofilm. This biofilm layer allows Legionella to protect itself from antimicrobial agents, due to this protection Legionella can multiply. The primary route of exposure to Legionella is through inhalation, making aerosols a concern.
Bio-aerosols produced during dental procedures pose a possible source for Legionella to be inhaled, this can lead to Legionnaire’s disease and Pontiac fever. Studies have confirmed that dental surgeons, nurses, and hygienists have raised levels of antibodies to Legionella. The small tubing of DUWL creates an optimal environment for Legionella species to attach and grow. In certain circumstances, amoeba will increase their uptake of bacteria providing a continuous supply of bacteria that can be released into the environment. The recognition of amoeba as a reservoir brings to light public health concerns.
Aerosols produced during dental treatment contain droplet nuclei particles which can remain in the environment for extended periods of time. These aerosols can be a source of infection, this calls to attention the need to monitor DUWL. Certain culturing methods can lead to a false-negative or underestimated bacterial count. PCR methods are preferred and can detect unculturable Legionella, those living with amoeba and doublets or chains of Legionella cells. Chains of Legionella cells are counted as 1 CFU when using the culture method, while PCR testing counts each individual cell giving a more accurate count.
Studies have indicated dentists and dental staff have a higher incidence of respiratory infections than the general public. Contaminated handpieces have been implicated as one of the contributing factors. The implementation of decontaminating dental handpieces was an attempt to reduce the likelihood of aerosol dissemination of bacteria. However, the decontamination of handpieces does not remove the risk of exposure if DUWL is not maintained. Exposure to pathogens can originate in the water lines and be disseminated through the decontaminated handpiece.
A dentist in Northern California died from Legionella infection, it was determined he acquired the infection from the dental unit. The American Dental Association has set the limit for heterotrophic bacteria at <200 cfu/mL, while the CDC guidelines for Infection Control in Dental Healthcare Settings suggests meeting the nationally recognized standard of <500 cfu/mL. The European and national directives standards are much more conservative, which are set at <100 cfu/mL.
The authors conclude by stating, “Clinicians can apply some preventative methods, as follows: flushing through the chair for 3 min at the start of each day; supplying the dental unit reservoir with sterile and good quality water; autoclaving dental handpieces after each use; regular maintenance of the dental unit system and regular chlorination of the water system of the dental clinic.”
- Carinci F, Scapoli L, Contaldo M, et al. Colonization of Legionella spp. In dental unit waterlines. J Biol Regul Homeost Agents. 2018;32(2 Suppl. 1):139-142.