The team have previously demonstrated that a photoactivatable therapeutic approach employing antibiotic-loaded, antibody-conjugated, polydopamine (PDA)-coated gold nanocages (AuNCs) could be used for the synergistic killing of bacterial cells within a biofilm. The approach was validated with a focus on Staphylococcus aureus using an antibody specific for staphylococcal protein A (Spa) and an antibiotic (daptomycin) active against Gram-positive cocci including methicillin-resistant S. aureus (MRSA). However, an important aspect of this approach is its potential therapeutic versatility.
In this report, the team evaluated this versatility by examining the efficacy of AuNC formulations generated with alternative antibodies and antibiotics targeting S. aureus and alternative combinations targeting the Gram-negative pathogen Pseudomonas aeruginosa.
Material & Methods Photothermal microscopy was performed as previously described, Briefly, a custom built platform based on an inverted Olympus IX73 using a 3-wavelength Wavelength Division Multiplexer was used to combine 488 nm, 532 nm from our (Laserglow Technologies) line and 635 nm laser beams into a single mode fibre. High resolution confocal fluorescence and PTM imaging were carried out simultaneously by steering laser beams using galvo-mirrors across the sample. Probe beam intensity was collected using 40× objective located above the sample and measured by amplified photodiode. PT signal was measured using a digital lock-in amplifier and recorded using custom software developed on the LabView platform. Conventional fluorescent imaging was carried out using a CCD camera DP80. PT and fluorescence images were merged and PT signal was quantified pixel-by-pixel using ImageJ software. Each AuNC sample was run in triplicate and 95% confidence intervals were obtained for labelling efficiency and signal intensity relative to a control of a sample of bacteria alone.
Results & Conclusions The results confirmed that daptomycin-loaded AuNCs conjugated to antibodies targeting two different S. aureus lipoproteins (SACOL0486 and SACOL0688) also effectively kill MRSA in the context of a biofilm. However, the results also demonstrate that antibiotic choice is critical. Specifically, ceftaroline and vancomycin-loaded AuNCs conjugated to anti-Spa antibodies were found to exhibit reduced efficacy relative to daptomycin-loaded AuNCs conjugated to the same antibody. In contrast, gentamicin-loaded AuNCs conjugated to an antibody targeting a conserved outer membrane protein were highly effective against P. aeruginosa biofilms.
These results confirm the therapeutic versatility of the approach taken by the team. However, to the extent that its synergistic efficacy is dependent on the ability to achieve both a lethal photothermal effect and the thermally controlled release of a sufficient amount of antibiotic, they also demonstrate the importance of carefully designing appropriate antibody and antibiotic combinations to achieve the desired therapeutic synergy.
Full access to the materials and methodology, can be found by clicking here.
Details on the 473 nm NIR Laser used in the research can be found by clicking here.