Date Published: June 21, 2018
Publisher: Public Library of Science
Author(s): Daniel Pletzer, Sarah C. Mansour, Robert E. W. Hancock, Michael R Yeaman.
With the antibiotic development pipeline running dry, many fear that we might soon run out of treatment options. High-density infections are particularly difficult to treat due to their adaptive multidrug-resistance and currently there are no therapies that adequately address this important issue. Here, a large-scale in vivo study was performed to enhance the activity of antibiotics to treat high-density infections caused by multidrug-resistant Gram-positive and Gram-negative bacteria. It was shown that synthetic peptides can be used in conjunction with the antibiotics ciprofloxacin, meropenem, erythromycin, gentamicin, and vancomycin to improve the treatment outcome of murine cutaneous abscesses caused by clinical hard-to-treat pathogens including all ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter cloacae) pathogens and Escherichia coli. Promisingly, combination treatment often showed synergistic effects that significantly reduced abscess sizes and/or improved clearance of bacterial isolates from the infection site, regardless of the antibiotic mode of action. In vitro data suggest that the mechanisms of peptide action in vivo include enhancement of antibiotic penetration and potential disruption of the stringent stress response.
ESKAPE pathogens (E. faecium, S. aureus, K. pneumoniae, A.baumannii, P. aeruginosa, E. cloacae) are recognized to be responsible for the majority of difficult-to-treat community-acquired, healthcare-associated, and nosocomial infections . Multidrug-resistant bacteria represent major therapeutic challenges and pose a great threat to human health . The increasing resistance to available antibiotics dampens treatment possibilities and there is a serious lack of adequate treatment options. Less discussed but of even greater concern are infections associated with high bacterial densities (>107 CFU/ml bacteria) especially biofilm and/or abscess infections. High bacterial densities lead to elevated MICs to multiple antibiotics  and are extremely difficult to treat with antibiotics . In this context, skin and soft tissue infections (SSTIs) are an emerging problem, a significant burden in health care facilities, and responsible for increased antibiotic administration . SSTIs such as abscesses form fluid, pus-filled pockets infiltrated by bacteria and immune cells , and are often highly resistant to antibiotic treatment. Indeed, abscesses are the most common indication for frequent (6–12 h), high-dose (up to 1 g/kg) and long term (>5 d)  intravenous (IV) broad-spectrum antibiotic administration . SSTIs have been traditionally thought to be largely caused by S. aureus and Streptococcus pyogenes but recent findings show that other microbes are very prevalent [8, 9]. Indeed, the SENTRY antimicrobial surveillance program (North America)  reported that the major pathogens isolated from SSTIs now include 10.8% P. aeruginosa, 8.2% Enterococcus sp., 7.0% E. coli, 5.8% Enterobacter sp., and 5.1% Klebsiella sp., as well as 45.9% S. aureus. Moreover, recently A. baumannii is increasingly recognized as an emerging cause of nosocomial infections and important cause of severe, life-threatening soft tissue infections . High bacterial numbers of greater than 108 CFU/ml isolated bacteria are present in soft-tissue and peritoneal infections , highlighting the importance of investigating high-bacterial density infections. However, standard in vitro susceptibility tests employ modest bacterial concentrations of 2–5 x 105 per ml which critically underestimates the strong impact on antibiotic susceptibility of the high concentrations of bacteria in such infections . Thus, it remains a major challenge to translate in vitro findings into in vivo efficacy and compounds that show excellent in vitro activity (e.g., low MIC in defined medium), often work poorly when tested under in vivo conditions.
To investigate abscess infections caused by the ESKAPE pathogens and E. coli, we extrapolated from our previously-developed cutaneous mouse infection model , prioritizing the study of resistant, recalcitrant host-adapted pathogens rather than commonly used laboratory strains. We identified clinical isolates that were able to cause chronic skin abscesses on the backs of CD-1 female mice after injection of a high bacterial dose (≥ 107 bacteria); each of these strains persisted throughout the course of a three day experiment and did not cause mortality in mice. MIC assays revealed that these strains had generally low antibiotic susceptibility and were resistant to antibiotics from at least three different classes (Table 1, S1 Table). Plasmid-encoded bioluminescently-tagged isolates were created to enable visualization and monitoring of the progress of disease using non-invasive techniques, and to provide evidence that the skin infection contained metabolically active bacteria; this enabled us to follow the infection for all strains (Fig 1).