Bioline Scholar Monthly: June 2012 Round-up – Focus on MRSA

The groundbreaking discovery by Sir Alexander Fleming of the first antibiotic penicillin in 1929, and its subsequent isolation by Ernst Chain and Sir Howard Florey in 1939, led to the award of the Nobel prize in 1945. This discovery revolutionized modern medicine, paving the way for the development of many more natural antibiotics such as chloramphenicol, streptomycin and tetracycline. These antibiotics were effective against the full array of bacterial pathogens including Gram-positive and Gram-negative bacteria, intracellular parasites and Mycobacterium tuberculosis.

However, over the past several decades, bacteria have developed resistance to existing drugs. Drug resistance by so-called “superbugs” is a severe public health problem, whilst development of new generation anti-microbials faces great challenges.

Earlier this year, the WHO released a sobering update on drug-resistant TB; it was reported to be at the highest rates ever recorded.

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most important nosocomial pathogens. For example, an estimated 89,785 invasive MRSA infections were associated with 15,249 deaths in 2008 in the United States. Even when the infection is successfully treated it can double the average length of a hospital stay and thereby increase healthcare costs. MRSA is able to produce a range of unique toxins causing severe infections such as endocarditis, osteomyelitis, pneumonia and septic shock in patients with open wounds, invasive devices and weakened immune systems.

One part of the problem is that bacteria and other microbial pathogens are very hardy and have developed different antimicrobial resistance mechanisms. Several interesting acronyms have been coined to describe the range of resistance from multi-drug resistant (MDR), extensively drug resistant (XDR), to totally drug resistant (TDR). The increasing use and misuse of existing antibiotics in human and veterinary medicine as well as agriculture has also exacerbated the problem. Antimicrobial resistance is genetically based; resistance is mediated by the acquisition of extrachromosomal genetic elements containing resistance genes, such as plasmids, transposable genetic elements and genomic islands, which are transferred between bacteria via horizontal gene transfer.

Last month, a new study published in the New England Journal of Medicine was able to identify which isolates of MRSA were part of a hospital outbreak using whole genome sequencing. Scientists from the Wellcome Trust Sanger Institute, University of Cambridge and Illumina sequenced the isolates within a timescale that could influence infection control and patient management. The authors created an artificial “resistome” of antibiotic-resistance genes and demonstrated concordance between it and the results of phenotypic susceptibility testing. They also created a “toxome” consisting of toxin genes.

Bioline offers a range products for use in microbial research and below we document some of the publications in which Bioline reagents were used to advance research.

The Petez-Osorio group used ImmoMix Red to describe a multiplex PCR method, the Mycobacterial IDentification and Drug Resistance Screen (MID-DRS) assay, which allows identification of members of the Mycobacterium tuberculosis complex (MTBC) and the simultaneous amplification of targets for sequencing-based drug resistance screening of rifampin-resistant (rifampinr), isoniazidr, and pyrazinamider TB. This MID-DRS assay reduces the time necessary for initial identification and drug resistance screening of TB specimens to as little as two days with reduced assay costs, preparation time and risks due to user errors.

Petez-Osorio, A. C., et al. J. Clin. Microbiol. 50(2): 326-336 (2012) Rapid Identification of Mycobacteria and Drug-Resistant Mycobacterium tuberculosis by Use of a Single Multiplex PCR and DNA Sequencing

SensiMix SYBR One-Step Kit
The Rosato group at the Center for Molecular and Translational Human Infectious Diseases Research, Methodist Hospital Research Institute in Houston, USA conducts translational research focusing on multidrug-resistant bacterial pathogens. The aim of their study was to identify the molecular genetic causes of drug resistance and resistance gene spread in MRSA.

In this paper, Dr Rosato’s group used DNA microarrays and qRT-PCR to evaluate differential gene expression during HeR-HoR selection and found increased expression of the agr two-component regulatory system. Their findings reinforce the concept that increased expression of agr during HeR-HoR selection plays a critical role in regulating the ß-lactam-induced increased mutation rate in very heterogeneous MRSA strains.

Plata, K. B., et al. Antimicrob. Agents Chemother. 55(7): 3176-3186 (2011) Fate of Mutation Rate Depends on agr Locus Expression during Oxacillin-Mediated Heterogeneous-Homogeneous Selection in Methicillin-Resistant Staphylococcus aureus Clinical Strains

In another exciting paper from Dr Rosato’s group at the Methodist Hospital Research Institute, Houston, they teamed up with researchers from Virginia Commonwealth University and Cubist Pharmaceuticals to investigate the molecular basis of resistance to daptomycin (DAP), a new class of cyclic lipopeptide antibiotic highly active against methicillin-resistant Staphylococcus aureus (MRSA) infections. Proposed drug mechanisms include disruption of the bacterial membrane wall.

Differential gene expression analysis using SensiMix One-Step Kit also showed up of the two-component regulatory system vraSR. Crucially, this effect was related to the impact of vraSR and mprF mutations in the cell wall. Their work underscores the suggestion that alterations in these two genes contributes to DAP-resistance in this group of clinical MRSA strains.

Mehta, S., et al. Antimicrob. Agents Chemother. 56(1): 92-102 (2012) VraSR Two-Component Regulatory System Contributes to mprF-Mediated Decreased Susceptibility to Daptomycin in In Vivo-Selected Clinical Strains of Methicillin-ResistantStaphylococcus aureus

BIO-X-ACT DNA Polymerase
The Morrissey group at the University of Leicester, UK conducts research into microbial genetics. One of their major areas of investigation is the Staphylococcus aureus adaptive response to antibacterial copper. Their results shows copper resistance varies considerably between clinical strains due to the carriage of an additional plasmid-encoded copper homeostasis mechanism, copBmco. Importantly, this plasmid has the potential to spread to other S. aureus strains. This is the first time that plasmid-encoded copper resistance has been reported and shown to be transferable between pathogenic bacteria isolated from humans.

Baker, J., et al. Environ. Microbiol. 13(9): 2495-2507 (2011) The Staphylococcus aureus CsoR regulates both chromosomal and plasmid-encoded copper resistance mechanisms

MangoTaq DNA Polymerase
The aim of this study at St. Vincent’s Hospital in Australia was to determine the utility of molecular methods compared to selective agars for MRSA detection. The conclusion of the van Hal group was that molecular detection methods for MRSA remain sensitive and rapid, but are associated with greater expense.

van Hal, S. J., et al. Euro. J. Clin. Microbiol. & Infectious Dis. 28(1): 47-53 (2010) MRSA detection: comparison of two molecular methods (BD GeneOhm PCR assay and Easy -Plex) with two selective MRSA agars (MRSA-ID and Oxoid MRSA) for nasal swabs

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