Post-surgical site infections place a significant burden on health provision worldwide, and one of the major causative pathogens of such infections is Staphylococcus aureus. It is estimated that there are approximately 150,000 post-surgical infections in the US alone per year, representing a health burden and cost of $10 billion per year (Zabaglo and Sharman, 2022; Seidelman et al., 2023). S. aureus is the pathogen responsible for most surgical site infections (SSIs) with approximately 80% of S. aureus post-surgical infections auto infected from S. aureus carried within the patient’s nose (von Eiff et al., 2001; Wertheim et al., 2004). As a result, many current guidelines recommend the nasal decolonization of patients prior to surgery (Sway et al., 2018; Bratzler et al., 2013; Engelman et al., 2019; Ling et al., 2019; National Institute for Health and Care Excellence, 2019). A number of key surgical guidelines in the US, Europe, and other countries recommend preventative treatment of all S. aureus in patients undergoing high risk surgeries (Bratzler et al., 2013; Sway et al., 2018; Engelman et al., 2019; Ling et al., 2019; National Institute for Health and Care Excellence, 2019). It is recognized that, although screening for S. aureus, including methicillin-resistant S. aureus (MRSA) carriage is effective in identifying the majority of carriers, screening has a number of drawbacks, including cost, time, isolation while awaiting screening result and an inability to identify all carriers due to assay sensitivity and the impact of false positives. A study comparing the impact of universal decolonization (UD) versus targeted decolonization or active surveillance in a high risk patient population (ICU patients) demonstrated that UD decolonization was superior to the other arms, resulting in a 37% reduction in MRSA clinical isolates and a 44% reduction in all-cause bloodstream infections (Huang et al., 2013). As a result, the 2022 SHEA/IDSA/APIC Practice Recommendation update (Popovich et al., 2023) recommends the use of UD for all patients in adult ICUs and that UD should also be considered to be used in neonatal ICUs, burns units and hemodialysis patients.

At present, the most widely used intranasal decolonization agent is mupirocin, a bacteriostatic antibiotic that, due to its high potential to generate resistance, is not recommended for multiple administration (GlaxoSmithKline, 1987). The global prevalence of mupirocin resistance in S. aureus strains has risen to 7.6% and when looking at mupirocin resistant MRSA, the rate increases to 13.8%. (Dadashi et al., 2020) Although universal decolonization using mupirocin has in some studies been shown to be superior to targeted decolonization (Huang et al., 2013) there is an underlying concern that this approach may drive the more rapid development of mupirocin resistant staphylococcal strains to evolve and undermine the ability to deliver safe and effective infection prophylaxis and mupirocin resistance remains an unresolved issue in the 2022 SHEA/IDSA/APIC Practice Recommendation update (Popovich et al., 2023). There is an urgent need to identify and adopt the use of new antimicrobial agents, which can address resistance, thus allowing long-term and widespread effective decolonization.

XF-73 is a novel, synthetic di-cationic porphyrin derivative with rapid, potent Gram positive bactericidal properties which is being developed clinically as a gel for the nasal decolonization of S. aureus to prevent post-operative staphylococcal infections. XF-73 has been awarded Qualifying Infectious Disease Product status (Link) and Fast Track designation (Link) by the US Food and Drug Administration, identifying it as a product that is seeking to address a priority infectious disease. A recent Phase 2 study has been completed (Mangino et al., 2023) investigating the safety and microbiological efficacy of XF-73 in reducing S. aureus nasal carriage prior to open chest cardiac surgery, which demonstrated that patients administered intranasal XF-73 gel within 24 hours of cardiac surgery had significantly reduced nasal burden of S. aureus one hour before surgery compared to a placebo control.

The antimicrobial properties of XF-73 have been investigated and found to have broad spectrum activity against all Gram positive bacteria tested to date, including methicillin-sensitive S. aureus (MSSA) and antibiotic resistant strains such as MRSA and mupirocin-resistant S. aureus isolates (Farrell et al., 2010). XF-73 has also been demonstrated to be active against cultures of S. aureus in stationary phase, non-growing cultures expressing the stringent response and in cold cultures, demonstrating that antimicrobial activity is not dependent on metabolic or growth activity (Ooi et al., 2010). XF-73 was found to have a low propensity to generate resistance, with no mutational resistance observed after 55 serial passages at sub-inhibitory concentrations (Farrell et al., 2011; MacLean, 2020) whilst mupirocin, which was also included in the same study, rapidly generated mutational resistance in all four MRSA isolates tested, with MICs up to 512 µg/ml generated. Importantly, there was no evidence of cross-resistance with XF-73 in these mupirocin resistant isolates generated (Farrell et al., 2011). Furthermore, the antimicrobial activity of XF-73 has been demonstrated to be independent of all known existing antimicrobial resistance mechanisms (Farrell et al., 2010). The presence of existing cell wall, DNA and protein synthesis resistance mechanisms were not found to affect the susceptibility of antibiotic-resistant bacteria to XF-73 compared to antibiotic-sensitive isolates, suggesting that the mechanism of action (MOA) of XF-73 is novel. Investigations into the MOA of XF-73 has demonstrated that XF-73 exerts a rapid bacterial membrane-perturbing activity resulting in substantial loss of potassium and adenosine triphosphate from the cells, complete inhibition of DNA, RNA and protein synthesis but without inducing bacterial lysis (Ooi et al., 2009).

Here, we report the antimicrobial susceptibility of over 2,500 Coagulase-positive and -negative Staphylococcus species clinical isolates to XF-73. The majority of isolates tested were S. aureus but the panel also included examples of 15 other Staphylococcus species, including ones that have been previously reported to be isolated from the nose and examples of pathogens responsible for infections including endocarditis and bacteremia. The Staphylococcus species panel was constructed to included antibiotic resistant and sensitive isolates to further investigate the effect of existing antibiotic resistance mechanisms on the activity of XF-73.

SSIs are among the most common healthcare-associated infections and are associated with longer post-operative hospital stays, additional surgical procedures, treatment in intensive care units and higher mortality. Staphylococcus species are responsible for a substantial percentage of SSIs and therefore are a major focus in the prevention of SSIs. A recent European Centre for Disease Prevention and Control Report from 2023 identified that S. aureus and Coagulase-negative staphylococci were reported in 29.2% of all SSIs in a survey of 7,816 SSIs from 11 countries (ECDC, 2023).

The results reported in this study demonstrate that XF-73 has similar antimicrobial activity against all of the staphylococcal species tested, irrespective of the existing antibiotic resistance profile. A screen of 3,929 clinical isolates of MRSA and MSSA from 12 different countries against linezolid, tigecycline and vancomycin reported MIC₅₀ and MIC₉₀ values (Karlowsky et al., 2017) that were either identical, or within one MIC dilution factor, to those reported in this study. Similarly, another screen against 756 MRSA and MSSA clinical isolates obtained from across Europe and Russia against the same three antibiotics (Riccobono et al., 2022) also reported similar MIC₅₀ and MIC₉₀ ranges. The manuscript by Riccobono et al. (2022) also report MIC₅₀ and MIC₉₀ values for the same three antibiotics against 218 Coagulase negative Staphylococcus species isolates, which are also either identical, or within one MIC dilution factor, to those reported in this study. The comparator antibiotic MIC₅₀ and MIC₉₀ results in this study are therefore similar to other results reported in the literature.

The data presented here expands significantly on the antimicrobial activity of XF-73 against staphylococcal isolates previously reported (Farrell et al., 2010). In total, over 2,500 staphylococcal clinical isolates were tested within this study consisting of 16 different Staphylococcus species from 33 different countries and isolated from 5 different clinical presentations. The MIC range for XF-73 against all of the Staphylococcus species was within a narrow range (≤0.12 – 4 µg/ml) with an MIC₅₀ and MIC₉₀ (where determined) of 0.5 and 1 µg/ml respectively, demonstrating that XF-73 has potent antimicrobial activities against all 16 Staphylococcus species tested.

The Staphylococcus species isolates were carefully selected to include antibiotic sensitive and resistant isolates, including MRSA; MSSA; antibiotic-susceptible and resistant Staphylococcus epidermidis and examples of Staphylococcus species clinical isolates resistant to cell wall synthesis inhibitors (β-lactams, carbapenems, glycopeptides and cephalosporins), protein synthesis inhibitors (oxazolidinones, macrolides and tetracyclines) and DNA synthesis inhibitors (fluoroquinolones) and folate synthesis inhibitors (antifolate). The results demonstrate that all pre-existing antibiotic resistance mechanisms were not found to impact the antimicrobial activity of XF-73. The inclusion of MDR S. aureus isolates in the test panel confirmed that XF-73 has equivalent activity even against these highly antibiotic resistant isolates, which demonstrates that nasal decolonization using XF-73 is likely to be equally effective even when such S. aureus isolates are present in the nares. Such an attribute could disrupt the spread of such resistant isolates, as well as reducing the incidence of nasal decolonization treatment failures.

The broad anti-staphylococcal activity of XF-73, which includes 12 Staphylococcus species that have been reported to be isolated from the nose suggests that in addition to the ability of XF-73 to significantly reducing the nasal burden of S. aureus which has been demonstrated in clinical studies, including those in patients (Mangino et al., 2023), the application of XF-73 to the anterior nares is also likely to significantly reduce the nasal burden of all Staphylococcus species that may be present, providing additional protection against surgical infections caused by other Staphylococcus species. The mutational resistance profile for XF-73 has been previously demonstrated to be superior to that of mupirocin in in vitro multi-passage studies (Farrell et al., 2011), and a recent study has demonstrated that XF-73 has equal in vitro and in vivo antimicrobial activity against S. aureus isolates that are mupirocin-sensitive, low-level (MIC 16 µg/ml) and high-level (4,096 µg/ml) mupirocin resistant (Zhang et al., 2023), therefore XF-73 is a potential new candidate for nasal decolonization and the clinical trials undertaken to date have demonstrated an appropriate safety profile and rapid reduction in S. aureus nasal load (Yendewa et al., 2020; Mangino et al., 2023).

The attributes of XF-73 also suggest that it could have utility for the treatment of a range of S. aureus infections, and XF-73 is also currently being developed for the treatment of superficial skin infections and for the prevention and treatment of serious infections associated with burns and open wounds such as diabetic foot ulcers.

These findings support the continued clinical development of XF-73 as an effective anti-staphylococcal drug.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

WR: Conceptualization, Project administration, Supervision, Writing – original draft, Writing – review & editing. HG: Formal Analysis, Writing – review & editing. WL: Conceptualization, Funding acquisition, Writing – review & editing.