Fluid dynamics of syringe-based irrigation to optimise anti-biofilm efficacy in root-canal disinfection

A mature bacterial biofilm is composed of multiple layers of bacteria embedded in a self-made matrix formed of extracellular polymeric substance. This substance has the potential to modify the response of the resident bacteria to antimicrobials by acting as a shield against the chemical effects of antimicrobials. There is also a localised high density of bacterial cells in a biofilm structure. This spatial arrangement will expose the cells in the deeper layers of the biofilm to less nutrients and redox potential than the cells on the biofilm surface. Since the degree of nutrient and gas gradients increases with the thickness and maturity of a biofilm, the influence of growth rate and oxygen on the antimicrobial resistance is particularly marked in aged biofilm. The resistance associated with biofilm bacteria is further associated with the slow growth rate (starvation) and/or due to the adoption of resistant phenotypes in bacteria residing in a biofilm. It is recognised that no single mechanism may account for the general resistance to antimicrobials in a biofilm. It is apparent that different mechanisms may act in concert within the biofilm, and amplify the effect of small variations in the susceptible phenotypes (Dunne et al. 1993; Costerton et al. 1994). Thus from a clinical perspective, bacteria are observed to demonstrate considerably high resistance to antimicrobials when they are in a biofilm (Kishen 2012).