Biological counterstrike: antibiotic resistance mechanisms of Gram-positive cocci

Antibacterial discovery and development--the failure of success?

A combination of approaches and compounds-many of which failed to yield immediate results in the past-will ultimately prove invaluable to the drug industry in the ongoing battle against infectious disease.

In vitro activities of the lipopeptides palmitoyl (Pal)-Lys-Lys-NH(2) and Pal-Lys-Lys alone and in combination with antimicrobial agents against multiresistant gram-positive cocci

The in vitro activities of the lipopeptides palmitoyl (Pal)-Lys-Lys-NH(2) and Pal-Lys-Lys against gram-positive cocci were investigated. Enterococci and streptococci demonstrated higher susceptibilities than staphylococci and Rhodococcus equi. A positive interaction was shown when the lipopeptides were combined with beta-lactams and vancomycin. These results suggest that lipopeptides are promising candidates for antimicrobial therapy for infections caused by gram-positive organisms.

The emergence of antibiotic resistance by mutation

The emergence of mutations in nucleic acids is one of the major factors underlying evolution, providing the working material for natural selection. Most bacteria are haploid for the vast majority of their genes and, coupled with typically short generation times, this allows mutations to emerge and accumulate rapidly, and to effect significant phenotypic changes in what is perceived to be real-time. Not least among these phenotypic changes are those associated with antibiotic resistance. Mechanisms of horizontal gene spread among bacterial strains or species are often considered to be the main mediators of antibiotic resistance. However, mutational resistance has been invaluable in studies of bacterial genetics, and also has primary clinical importance in certain bacterial species, such as Mycobacterium tuberculosis and Helicobacter pylori, or when considering resistance to particular antibiotics, especially to synthetic agents such as fluoroquinolones and oxazolidinones. In addition, mutation is essential for the continued evolution of acquired resistance genes and has, e.g., given rise to over 100 variants of the TEM family of beta-lactamases. Hypermutator strains of bacteria, which have mutations in genes affecting DNA repair and replication fidelity, have elevated mutation rates. Mutational resistance emerges de novo more readily in these hypermutable strains, and they also provide a suitable host background for the evolution of acquired resistance genes in vitro. In the clinical setting, hypermutator strains of Pseudomonas aeruginosa have been isolated from the lungs of cystic fibrosis patients, but a more general role for hypermutators in the emergence of clinically relevant antibiotic resistance in a wider variety of bacterial pathogens has not yet been proven.

blaVIM-2 and blaVIM-7 carbapenemase-producing Pseudomonas aeruginosa isolates detected in a tertiary care medical center in the United States: report from the MYSTIC program

Two Pseudomonas aeruginosa strains resistant to beta-lactams, fluoroquinolones, aminoglycosides, tetracyclines, and carbapenems and susceptible only to polymyxin B (MIC <or= 2 microg/ml) were identified as part of the Meropenem Yearly Susceptibility Test Information Collection program. Metallo-beta-lactamase screening tests were positive, PCR yielded products with blaVIM primers, and sequence analysis revealed blaVIM-7 and blaVIM-2. The isolates had distinct ribotype and pulsed-field gel electorphoresis patterns and appeared independently, remote in time and location, at the same cancer center.

In vitro activity of tigecycline against multiple-drug-resistant, including pan-resistant, gram-negative and gram-positive clinical isolates from Greek hospitals

The in vitro activities of tigecycline and selected antimicrobials were evaluated against a variety of multiple-drug-resistant clinical isolates, including extended-spectrum beta-lactamase- and/or metallo-beta-lactamase-producing gram-negative strains, colistin-resistant strains, vancomycin- and/or linezolid-resistant enterococci, and methicillin-resistant Staphylococcus aureus (MRSA). Tigecycline showed excellent activity against a collection of difficult-to-treat pathogens currently encountered in the hospital setting.

Rapid detection and identification of metallo-beta-lactamase-encoding genes by multiplex real-time PCR assay and melt curve analysis

Metallo-beta-lactamase enzymes (MbetaL) are encoded by transferable genes, which appear to spread rapidly among gram-negative bacteria. The objective of this study was to develop a multiplex real-time PCR assay followed by a melt curve step for rapid detection and identification of genes encoding MbetaL-type enzymes based on the amplicon melting peak. The reference sequences of all genes encoding IMP and VIM types, SPM-1, GIM-1, and SIM-1 were downloaded from GenBank, and primers were designed to obtain amplicons showing different sizes and melting peak temperatures (Tm). The real-time PCR assay was able to detect all MbetaL-harboring clinical isolates, and the Tm-assigned genotypes were 100% coincident with previous sequencing results. This assay could be suitable for identification of MbetaL-producing gram-negative bacteria by molecular diagnostic laboratories.

Molecular evolution of metallo-beta-lactamase-producing Pseudomonas aeruginosa in a nosocomial setting of high-level endemicity

An outbreak of multidrug-resistant Pseudomonas aeruginosa strains producing VIM-type metallo-beta-lactamases (MBLs) has occurred in an Italian hospital since 2000 (C. Lagatolla, E. A. Tonin, C. Monti-Bragadin, L. Dolzani, F. Gombac, C. Bearzi, E. Edalucci, F. Gionechetti, and G. M. Rossolini, Emerg. Infect. Dis. 10:535-538, 2004). In this work, using molecular methods, we characterized 128 carbapenem-resistant isolates (including 98 VIM-positive isolates) collected from that hospital from 2000 to 2002 to investigate the dynamics of the dissemination of MBL producers in the clinical setting. Genotyping by random amplification of polymorphic DNA and pulsed-field gel electrophoresis showed that most VIM-positive isolates belonged to two different clonal lineages, producing either a VIM-1- or a VIM-2-like MBL, whose ancestors were detected for the first time in the hospital in 1999, suggesting that clonal expansion played a predominant role in the dissemination of these isolates. The 86 clonally related isolates carrying a blaVIM-1-like gene on an In70-like integron were clearly related to a VIM-1-positive P. aeruginosa clone circulating in various Italian hospitals since the late 1990s. VIM-negative P. aeruginosa strains related to the VIM-1-positive clone were detected during the same period, suggesting that the latter strain was derived from a clonal lineage already circulating in the hospital. In the VIM-2-like positive clone, the MBL gene was carried by an unusual class 1 integron, named In71, lacking the 3' conserved sequence region typical of sul1-associated integrons. A different class 1 integron with an original structure carrying a blaVIM-2 determinant, named In74, was detected in a sporadic isolate. A retrospective investigation did not reveal the presence of strains related to any of the VIM-producing isolates earlier than 1997.

Establishing clonal relationships between VIM-1-like metallo-beta-lactamase-producing Pseudomonas aeruginosa strains from four European countries by multilocus sequence typing

Ten multidrug-resistant Pseudomonas aeruginosa strains producing VIM-1-like acquired metallo-beta-lactamases (MBLs), isolated from four European countries (Greece, Hungary, Italy, and Sweden), were analyzed for genetic relatedness by several methodologies, including fliC sequence analysis, macrorestriction profiling of genomic DNA by pulsed-field gel electrophoresis (PFGE), random amplification of polymorphic DNA (RAPD), and multilocus sequence typing (MLST). The four approaches yielded consistent results overall but showed different resolution powers in establishing relatedness between isolates (PFGE>RAPD>MLST>fliC typing) and could usefully complement each other to address issues in the molecular epidemiology of P. aeruginosa strains producing acquired MBLs. In particular, the recently developed MLST approach was useful in revealing clonal relatedness between isolates when this was not readily apparent using RAPD and PFGE, and it suggested a common ancestry for some of the VIM-1-like MBL-positive P. aeruginosa strains currently spreading in Europe. The MBL producers belonged in three clonal complexes/burst groups (BGs). Of these, one corresponded to the previously described BG4 and included serotype O12 strains from Hungary and Sweden, while the other two were novel and included serotype O11 or nonserotypable strains from Greece, Sweden, and/or Italy. Comparison of the integrons carrying blaVIM-1-like cassettes of various isolates revealed a remarkable structural heterogeneity, suggesting the possibility that multiple independent events of acquisition of different blaVIM-containing integrons had occurred in members of the same clonal lineage, although a contribution of integrase-mediated cassette shuffling or other recombination mechanisms during the evolution of similar strains could also have played a role in determining this variability.

Antibiotic resistance mechanisms among pediatric respiratory and enteric pathogens: A current update

Antibiotic resistance is a continually increasing problem that has, to a greater or lesser extent, affected virtually every area of the world. The scientific literature is abundant with papers related to antibiotics and antibiotic resistance. Many excellent papers and reviews have been published during the past few years, and the literature base continues to expand at rapid speed. This review is meant to provide a recent update on antibiotic resistance among respiratory and enteric pathogens, with a focus on infections in children. Not a small task, but this paper is not meant to be exhaustive. Rather, the intention is to highlight the key antibiotics and antibiotic resistance mechanisms that are currently the most relevant to pediatrics. The most recently published literature is used wherever possible, and the reader is encouraged to explore specific topics of interest further by reviewing the referenced literature.

Immunochemical characterization of polysaccharide antigens from six clinical strains of Enterococci

Background

Enterococci have become major nosocomial pathogens due to their intrinsic and acquired resistance to a broad spectrum of antibiotics. Their increasing drug resistance prompts us to search for prominent antigens to develop vaccines against enterococci. Given the success of polysaccharide-based vaccines against various bacterial pathogens, we isolated and characterized the immunochemical properties of polysaccharide antigens from five strains of Enterococcus faecalis and one strain of vancomycin-resistant E. faecium.

Results

We cultured large batches of each strain, isolated sufficient quantities of polysaccharides, analyzed their chemical structures, and compared their antigenic specificity. Three classes of polysaccharides were isolated from each strain, including a polyglucan, a teichoic acid, and a heteroglycan composed of rhamnose, glucose, galactose, mannosamine, and glucosamine. The polyglucans from all six strains are identical and appear to be dextran. Yields of the teichoic acids were generally low. The most abundant polysaccharides are the heteroglycans. The six heteroglycans are structurally different as evidenced by NMR spectroscopy. They also differ in their antigenic specificities as revealed by competitive ELISA. The heteroglycans are not immunogenic by themselves but conjugation to protein carriers significantly enhanced their ability to induce antibodies.

Conclusion

The six clinical strains of enterococci express abundant, strain-specific cell-surface heteroglycans. These polysaccharides may provide a molecular basis for serological typing of enterococcal strains and antigens for the development of vaccines against multi-drug resistant enterococci.

Treatment of intra-abdominal and skin and soft tissue infections: The role of the glycylcyclines

The need for new, effective agents to treat multidrug-resistant infections continues to grow as more and more bacteria develop resistance that may result in clinical therapeutic failure. This is particularly true for common surgical infections, such as complicated intra-abdominal infections, which frequently involve multiple pathogens, making therapy with a broad-spectrum antibiotic an important treatment intervention, and also for complicated skin infections, which often involve methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). With treatment options limited, it has become critical to identify antibiotics with novel mechanisms of activity. Several new drugs have emerged as possible therapeutic alternatives: linezolid, quinupristin-dalfopristin and most recently daptomycin have all been FDA-approved for the treatment of skin and skin structure infections. This review examines the potential role of a new class of investigational agents, the glycylcyclines, also recently FDA-approved and currently under review for European licensing, in the treatment of complicated skin infections and intra-abdominal infections. Tigecycline, the first of the glycylcyclines, has shown excellent activity in Phase III studies of these infections, achieving clinical success rates ranging from 70% to 91%. Furthermore, it has a good safety profile, suggesting it will be a clinical useful addition to current therapeutic options for the treatment of complicated skin infections and intra-abdominal infections.

The importance of the development of antibiotic resistance in Staphylococcus aureus

Hospital- and community-acquired Staphylococcus aureus infections pose a substantial burden in terms of morbidity, mortality and health care costs. The introduction of new antibiotics to counter this pathogen has frequently been closely followed by the emergence of resistant strains. Most significantly, S. aureus isolates resistant to beta-lactams have become common, and many of these are also resistant to beta-lactamase-resistant penicillins. The rapid spread of methicillin-resistant S. aureus (MRSA) clones across the world often results in hospital outbreaks, but implementation of appropriate control measures usually reduces prevalence to sporadic levels. However, the recent emergence of MRSA infections in the community, affecting patients with no established risk factors for MRSA acquisition, is likely to impact significantly on future strategies for control of nosocomial MRSA. In contrast to other antibiotic classes, S. aureus resistance to glycopeptides did not emerge until nearly 40 years after their clinical introduction, and as a result this drug class has remained the mainstay of treatment for MRSA infections. However, a number of vancomycin-intermediate S. aureus isolates have emerged worldwide and four fully resistant S. aureus isolates have been reported in the USA. This raises the concern that the current first-line treatment for MRSA infection may become ineffective in an increasing proportion of cases in the near future. New classes of antibiotic are urgently needed to treat infections with this growing population of multidrug-resistant S. aureus, and the recently introduced oxazolidinone linezolid and the cyclic lipopeptide daptomycin are welcome additions to the ever-narrowing range of therapies effective against this pathogen.

Comparison of genetic backgrounds of methicillin-resistant and -susceptible Staphylococcus aureus isolates from Portuguese hospitals and the community

In order to understand the origins of the dominant methicillin-resistant Staphylococcus aureus (MRSA) clones in Portuguese hospitals, we compared the genetic backgrounds of nosocomial MRSA with methicillin-susceptible S. aureus (MSSA) isolates from the same hospitals (n=155) and from the community (n=157) where they were located. Pulsed-field gel electrophoresis, spa typing, multilocus sequence typing, and agr type analysis revealed that the genetic backgrounds correspondent to the dominant MRSA clones in Portuguese hospitals during the last 15 years (Iberian ST 247, Brazilian ST 239, and EMRSA-15 ST 22) were scarcely or not found among the present MSSA collection. The four major MSSA clones encountered (A-ST 30, B-ST 34, C-ST 5, and H-ST 45) correspond, or are very similar, to the background of other international MRSA pandemic clones, i.e., EMRSA-16, New York/Japan, Pediatric, and Berlin clones. However, with the exception of the Pediatric clone, none of these MRSA clones has been detected in Portugal. Our findings suggest the three major MRSA clones identified in Portuguese hospitals have not originated from the introduction of SCCmec into dominant MSSA backgrounds present in the Portuguese nosocomial or community environment but were probably imported from abroad. In contrast, the MRSA Pediatric clone might have originated in our country by the acquisition of SCCmec type IV into MSSA clone C. Furthermore, we provide evidence that the introduction of SCCmec into sensitive clones is most likely a relatively infrequent event that seems to depend not exclusively on the presence of a successful MSSA lineage.

Rapidly progressive soft tissue infections

Skin and soft tissue infections are among the most common reasons for people to seek medical advice. They also represent one of the most common indications for antimicrobial therapy and account for approximately 7-10% of hospitalisations in North America. Although non-limb and non-life threatening infections may be treated on an out-patient basis with oral antibiotics, patients with more serious acute skin and soft tissue infections may require admission to hospital for management; this decision is especially true if the infection is rapidly progressive. We provide a concise overview of the differential diagnosis and approach to management of community-acquired rapidly progressive skin and soft tissue infections.

Molecular detection of antibiotic resistance: when and where?

Antibiotic resistance is a key issue affecting public health, and diagnostic bacteriology laboratories are essential for prompt recognition of resistant isolates. Determination of susceptibility or resistance using phenotypic tests is a 'gold standard' against which newer technologies are compared in terms of performance, cost and ease of use. Molecular methods for detecting resistance are myriad, and are used widely in academia and in reference laboratories, but gaining a significant foothold in diagnostic laboratories is proving more difficult. However, if used widely in a diagnostic setting, these techniques would impact more directly on patient care and would be valuable infection control tools, e.g. by rapidly confirming patients colonized by resistant bacteria. The cost of molecular assays may be considered prohibitive, and this is compounded by the daunting variety of proprietary platforms available; most diagnostic laboratories would prefer to invest their capital and to train their staff in a single versatile technology. In a market that has no clear leader, many laboratories are understandably reluctant to gamble on making the correct choice. If molecular detection of resistance is to achieve wide acceptance, manufacturers must broaden the repertoires of their technologies, develop more off-the-shelf applications with in-built quality control, and make them suitable for laboratory personnel with no specialist expertise in molecular biology.