β-Lactams Enhance Vancomycin Activity against Methicillin-Resistant Staphylococcus aureus Bacteremia Compared to Vancomycin Alone

Vancomycin (VAN) is often used to treat methicillin-resistant Staphylococcus aureus (MRSA) bacteremia despite a high incidence of microbiological failure. Recent in vitro analyses of β-lactams in combination with VAN demonstrated synergistic activity against MRSA. The goal of this study was to examine the impact of combination therapy with VAN and a β-lactam (Combo) on the microbiological eradication of MRSA bacteremia compared to VAN alone. This was a retrospective cohort study of patients with MRSA bacteremia who received Combo therapy or VAN alone. Microbiological eradication of MRSA, defined as a negative blood culture obtained after initiation of therapy, was used to evaluate the efficacy of each regimen. A total of 80 patients were included: 50 patients in the Combo group and 30 patients in the VAN-alone group. Microbiological eradication was achieved in 48 patients (96%) in the Combo group compared to 24 patients (80%) in the VAN-alone group (P = 0.021). In a multivariable model, the Combo treatment had a higher likelihood of achieving microbiological eradication (adjusted odds ratio, 11.24; 95% confidence interval, 1.7 to 144.3; P = 0.01). In patients with infective endocarditis (n = 22), 11/11 (100%) who received Combo therapy achieved microbiological eradication compared to 9/11 (81.8%) treated with VAN alone, but the difference was not statistically significant (P = 0.20). Patients with MRSA bacteremia who received Combo therapy were more likely to experience microbiological eradication of MRSA than patients who received VAN alone.     

β-Lactam Resistance in Methicillin-Resistant Staphylococcus aureus USA300 Is Increased by Inactivation of the ClpXP Protease

Methicillin-resistant Staphylococcus aureus (MRSA) has acquired the mecA gene encoding a peptidoglycan transpeptidase, penicillin binding protein 2a (PBP2a), which has decreased affinity for β-lactams. Quickly spreading and highly virulent community-acquired (CA) MRSA strains recently emerged as a frequent cause of infection in individuals without exposure to the health care system. In this study, we found that the inactivation of the components of the ClpXP protease substantially increased the β-lactam resistance level of a CA-MRSA USA300 strain, suggesting that the proteolytic activity of ClpXP controls one or more pathways modulating β-lactam resistance. These pathways do not involve the control of mecA expression, as the cellular levels of PBP2a were unaltered in the clp mutants. An analysis of the cell envelope properties of the clpX and clpP mutants revealed a number of distinct phenotypes that may contribute to the enhanced β-lactam tolerance. Both mutants displayed significantly thicker cell walls, increased peptidoglycan cross-linking, and altered composition of monomeric muropeptide species compared to those of the wild types. Moreover, changes in Sle1-mediated peptidoglycan hydrolysis and altered processing of the major autolysin Atl were observed in the clp mutants. In conclusion, the results presented here point to an important role for the ClpXP protease in controlling cell wall metabolism and add novel insights into the molecular factors that determine strain-dependent β-lactam resistance.     

Daptomycin-Oxacillin Combinations in Treatment of Experimental Endocarditis Caused by Daptomycin-Nonsusceptible Strains of Methicillin-Resistant Staphylococcus aureus with Evolving Oxacillin Susceptibility (the “Seesaw Effect”)

In vivo development of daptomycin resistance (DAP^r) among Staphylococcus aureus strains, especially methicillin-resistant S. aureus (MRSA) strains, in conjunction with clinical treatment failures, has emerged as a major problem. This has raised the question of DAP-based combination regimens to enhance efficacy against such strains. We studied five recent DAP-susceptible (DAP^s)/DAP^r clinical MRSA strain pairs obtained from patients who failed DAP monotherapy regimens, as well as one DAP^s/DAP^r MRSA strain pair in which the resistant strain was generated by in vitro passage in DAP. Of note, we identified a DAP-oxacillin (OX) “seesaw” phenomenon in vitro in which development of DAP^r was accompanied by a concomitant fall in OX resistance, as demonstrated by 3- to 4-fold decreases in the OX MIC, a susceptibility shift by population analyses, and enhanced early killing by OX in time-kill assays. In addition, the combination of DAP and OX exerted modest improvement in in vitro bactericidal effects. Using an experimental model of infective endocarditis and two DAP^s/DAP^r strain pairs, we demonstrated that (i) OX monotherapy was ineffective at clearing DAP^r strains from any target tissue in this model (heart valve, kidneys, or spleen) and (ii) DAP-OX combination therapy was highly effective in DAP^r strain clearances from these organs. The mechanism(s) of the seesaw effect remains to be defined but does not appear to involve excision of the staphylococcal cassette chromosome mec (SCCmec) that carries mecA.Daptomycin (DAP) is a cyclic lipopeptide antibiotic active against a wide range of Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin (VAN)-intermediate S. aureus (VISA), and vancomycin-resistant S. aureus (VRSA) strains (39, 54). However, an increasing number of reports has described the in vivo loss of DAP susceptibility in association with DAP clinical treatment failures in S. aureus infections (2, 17, 47). Clearly, innovative approaches to this problem need to be expeditiously applied to enhance/retain DAP efficacy, including dose escalations and combination therapy strategies.It has been previously reported that in some clinical VISA and VRSA strains, as well as in vitro-selected VISA strains, as the VAN MIC was observed to rise, the concomitant MIC to semisynthetic antistaphylococcal penicillins (e.g., oxacillin [OX] or nafcillin) progressively fell. This phenomenon was termed the “seesaw effect” (42-44). This loss of methicillin resistance has most often been due to excision of the staphylococcal cassette chromosome mec element (SCCmec) that carries mecA, the gene encoding penicillin-binding protein 2a (PBP2a), which is responsible for the MRSA phenotype (11, 12, 37). Although uncommon in vivo (3), the seesaw effect was recently observed in a clinical VISA isolate with a retained mecA gene obtained during VAN therapy; of interest, after discontinuation of VAN therapy, the methicillin resistance phenotype was restored (32). Importantly, the combination of VAN and nafcillin has been shown to exert synergistic killing activities in vitro and in an experimental model of infective endocarditis (IE) (8) with both VISA and VRSA strains (14).We have found a similar seesaw relationship in S. aureus strains that progressively acquire DAP^r during DAP exposure, and their respective OX MICs declined in parallel (29). In the current study, we investigated (i) the extent of this seesaw effect in vitro by utilizing five clinical DAP^s/DAP^r strain pairs and (ii) whether combination regimens of DAP and OX would enhance both the in vitro and in vivo efficacies of these single agents against DAP^r strains.Note that although the currently accepted term for reduced in vitro susceptibility to daptomycin is “nonsusceptible,” we use the terms “daptomycin resistant” (DAP^r) and “daptomycin susceptible” (DAP^s) in this paper for a more facile presentation.     

Prevention and Treatment of Staphylococcus aureus Pneumonia with a β-Cyclodextrin Derivative

Staphylococcus aureus pneumonia is a common, potentially life-threatening infection caused by this human pathogen. The only therapies available to treat S. aureus pneumonia are antibiotics, a modality that is jeopardized by the organism''s remarkable ability to acquire antimicrobial resistance. S. aureus alpha-hemolysin is a pore-forming cytotoxin that is essential for the pathogenesis of pneumonia. Strains lacking this cytotoxin are avirulent in a murine model of pneumonia; likewise, vaccine-based strategies that antagonize the toxin afford protection against lethal disease. Disruption of the function of this toxin therefore provides a potent mechanism to prevent and/or treat S. aureus pneumonia. β-Cyclodextrin derivatives are small molecules with a sevenfold symmetry that mirrors the heptameric alpha-hemolysin. These compounds block the assembled alpha-hemolysin pore, compromising toxin function. We report that a modified β-cyclodextrin compound, IB201, prevents alpha-hemolysin-induced lysis of human alveolar epithelial cells. This protective effect does not result from the ability of the β-cyclodextrin to impair formation of the oligomeric alpha-hemolysin on the cell surface, supporting a role for this molecule in blockade of the lytic pore. An examination of IB201 in murine S. aureus pneumonia demonstrated that administration of this compound prevents and treats disease, protecting against mortality. Consistent with the vital importance of alpha-hemolysin in pneumonia caused by methicillin-sensitive and highly virulent methicillin-resistant S. aureus strains, IB201 protects against lethal challenge with both types of isolates. These observations, coupled with a favorable safety profile of β-cyclodextrin compounds, provide a novel strategy that may be developed to combat S. aureus pneumonia.Staphylococcus aureus is a gram-positive human pathogen that is able to cause a multitude of diseases ranging in severity (21). Of the many infections and toxinoses mediated by S. aureus, pneumonia is among the most prominent, accounting for an estimated 50,000 staphylococcal infections per year in the United States alone (18, 20). As one of the leading etiologic agents of ventilator-associated pneumonia, S. aureus has long plagued the intensive care environment (14, 19). Further, this pathogen is now increasingly recognized as an important cause of community-acquired pneumonia, displaying the capacity to infect a population of otherwise healthy adults and children (9, 10, 19). A number of these reported cases of community-acquired disease have occurred on the backdrop of an intercurrent respiratory viral infection, often caused by influenza (8, 9, 15). The fulminant nature of coinfection with influenza and S. aureus is evident in mortality rates that approach or exceed 50%, highlighting an apparent synergy of these pathogens in the lung environment. Both the reliance of an aging population on intensive care therapies and the current threat of epidemic or pandemic influenza underscore the large populations of diverse individuals that are at significant risk for the development of S. aureus pneumonia.Complicating the clinical management of staphylococcal pneumonia is the fact that over half of S. aureus isolates are currently classified as methicillin-resistant S. aureus (MRSA), harboring genes that render these isolates insensitive to a once potent class of antimicrobial agents (14). Recent clinical observations have documented that mortality from MRSA pneumonia can exceed 50%, defining the severity of disease caused by this organism (1). The estimated direct medical cost to treat a patient suffering with S. aureus pneumonia is in excess of $35,000, imparting a significant burden on the economy (30). The combined risks of a changing disease epidemiology and widespread drug resistance among S. aureus strains mandate the development of novel strategies to both prevent and treat disease. Recent investigations highlighting the pore-forming cytotoxin alpha-hemolysin (Hla) as essential for the pathogenesis of S. aureus pneumonia have provided opportunities to design and investigate new strategies to combat this disease (5, 6, 28).S. aureus strains lacking Hla display a profound defect in virulence in a murine model of pneumonia (4, 5). Consistent with this observation, antagonism of the toxin through a number of distinct immunization strategies has been demonstrated to provide protection against disease (6, 28). Hla displays a detrimental effect on the lung epithelium, resulting in cellular injury and death as well as the generation of proinflammatory mediators (23, 29). The toxin also has the capacity to target the pulmonary vascular endothelium, as treatment of isolated pulmonary arteries ex vivo results in increased vascular resistance and vascular leakage (31, 32). In addition to its role in the lung, this toxin plays a critical role in S. aureus pathogenesis in intraperitoneal, intramammary, and corneal models of infection (3, 7, 27). Hla is secreted by the majority of S. aureus strains as a water-soluble monomer (2, 26). This monomeric form binds to susceptible host cell membranes and through a well-detailed series of intra- and intermolecular interactions, subsequently assembles into a stable homoheptameric transmembrane pore with a 2-nm internal diameter (11, 17, 33).With the essential nature of alpha-toxin in mind, Karginov et al. used structure-inspired drug design to demonstrate that a hepta-6-substituted β-cyclodextrin derivative, termed IB201, is able to prevent alpha-toxin-mediated hemolysis of rabbit red blood cells (rRBCs), a cell type that is highly sensitive to the lytic action of the toxin (16). Previous investigations had demonstrated the utility of unsubstituted β-cyclodextrin as an adapter molecule, capable of lodging in the central pore of alpha-hemolysin and facilitating the use of the toxin as a biosensor (12, 13). An investigation of IB201 revealed that this molecule blocks ion conductance through the assembled hemolysin pore, consistent with the ability of the cyclodextrins to insert into the pore itself. The inhibitory effect of IB201 on ion conductance and red blood cell hemolysis are both observed in the low micromolar concentration range, demonstrating the potency of this molecule as an inhibitor of the S. aureus alpha-hemolysin. Interestingly, McCormick et al. have recently utilized methyl-β-cyclodextrin plus cholesterol to inhibit the activity of S. aureus alpha-hemolysin, revealing that this treatment affords protection against toxin-induced corneal erosions in a rabbit model of S. aureus keratitis (22). We report herein that the β-cyclodextrin derivative IB201 is able to prevent alpha-toxin-mediated alveolar epithelial cell lysis in vitro and is also able to prevent mortality associated with S. aureus pneumonia in a murine model of infection.     

Is methicillin-resistant Staphylococcus aureus replacing methicillin-susceptible S. aureus?

Despite extensive research on the emergence of and treatments for methicillin-resistant Staphylococcus aureus (MRSA), prior studies have not rigorously evaluated the impact of methicillin resistance on the overall incidence of S. aureus infections. Yet, there are direct clinical and research implications of determining whether methicillin-susceptible S. aureus (MSSA) infection rates remain stable in the face of increasing MRSA prevalence or whether MSSA will be replaced over time. A synthesis of prior studies indicates that the emergence of healthcare-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA) has led to an increase in the overall incidence of S. aureus infections, with MRSA principally adding to, rather than replacing, MSSA. However, colonization with CA-MRSA may at least partially replace colonization with MSSA. So far, evidence indicates that MSSA still accounts for many infections. Therefore, eradication of MRSA alone is not sufficient to address the public health burden of S. aureus.     

18β-Glycyrrhetinic Acid Inhibits Methicillin-Resistant Staphylococcus aureus Survival and Attenuates Virulence Gene Expression

Methicillin-resistant Staphylococcus aureus (MRSA) has become a major source of infection in hospitals and in the community. Increasing antibiotic resistance in S. aureus strains has created a need for alternative therapies to treat disease. A component of the licorice root Glycyrrhiza spp., 18β-glycyrrhetinic acid (GRA), has been shown to have antiviral, antitumor, and antibacterial activity. This investigation explores the in vitro and in vivo effects of GRA on MRSA pulsed-field gel electrophoresis (PFGE) type USA300. GRA exhibited bactericidal activity at concentrations exceeding 0.223 μM. Upon exposure of S. aureus to sublytic concentrations of GRA, we observed a reduction in expression of key virulence genes, including saeR and hla. In murine models of skin and soft tissue infection, topical GRA treatment significantly reduced skin lesion size and decreased the expression of saeR and hla genes. Our investigation demonstrates that at high concentrations GRA is bactericidal to MRSA and at sublethal doses it reduces virulence gene expression in S. aureus both in vitro and in vivo.     

An antidote for Staphylococcus aureus pneumonia?

Methicillin-resistant Staphylococcus aureus (MRSA) is the leading cause of bacterial infections in the United States. Severe invasive MRSA infections, which include pneumonia, are difficult to treat because the bacteria are resistant to antibiotics. A new report now shows that immunization against alpha-hemolysin (Hla), a cytolytic toxin secreted by most S. aureus strains, protects mice against lethal pneumonia. This finding represents the first successful vaccine strategy for the treatment of staphylococcal pneumonia.     

Methicillin-resistant Staphylococcus aureus and animals: zoonosis or humanosis?

Methicillin-resistant Staphylococcus aureus (MRSA) is increasing worldwide. Occasionally, animals are colonized or infected incidentally with human strains. Recently, however, new strains of MRSA emerging from within the animal kingdom, particularly in pigs, are causing human infection. MRSA has been reported in species as diverse as companion animals, horses and pigs, through to chinchillas, bats and parrots. In contrast, whereas strains of community-associated MRSA, the majority of which carry genes encoding Panton-Valentine leucocidin, are spreading rapidly in human populations, only sporadic cases have been reported in animals to date. Although MRSA has been found in some meat products, the implications for human infection through consumption are unclear. This review examines the epidemiology of MRSA in animals and human attendants/owners, the diagnosis and management of MRSA colonization, infection and infection control strategies in animals.     

Treatment of severe infections caused by Staphylococcus aureus: a change in the vancomycin paradigm?

Staphylococcus aureus is a frequent agent of serious intensive care unit infections with high associated mortality rates. Resistance to methicillin is frequent, exceeding 50% in many countries, and for these strains, vancomycin is the agent of choice. Resistance to this drug is now frequent for Enterococcus, but low rates are reported for S. aureus. Nonetheless, treatment failures with vancomycin for infections caused by strains considered susceptible have been reported during recent years. This article aims to review the possible explanations for this phenomenon, which is challenging the old paradigm of S. aureus infections and vancomycin use.     

Methicillin-Resistant Staphylococcus aureus Septic Internal Jugular Thrombophlebitis: Updates in the Etiology and Treatment of Lemierre’s Syndrome

BackgroundLemierre’s syndrome is classically precipitated by oropharyngeal infections that progress to suppurative internal jugular vein thrombophlebitis via direct extension. Metastatic pneumonia from septic emboli is nearly universal and bacterial seeding frequently results in disseminated septic foci. Fusobacterium necrophorum is the most commonly reported etiologic agent, though methicillin-resistant Staphylococcus aureus (MRSA) is an emerging pathogen and a myriad of oropharyngeal flora must be covered until blood cultures return. Prompt identification is paramount to minimizing morbidity. Empiric treatment with antibiotics exhibiting predominantly anaerobic activity has been standard, but now may be insufficient, given an evolving microbial landscape. Anticoagulation continues to be debated.Case ReportWe describe an uncommon presentation of Lemierre’s syndrome in a diabetic patient secondary to MRSA, where the only identifiable source of entry was atraumatic post-auricular cellulitis. Why Should an Emergency Physician Be Aware of This? Given the evolving landscape of organisms implicated in septic internal jugular thrombophlebitis, empiric treatment should entail consideration of MRSA. Patients at an elevated risk include those who are undomiciled or incarcerated, injection drug users, human immunodeficiency virus–positive, and have recently been hospitalized or completed a course of antibiotics. The existing evidence evaluating empiric anticoagulation is low-powered and retrospective and would benefit from randomized controlled trials. Although it does not appear valuable for most, those with thrombus extension, persistent bacteremia, or central venous thrombosis may benefit.     

Automobile Versus Pedestrian Injuries: Does Gender Matter?

Background: Automobile vs. pedestrian (AVP) injuries cause substantial morbidity and mortality. Gender may be an important factor in determining the anatomic distribution and severity of these injuries. The objective of this study was to examine the effect of gender on the nature and severity of automobile vs. pedestrian injuries and the outcome. Methods: Trauma registry study that included all AVP pedestrian injuries admitted during a 14-year period to a Level I trauma center. The following variables were included in an Excel (Microsoft Corporation, Redmond, WA) file for the purpose of this study: age, gender, body area Abbreviated Injury Score, Injury Severity Score, specific fractures (pelvic, spine, femur, tibia), survival, and intensive care unit (ICU) and hospital length of stay. Results: The study population included 6965 patients, 67.3% of whom were male. Overall, 20.7% were in the age group < 15 years, 60.5% in the age group 15–55 years, 7.6% in the age group 56–65 years, and 11.1% in the age group > 65 years. Pelvic fractures were significantly more common in females than males (20.7% vs. 11.4%, respectively, p < 0.0001). This difference was present in all age groups, but especially in the groups 56–65 years (28.5% vs. 12.3%, respectively, p < 0.0001) and > 65 years (32.5% vs. 15.7%, respectively, p < 0.0001). Males in the age group 15–55 years were significantly more likely to suffer tibia fractures (31.8% vs. 25.7%, respectively, p < 0.001). Multivariate analysis showed no difference in survival or ICU stay between the two genders, but there was a significantly longer hospital stay in males 15–65 years. Conclusions: Gender plays a significant role in the incidence of pelvic and tibial fractures but has no effect on survival or ICU stay, but male patients in the age group 15–65 years had a significantly longer hospital stay.