BackgroundStaphylococcus aureus and Pseudomonas aeruginosa are often found together in the airways of cystic fibrosis (CF) patients. It was previously shown that the P. aeruginosa exoproduct 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) suppresses the growth of S. aureus and provokes the emergence of small-colony variants (SCVs). The presence of S. aureus SCVs as well as biofilms have both been associated with chronic infections in CF.ResultsWe demonstrated that HQNO stimulates S. aureus to form a biofilm in association with the formation of SCVs. The emergence of SCVs and biofilm production under HQNO exposure was shown to be dependent on the activity of the stress- and colonization-related alternative sigma factor B (SigB). Analysis of gene expression revealed that exposure of a prototypical S. aureus strain to HQNO activates SigB, which was leading to an increase in the expression of the fibronectin-binding protein A and the biofilm-associated sarA genes. Conversely, the quorum sensing accessory gene regulator (agr) system and the α-hemolysin gene were repressed by HQNO. Experiments using culture supernatants from P. aeruginosa PAO1 and a double chamber co-culture model confirmed that P. aeruginosa stimulates biofilm formation and activates SigB in a S. aureus strain isolated from a CF patient. Furthermore, the supernatant from P. aeruginosa mutants unable to produce HQNO induced the production of biofilms by S. aureus to a lesser extent than the wild-type strain only in a S. aureus SigB-functional background.ConclusionsThese results suggest that S. aureus responds to HQNO from P. aeruginosa by forming SCVs and biofilms through SigB activation, a phenomenon that may contribute to the establishment of chronic infections in CF patients.

Pseudomonas aeruginosa and Staphylococcus aureus are the most prevalent pathogens in airway infections of cystic fibrosis (CF) patients. We studied how these pathogens coexist and interact with each other. Clinical isolates of both species were retrieved from adult CF patients. Culture supernatants from 63 P. aeruginosa isolates triggered a wide range of biofilm-stimulatory activities when added to the culture of a control S. aureus strain. The extent of biofilm formation by S. aureus was positively correlated to the levels of the 2-alkyl-4-(1H)-quinolones (AQs) Pseudomonas Quinolone Signal (PQS) and 2-heptyl-4-hydroxy quinoline N-oxide (HQNO) produced by the P. aeruginosa isolates. Supernatants from P. aeruginosa isogenic mutants deficient in PQS and HQNO production stimulated significantly less biofilm formation by S. aureus than that seen with the parental strain PA14. When studying co-isolated pairs of P. aeruginosa and S. aureus retrieved from patients showing both pathogens, P. aeruginosa supernatants stimulated less biofilm production by the S. aureus counterparts compared to that observed using the control S. aureus strain. Accordingly, some P. aeruginosa isolates produced low levels of exoproducts and also some of the clinical S. aureus isolates were not stimulated by their co-isolates or by PA14 despite adequate production of HQNO. This suggests that colonization of the CF lungs promotes some type of strain selection, or that co-existence requires specific adaptations by either or both pathogens. Results provide insights on bacterial interactions in CF.

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These results show that tomatidine is an aminoglycoside potentiator that also acts as an anti-virulence agent targeting both antibiotic-susceptible and antibiotic-resistant S. aureus.

Antibacterial agents that kill nondividing bacteria may be of utility in treating persistent infections. Oritavancin and dalbavancin are bactericidal lipoglycopeptides that are approved for acute bacterial skin and skin structure infections in adults caused by susceptible Gram-positive pathogens. Using time-kill methodology, we demonstrate that oritavancin exerts bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA) isolates that are maintained in a nondividing state in vitro, whereas dalbavancin and the glycopeptide vancomycin do not. Persistent Staphylococcus aureus infections may harbor bacteria in a nondividing state in which killing by bactericidal agents is reduced relative to the killing of actively dividing bacteria (1). Hence, the use of antibacterial agents with activity against such nondividing bacteria may potentially decrease the duration of therapy that is required to treat these infections and ultimately be of benefit in their clinical management.Oritavancin and dalbavancin are long-acting lipoglycopeptides with activity against Gram-positive bacteria (2, 3). Oritavancin has multiple mechanisms of action, and its rapid concentration-dependent bactericidal activity against S. aureus isolates in vitro results from a combination of cell wall synthesis inhibition and perturbation of membrane barrier function (4, 5). The timedependent bactericidal activity of dalbavancin results from inhibition of cell wall synthesis via a mechanism of action that is shared with the prototypic glycopeptide vancomycin (6-8). The differences in the mechanisms of action among these agents may be important determinants of their activities against bacteria in a nondividing state. For example, oritavancin has been shown to maintain bactericidal activity in vitro against stationary-phase isolates of S. aureus in a nutrient-depleted medium, a condition in which bacterial killing by vancomycin was attenuated (a consequence of diminished cell wall synthesis) (5). In a different study, exposure of a methicillin-resistant S. aureus (MRSA) isolate to dalbavancin required 48 h to exert a Ն3-log kill against cells in a nondividing state (9). To date, no studies have directly compared the antibacterial activities of these long-acting lipoglycopeptides against either actively dividing or nondividing cells under the same test conditions in vitro. In light of this, we compare the antibacterial activities of oritavancin, dalbavancin, and vancomycin against MRSA isolates that are either actively dividing or in a nondividing state in vitro to reveal differences in the mechanisms of action of these agents that may lead to optimal therapies for infections harboring bacteria in a nondividing state.(Part of this work was presented at the Joint 55th Interscience Conference on Antimicrobial Agents and Chemotherapy and 28th International Congress of Chemotherapy Meeting, San Diego, CA, 17 to 21 September 2015 [10].)The four S. aureus isolates used in this study were MRSA ATCC 43300, MRSA NRS384, MRSA-heterogeneous vancom...