Our study is the first to utilize TEM and SEM to document biofilm formation and investigate the bactericidal effects of PAA. Our in vitro biofilm models exhibited mature biofilm development inside a narrow catheter lumen, as reported previously, thus mimicking biofilm formation inside the endoscope channel [
23]. Notably, different HLDs presented varying bactericidal effects on
S. aureus and
P. aeruginosa biofilms in our in vitro biofilm models. Accordingly, PAA had the most rapid bactericidal activity (< 1 min), in line with a report by Tote et al. [
24]. In contrast, OPA required >10 min and 30 min to completely eradicate
S. aureus and
P. aeruginosa biofilms, respectively. These results indicate that
P. aeruginosa shows reduced susceptibility to OPA due to overproduction of biofilm matrix components [
25]. The
P. aeruginosa biofilm is comprised of bacteria embedded in a matrix of extracellular polymeric substances, which functions as both a structural scaffold and/or as a protective barrier against harsh environments. Alginate or other polysaccharides have been identified as the main matrix ingredients in
P. aeruginosa biofilms, with important roles in structural maintenance and AMR [
2]. Additional compounds affecting biofilm matrix and tolerance of
P. aeruginosa biofilms to OPA are currently being investigated. Polysaccharide intercellular adhesin has also been described as a major proteinaceous component of the
S. aureus biofilm matrix, which is also rich in teichoic acids [
4]. Other cellular components are likely to be present, and await further investigation. To address a study limitation, our future work will investigate the influence of HLDs on several known biofilm matrices from other bacterial strains. However, as disinfection exposure time recommended by guidelines for endoscope reprocessing may differ by biofilm matrix, as indicated by testing OPA in this study, it would be necessary to select an HLD with stronger and fast-acting bactericidal effects, such PAA, in practical use.
Although mature
S. aureus and
P. aeruginosa biofilms are characterized by production of matrix components that hinder biofilm killing, PAA appears to be an effective disinfectant. TEM and SEM observations enabled a comparative analysis of the mechanism of action of PAA, revealing changes in shape after 5 min and structural damage after 30 min. In the current report, mature
P. aeruginosa biofilms aged 96 h were eradicated at 3000 ppm (0.3%) of PAA after 5-min exposure [
26]. Our findings indicate that the exposure time for effective bactericidal activity is 5 min under high concentrations of HLD, e.g., 0.3% PAA.
These alterations might be caused by a sharp decrease in bacterial inner pressure resulting from high permeability to PAA for a short period. Additionally, PAA is thought to act as an oxidizing agent by releasing hydroxyl radicals, which subsequently attack essential biofilm matrix components [
27‐
29]. To date, no PAA-resistance mechanisms have been reported for either cell suspensions or biofilms. Here, spectroscopic measurements of bacterial survival correlated well with TEM and SEM observations of biofilm structures and surfaces. This in vitro biofilm model associated with electron microscopy represents an effective tool to investigate the mechanistic action of disinfectants such as PAA. Future work on biofilm formation should help elucidate the nature of interactions under changing environmental conditions (e.g., pH) or in the presence of plasma for various durations of exposure.
In summary, we found that PAA is a useful HLD for endoscope channel reprocessing, even in the presence of strongly adhering bacterial biofilms. The current developments should allow for shorter exposure time for effective bactericidal activity during endoscope reprocessing in healthcare settings. This is expected to enable the prevention of endoscopy-related infections resulting from potential contamination by S. aureus and P. aeruginosa biofilms.