Towards squalamine mimics: synthesis and antibacterial activities of head-to-tail dimeric sterol-polyamine conjugates.

Four dimeric sterol-polyamine conjugates have been synthesized from the homo- and hetero-connection of monomeric sterol-polyamine analogs in a head-to-tail manner. These dimeric conjugates show strong antibacterial activity against a broad spectrum of Gram-positive bacteria, whereas their corresponding activities against Gram-negative bacteria are relatively moderate. Though no significant difference was observed in the activities of these conjugates, cholic acid-containing dimeric conjugates generally exhibit higher activities than the corresponding deoxycholic acid-derived analogs. This is in contrast to the finding that a monomeric deoxycholic acid-spermine conjugate was more active than the corresponding cholic acid-derived analog.

Serine/threonine phosphatase Stp1 contributes to reduced susceptibility to vancomycin and virulence in Staphylococcus aureus.

The genetic mechanisms that contribute to reduced susceptibility to vancomycin in Staphylococcus aureus are complex and heterogeneous. In addition, debate is emerging as to the true effect of reduced susceptibility to vancomycin on staphylococcal virulence. To investigate this, comparative genomics was performed on a collection of vancomycin-exposed isogenic S. aureus pairs (14 strains in total). Previously described mutations were observed in genes such as vraG, agrA, yvqF, and rpoB; however, a new mechanism was identified involving a serine/threonine phosphatase, Stp1. After constructing an stp1 deletion mutant, we showed that stp1 is important in vancomycin susceptibility and cell wall biosynthesis. Gene expression studies showed that stp1 also regulates virulence genes, including a hemolysin, superantigen-like protein, and phenol-soluble modulin, and that the deletion mutant is attenuated in virulence in vivo. Stp1 provides a new link between vancomycin susceptibility and virulence in S. aureus.

MRSA: the first half century.

Fifty years ago methicillin-resistant Staphylococcus aureus (MRSA) first revealed themselves to the medical community, having been described in a landmark article published in the British Medical Journal. Among other things, their discovery set off a major response from the scientific and medical professions to control or even eliminate them as major human pathogens. Despite these efforts, however, MRSA have spread throughout the world and a half century after they burst upon the scene they continue to pose major challenges to research scientists and clinicians alike. In a very real sense, this year marks the 'birthday' of a remarkably successful pathogen. The major reasons for the ability of MRSA to prosper and cause disease in settings inimical to its survival form the basis of this article.

Drug interactions modulate the potential for evolution of resistance.

Antimicrobial treatments increasingly rely on multidrug combinations, in part because of the emergence and spread of antibiotic resistance. The continued effectiveness of combination treatments depends crucially on the frequency with which multidrug resistance arises. Yet, it is unknown how this propensity for resistance depends on cross-resistance and on epistatic interactions-ranging from synergy to antagonism-between the drugs. Here, we analyzed how interactions between pairs of drugs affect the spontaneous emergence of resistance in the medically important pathogen Staphylococcus aureus. Resistance is selected for within a window of drug concentrations high enough to inhibit wild-type growth but low enough for some resistant mutants to grow. Introducing an experimental method for high-throughput colony imaging, we counted resistant colonies arising across a two-dimensional matrix of drug concentrations for each of three drug pairs. Our data show that these different drug combinations have significantly different impacts on the size of the window of drug concentrations where resistance is selected for. We framed these results in a mathematical model in which the frequencies of resistance to single drugs, cross-resistance, and epistasis combine to determine the propensity for multidrug resistance. The theory suggests that drug pairs which interact synergistically, preferred for their immediate efficacy, may in fact favor the future evolution of resistance. This framework reveals the central role of drug epistasis in the evolution of resistance and points to new strategies for combating the emergence of drug-resistant bacteria.

Prokaryote-eukaryote interactions identified by using Caenorhabditis elegans.

Prokaryote-eukaryote interactions are ubiquitous and have important medical and environmental significance. Despite this, a paucity of data exists on the mechanisms and pathogenic consequences of bacterial-fungal encounters within a living host. We used the nematode Caenorhabditis elegans as a substitute host to study the interactions between two ecologically related and clinically troublesome pathogens, the prokaryote, Acinetobacter baumannii, and the eukaryote, Candida albicans. After co-infecting C. elegans with these organisms, we observed that A. baumannii inhibits filamentation, a key virulence determinant of C. albicans. This antagonistic, cross-kingdom interaction led to attenuated virulence of C. albicans, as determined by improved nematode survival when infected with both pathogens. In vitro coinfection assays in planktonic and biofilm environments supported the inhibitory effects of A. baumannii toward C. albicans, further showing a predilection of A. baumannii for C. albicans filaments. Interestingly, we demonstrate a likely evolutionary defense by C. albicans against A. baumannii, whereby C. albicans inhibits A. baumannii growth once a quorum develops. This counteroffensive is at least partly mediated by the C. albicans quorum-sensing molecule farnesol. We used the C. elegans-A. baumannii-C. albicans coinfection model to screen an A. baumannii mutant library, leading to the identification of several mutants attenuated in their inhibitory activity toward C. albicans. These findings present an extension to the current paradigm of studying monomicrobial pathogenesis in C. elegans and by use of genetic manipulation, provides a whole-animal model system to investigate the complex dynamics of a polymicrobial infection.

Lack of bactericidal antagonism or synergism in vitro between oxacillin and vancomycin against methicillin-susceptible strains of Staphylococcus aureus.

With the current high prevalence of infection caused by methicillin-resistant Staphylococcus aureus (MRSA) strains but in light of the general belief that beta-lactam antibiotics are more effective than vancomycin against infections caused by methicillin-susceptible S. aureus (MSSA) isolates, clinicians may utilize antistaphylococcal penicillins in combination with vancomycin for the empirical treatment of S. aureus infections. Vancomycin is considered to kill MSSA more slowly than oxacillin. Thus, we sought to evaluate the interaction of the combination of oxacillin and vancomycin on bacterial killing in vitro. Ten clinical isolates of MSSA isolated in the year 2000 were investigated. The killing observed at 24 h by vancomycin at 20 microg/ml, oxacillin at 16 microg/ml, or the combination did not differ (approximately 2.5 to 3.5 log10 CFU/ml). In a separate experiment, we assessed bacterial killing in a dynamic model simulating the free plasma concentration profiles expected following the administration of a combination of vancomycin at 1 g every 12 h and oxacillin at 1 g every 6 h. The time-kill profiles of these regimens against S. aureus ATCC 29213 were comparable to those observed in the fixed-concentration experiments. Using these methods, we found no evidence that vancomycin antagonized the bactericidal effect of oxacillin or that there was any benefit from use of the combination.

The problem of complicated skin and skin structure infections: the need for new agents.

Complicated skin and skin structure infections (cSSSIs) continue to pose a significant clinical challenge. The most frequent cause of these infections is Staphylococcus aureus, although other organisms, including Streptococcus pyogenes and, in certain circumstances, Enterobacteriaceae, are also involved. The relentless increase in methicillin resistance among S. aureus isolated in hospitals throughout the world has made it important to provide coverage for these organisms when treating cSSSIs in hospitals. More recently, however, there has been a striking increase in methicillin resistance among staphylococci isolated from infections acquired in the community, particularly in the USA. As a result, previous recommendations for empirical therapy of these important infections are now outdated. The papers in this Supplement detail the properties of a new broad-spectrum cephalosporin that has activity against MRSA and is, thus, an outstanding candidate for empirical therapy of cSSSIs. The papers included provide data on the in vitro activity, pharmacokinetics and pharmacodynamics as well as the clinical efficacy of ceftaroline fosamil, which is a welcome addition to our therapeutic armamentarium against cSSSIs.

Development of reduced vancomycin susceptibility in methicillin-susceptible Staphylococcus aureus.

BACKGROUND: Most cases of reduced vancomycin susceptibility in Staphylococcus aureus reported in the literature have been in methicillin-resistant strains. We report the development of reduced vancomycin susceptibility in a series of clonally related, methicillin-susceptible S. aureus (MSSA) clinical isolates. This isogenic series permitted us to determine whether the evolution of reduced vancomycin susceptibility in MSSA is similar to that seen in MRSA. METHODS: Differences in vancomycin population analysis profiles; chemical autolysis; vancomycin, oxacillin, and daptomycin minimum inhibitory concentrations; and bactericidal activities were examined. RESULTS: Progressive vancomycin resistance correlated with increasing daptomycin nonsusceptibility. Chemical autolysis and the bactericidal activity of vancomycin, oxacillin, and daptomycin were reduced in the final, vancomycin-intermediate S. aureus isolate, compared with the vancomycin-susceptible MSSA progenitor. CONCLUSIONS: Clinicians should recognize that reduced vancomycin susceptibility can occur in S. aureus irrespective of background methicillin susceptibility and that development of intermediate vancomycin susceptibility in MSSA may result in increased tolerance to several classes of anti-staphylococcal antibiotics.

Modeling the invasion of community-acquired methicillin-resistant Staphylococcus aureus into hospitals.

BACKGROUND: Methicillin-resistant Staphylococcus aureus (MRSA) has traditionally been associated with infections in hospitals. Recently, a new strain of MRSA has emerged and rapidly spread in the community, causing serious infections among young, healthy individuals. Preliminary reports imply that a particular clone (USA300) of a community-acquired MRSA (CA-MRSA) strain is infiltrating hospitals and replacing the traditional hospital-acquired MRSA strains. If true, this event would have serious consequences, because CA-MRSA infections in hospitals would occur among a more debilitated, older patient population. METHODS: A deterministic mathematical model was developed to characterize the factors contributing to the replacement of hospital-acquired MRSA with CA-MRSA and to quantify the effectiveness of interventions aimed at limiting the spread of CA-MRSA in health care settings. RESULTS: The model strongly suggests that CA-MRSA will become the dominant MRSA strain in hospitals and health care facilities. This reversal of dominant strain will occur as a result of the documented expanding community reservoir and increasing influx into the hospital of individuals who harbor CA-MRSA. Competitive exclusion of hospital-acquired MRSA by CA-MRSA will occur, with increased severity of CA-MRSA infections resulting in longer hospitalizations and a larger in-hospital reservoir of CA-MRSA. CONCLUSIONS: Improving compliance with hand hygiene and screening for and decolonization of CA-MRSA carriers are effective strategies. However, hand hygiene has the greatest return of benefits and, if compliance is optimized, other strategies may have minimal added benefit.

Vancomycin therapeutic guidelines: a summary of consensus recommendations from the infectious diseases Society of America, the American Society of Health-System Pharmacists, and the Society of Infectious Diseases Pharmacists.

Practice guidelines for therapeutic monitoring of vancomycin treatment for Staphylococcus aureus infection in adult patients were reviewed by an expert panel of the Infectious Diseases Society of America, the American Society of Health-System Pharmacists, and the Society of Infectious Diseases Pharmacists. A literature review of existing evidence regarding vancomycin dosing and monitoring of serum concentrations, in addition to patient outcomes combined with expert opinion regarding the drug's pharmacokinetic, pharmacodynamic, and safety record, resulted in new recommendations for targeting and adjustment of vancomycin therapy.

Clinical consensus conference: survey on Gram-positive bloodstream infections with a focus on Staphylococcus aureus.

The increased incidence over the past decade of bloodstream infections (BSIs) caused by gram-positive bacteria, particularly methicillin-resistant Staphylococcus aureus, highlights the critical need for a consistent approach to therapy. However, there is currently no international consensus on the diagnosis and management of gram-positive BSIs. The Clinical Consensus Conference on Gram-Positive Bloodstream Infections was convened as a session at the 9th International Symposium on Modern Concepts in Endocarditis and Cardiovascular Infections held in 2007. Participants discussed various aspects of the practical treatment of patients who present with gram-positive BSI, including therapeutic options for patients with BSIs of undefined origin, the selection of appropriate empirical therapy, and treatment of complicated and uncomplicated BSIs. The opinions of participants about these key issues are reflected in this article.

Galleria mellonella as a model system to study Acinetobacter baumannii pathogenesis and therapeutics.

Nonmammalian model systems of infection such as Galleria mellonella (caterpillars of the greater wax moth) have significant logistical and ethical advantages over mammalian models. In this study, we utilize G. mellonella caterpillars to study host-pathogen interactions with the gram-negative organism Acinetobacter baumannii and determine the utility of this infection model to study antibacterial efficacy. After infecting G. mellonella caterpillars with a reference A. baumannii strain, we observed that the rate of G. mellonella killing was dependent on the infection inoculum and the incubation temperature postinfection, with greater killing at 37 degrees C than at 30 degrees C (P = 0.01). A. baumannii strains caused greater killing than the less-pathogenic species Acinetobacter baylyi and Acinetobacter lwoffii (P < 0.001). Community-acquired A. baumannii caused greater killing than a reference hospital-acquired strain (P < 0.01). Reduced levels of production of the quorum-sensing molecule 3-hydroxy-C(12)-homoserine lactone caused no change in A. baumannii virulence against G. mellonella. Treatment of a lethal A. baumannii infection with antibiotics that had in vitro activity against the infecting A. baumannii strain significantly prolonged the survival of G. mellonella caterpillars compared with treatment with antibiotics to which the bacteria were resistant. G. mellonella is a relatively simple, nonmammalian model system that can be used to facilitate the in vivo study of host-pathogen interactions in A. baumannii and the efficacy of antibacterial agents.

Current treatment options for community-acquired methicillin-resistant Staphylococcus aureus infection.

During the past decade, there has been a marked increase in the prevalence of community-acquired methicillin-resistant Staphylococcus aureus infection in the United States and elsewhere. The most common such infections are those involving the skin and skin structures. Although a number of these lesions (including small furuncles and abscesses) respond well to surgical incision and drainage, oral antimicrobial agents are commonly used to treat these infections in outpatients. Unfortunately, with the exception of linezolid, none of the agents presently being used in this fashion has been subjected to rigorous clinical trial. Thus, current therapy is based largely on anecdotal evidence. For more-serious infections requiring hospitalization, parenteral antimicrobials such as vancomycin, teicoplanin, daptomycin, linezolid, and tigecycline are presently available and have demonstrated effectiveness in randomized, prospective, double-blind trials.

Increasing antibiotic resistance among methicillin-resistant Staphylococcus aureus strains.

Vancomycin use has increased dramatically worldwide since the mid-1980s, largely as a result of empirical and directed therapy against burgeoning methicillin-resistant Staphylococcus aureus (MRSA) infections. With limited choices, clinicians have traditionally relied on vancomycin alone in the management of serious MRSA infections and have enjoyed a significant period free of vancomycin resistance in S. aureus. Even now, 5 decades after its introduction, vancomycin resistance among S. aureus strains, as currently defined microbiologically, remains rare. Yet it is becoming clear that vancomycin is losing potency against S. aureus, including MRSA. Serious infections due to MRSA defined as susceptible in the laboratory are not responding well to vancomycin. This is demonstrated by increased mortality seen in patients with MRSA infection and markedly attenuated vancomycin efficacy caused by vancomycin heteroresistance in S. aureus. Therefore, it appears that our definition of vancomycin susceptibility requires further scrutiny as applied to serious MRSA infections, such as bacteremia and pneumonia.

Evaluation of endocarditis caused by methicillin-susceptible Staphylococcus aureus developing nonsusceptibility to daptomycin.

We examined sequential methicillin-susceptible Staphylococcus aureus isolates from a patient with mitral valve endocarditis recovered during persistent bacteremia on standard therapy and relapse after treatment with daptomycin. An isolate obtained after 5 days of antimicrobial therapy, but before exposure to daptomycin, showed subtle physiological changes in response to daptomycin, with significant regrowth in the daptomycin killing assay compared to the treatment-naive strain. Once daptomycin was started, the population became more heterogeneous and tested as nonsusceptible. These organisms were examined in a simulated-vegetation in vitro pharmacodynamic model, which confirmed progressive decreases in killing with daptomycin concentrations that simulate those attained in humans with 6-mg/kg of body weight daily dosing. Early surgical intervention or combination therapy or both might have prevented the loss of daptomycin susceptibility.

A 39-year-old man with a skin infection.

The case of Mr M, a previously healthy 39-year-old man with erythema and swelling of his finger, illustrates the issues involved in treating community-acquired skin and soft tissue infections since the emergence of methicillin-resistant Staphylococcus aureus (MRSA) in the community. Most community-acquired infections of the skin and soft tissues are caused by S aureus or Streptococcus pyogenes. Until recently, infections due to such organisms in the United States could safely be treated with an oral antistaphylococcal penicillin or an oral first-generation cephalosporin. However, the emergence of methicillin-resistant staphylococci as community-acquired pathogens has changed the picture as far as empirical therapy is concerned. Not only do community-acquired MRSA bacteria cause furunculitis and cellulitis, they have also been involved in a variety of more serious and life-threatening infections. Most of these organisms are susceptible to trimethoprim-sulfamethoxazole, minocycline, doxycycline, and rifampin, and these agents, along with clindamycin, have been used in the therapy of such infections, even though no clinical trials have proven their efficacy. For more serious, life-threatening infections, linezolid or parenteral agents such as vancomycin or daptomycin should be considered.