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An increasing microbial resistance to known antibiotics raises a demand for new antimicrobials. In this study the antimicrobial properties of a series of new N-Alkylpyridinium quaternary ammonium compounds (QACs) with varying alkyl chain lengths were evaluated for several nosocomial pathogens. The chemical identities of the new QACs were determined by NMR, LC-MS, and HRMS. All the planktonic bacteria tested were susceptible to the new QACs as evaluated by MIC and MBC assays. The antimicrobial effect was most pronounced against Staphylococcus aureus clinical isolates. Live/dead staining CLSM was used to test the effectiveness of the QACs in biofilms. The effectiveness was up to 10-fold lower than in the plankton. When QACs were used as irrigants in Er:YAG-SSP photoacoustic steaming, their effectiveness significantly increased. The combined use of irrigants and photoacoustic streaming increased biofilm removal from the surface and increased the killing rate of the cells remaining on the surface. This may allow for a shorter chemical exposure time and lower dosage of QACs used in applications. The results demonstrate that the new QACs have potential to be applied as antibacterial compounds effective against planktonic and biofilm bacteria as well as irrigants in removal of difficult-to-reach biofilms.

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<p><strong>Introduction:</strong> An increasing microbial resistance to known antibiotics raises the demand for new antimicrobials. New antibacterial agent should have a good activity against planktonic as well as biofilm bacteria. Quaternary ammonium compounds (QACs), are widely used in medicine, have proven antimicrobial properties, and are low toxic and low irritating. In this study new QACs were evaluated for their biofilm eradication efficiency as antibacterial compounds and as irrigants in combination with Er:YAG photoacoustic streaming.</p> <p><strong>Aims:</strong> To evaluate the effectiveness of new QACs against E. faecalis biofilms and to increase the effectiveness of QACs with laser treatment.</p> <p><strong>Method:</strong> The biofilm of E. faecalis were grown on titanium surface. The fraction of the dead cells and the biofilm surface coverage was determined with LIVE/DEAD<sup>TM</sup> using confocal microscopy (CLSM) before and after QACs treatment. To enhance to effectiveness the biofilm samples were pretreated with QACs followed by short laser Er:YAG photoacoustic streaming treatment.</p> <p><strong>Results: </strong>All tested QACs were effective against the E. faecalis biofilms. The best anti-biofilm compounds were N-Alkylimidazolinium derivatives. Compared to planktonic bacteria the bacteria in the biofilm were up to 10 fold more resistant. The fraction of the dead bacteria that were treated with QACs followed by Er:YAG photoacoustic streaming increased significantly compared to the chemical treatment alone. In addition, the biofilm surface coverage decreased after laser treatment.</p> <p><strong>Conclusions:</strong> The results suggest that new QACs have a great potential as antibacterial compounds effective against biofilms of E. faecalis. The laser treatment can significantly improve the effectiveness of QACs treatment.</p>