Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen, which forms biofilms on different surfaces such as implanted medical devices and tissues causing severe and chronic infections. Biofilms are 1000 times more resistant to antibiotics than bacterial cells in planktonic form and because of this, infections caused by biofilms are a major health problem and treatments often fail. However, recent studies have shown that P. aeruginosa planktonic cells undergo a rapid conversion from the normal rod shape to a cell wall defective (CWD) spherical morphology when exposed to β-lactam antibiotics. Moreover, the addition of antimicrobial peptides (AMPs) to β-lactam-induced CWD cells has an effective and rapid killing effect.
Based on these findings, we want to explore whether P. aeruginosa biofilms harbour CWD cells upon exposure to β-lactam antibiotics and consequently whether they can be killed by the activity of AMPs. Static biofilms were cultured in WPI dishes for 24h and treated with 5xMIC meropenem (10mg/mL) for 24h. The biofilms were stained with the fluorescent membrane stain FM1-43 to visualise the morphology of bacterial cells using confocal scanning laser microscopy (CSLM). Subsequently, the activity of meropenem combined with the antimicrobial peptide nisin was tested for the determination of possible concentrations that could eradicate the biofilm by using a checkerboard assay.
These assays have shown that meropenem-treated biofilms exhibit micro-colonies with spherical cells after 24h treatment. In addition, meropenem-nisin combinations show eradication of Pseudomonas biofilms. Considering these observations, it may be possible to develop a combined therapy that exploits this β-lactam antibiotic tolerance mechanism (i.e. en masse transition into CWD bacteria) to eradicate P. aeruginosa biofilms. Therefore, this approach could be developed as a novel therapeutic treatment for P. aeruginosa infections in an era where current therapies fail.