Infections involving antibiotic resistant Staphylococcus aureus (S. aureus) represent a major challenge to successful treatment. Further, although bacteriophages (phages) could be an alternative to antibiotics, there exists a lack of correlation in phage susceptibility results between conventional in vitro and in vivo assays. This discrepancy may hinder the potential implementation of bacteriophage therapy. In this study, the susceptibility of twelve S. aureus strains to three commercial phage cocktails and two single phages was assessed. These S. aureus strains (including ten clinical isolates, five of which were methicillin-resistant) were compared using four assays: the spot test, efficiency of plating (EOP), the optical density assay (all in culture media) and microcalorimetry in human serum. In the spot test, EOP and optical density assay, all cocktails and single phages lysed both methicillin susceptible and methicillin resistant S. aureus strains. However, there was an absence of phage-mediated lysis in high concentrations of human serum as measured using microcalorimetry. As this microcalorimetry-based assay more closely resembles in vivo conditions, we propose that microcalorimetry could be included as a useful addition to conventional assays, thereby facilitating more accurate predictions of the in vivo susceptibility of S. aureus to phages during phage selection for therapeutic purposes.

Burn wounds are a major burden, with high mortality rates due to infections. Staphylococcus aureus is a major causative agent of burn wound infections, which can be difficult to treat because of antibiotic resistance and biofilm formation. An alternative to antibiotics is the use of bacteriophages, viruses that infect and kill bacteria. We investigated the efficacy of bacteriophage therapy for burn wound infections, in both a porcine and a newly developed human ex vivo skin model. In both models, the efficacy of a reference antibiotic treatment (fusidic acid) and bacteriophage treatment was determined for a single treatment, successive treatment, and prophylaxis. Both models showed a reduction in bacterial load after a single bacteriophage treatment. Increasing the frequency of bacteriophage treatments increased bacteriophage efficacy in the human ex vivo skin model, but not in the porcine model. In both models, prophylaxis with bacteriophages increased treatment efficacy. In all cases, bacteriophage treatment outperformed fusidic acid treatment. Both models allowed investigation of bacteriophage-bacteria dynamics in burn wounds. Overall, bacteriophage treatment outperformed antibiotic control underlining the potential of bacteriophage therapy for the treatment of burn wound infections, especially when used prophylactically.

Left-ventricular assist devices (LVADs) are increasingly used as a bridge to heart transplantation and destination therapy. These devices, especially the driveline, are susceptible to difficult-to-treat infections, associated with high morbidity and mortality rates. Staphylococcus aureus ( S. aureus ) is a major causative pathogen of LVAD infections. Antibiotic resistance and biofilm formation can complicate the treatment of these infections. A novel in vitro assay was developed to study the antibiotic susceptibility of S. aureus biofilm grown on LVAD drivelines. Besides antibiotic monotherapy, the effect of various antibiotics combined with rifampicin was studied. Additionally, we explored the efficacy of four individual phages and phage-antibiotic combinations as potential treatment strategies. Our data showed a decrease of susceptibility of the S. aureus biofilms to antibiotic monotherapy compared to planktonic S. aureus . With only rifampicin and erythromycin monotherapy resulting in full bacterial clearance. Combining antibiotics with rifampicin showed similar antimicrobial efficacy against S. aureus biofilms as rifampicin monotherapy. While both individual phages and a phage cocktail were effective against planktonic bacteria, phage efficacy was limited against S. aureus in biofilm. Combining phages with antibiotics did not clearly improve treatment efficacy, compared to antibiotic monotherapy. Contrarily, it even increased bacterial growth when phage administration preceded antibiotic treatment. Here, both antibiotic- and phage monotherapy showed reduced efficacy on LVAD-driveline biofilms. Additionally, phages did not show an additive value to antibiotic treatment of LVAD driveline infections. Further studies are needed to elucidate optimal treatment strategies for LVAD-driveline infections. IMPORTANCE: Current treatment strategies for S. aureus LVAD-driveline infections are based on in vitro antibiotic susceptibility of planktonic bacteria. However, LVAD infections are most often biofilm-related, which decreases antibiotic susceptibility significantly, resulting in discrepancies between in vitro antibiotic susceptibility and in vivo treatment success. Here, we have developed a novel in vitro assay to determine antibiotic susceptibility of S. aureus biofilm, grown in conditions relevant to LVAD-driveline infections. Next to antibiotic susceptibility, the susceptibility of this biofilm to bacteriophage mono- and combination treatment with antibiotics was evaluated as an alternative treatment strategy. In the future, this assay can be used to provide a better insight in in vivo antibiotic- and bacteriophage susceptibility of LVAD-driveline biofilms. Thereby improving in vivo treatment strategies for LVAD-driveline infections.