Bacteria in wet environments interact with solid surfaces such as dental unit waterlines (DUWL) or teeth to form a tenacious, adherent biofilm, which in the mouth is more commonly known as dental plaque.


A biofilm has similar properties wherever it is formed. They are comprised of a complex community of bacteria and other microbes living together within an extracellular polysaccharide matrix.

Bacteria grow in biofilms rather than existing as a free-living form (planktonic form) as a strategy to aid their survival in harsh environments such as the mouth or DUWL.

In the mouth, bacteria in the absence of a matrix would be washed away by saliva. In the case of DUWL bacteria freely suspended in the water are removed by flushing, and destroyed readily by disinfectants.

The practical impact for the dental practice of biofilm formation is that bacteria growing within biofilms are more resistant to treatment (disinfectants, antibiotics, ultraviolet light, metal toxicity, acid exposure, dehydration and phagocytosis by cells of the human immune system) than their corresponding free-living planktonic cells.

Bacteria in biofilms require 100 to 1,000 times the concentration of a disinfectant to destroy them. 

Biofilm Formation and Properties

Biofilm formation occurs in a series of defined stages. Biofilms are structured and contain a succession of different bacteria.

Stages of biofilm formation:

  • Pellicle formation (organic molecules derived from dental water or saliva that aid bacterial adhesion)
  • Adhesion of pioneer species of bacteria that produce attachment structures (fimbriae)
  • Replication and recruitment of other species
  • Matrix formation
  • Maturation

Oxygen and nutrient gradients develop within the mature biofilm, and organisms with differing growth requirements are able to take advantage of these microhabitats.

A number of complex interactions and genetic mutations occur within the biofilm that are resistant to antibiotics or disinfectants.

Bacteria living within the biofilm community are able to exchange genetic material including virulence factors that can make them resistant to antibiotics and disinfectants. Resistant strains of bacteria can persist and multiply within the biofilm.

Bacterial cells within a biofilm population use molecular signals in order to communicate with each other. This allows them to sense their density and number (quorum sensing), which is an important component of biofilm formation, development and stability, and reduces competition for nutrients. 

Management of Biofilms in DUWL

In DUWLs the biofilm presents more of a cross infection problem than the planktonic bacteria floating free in the waterline.

The biofilm provides a continuous source of bacteria that are shed from the surface. Once a new dental unit system is connected to mains water supply a biofilm is able to form within 8 hours and the microbes are embedded in a protective mature matrix within 6 days.

To override the protective properties of the matrix, higher concentrations of biocide disinfectants are required than would be needed if the bacteria were growing in their planktonic form.

Biocide disinfectants have to be used either continuously or on a regular (daily) basis to prevent biofilm re-formation in the DUWL.

In the mouth, the situation is much easier as the biofilm is readily removed by daily tooth brushing which mechanically removes the biofilm. This option is not available in DUWL as the tubing is too narrow.