21 October 2016

Staphylococcus aureus (S. aureus) is a gram positive bacterium that can live a double life. Widespread in the environment as a commensal, S. aureus can also be a versatile and dangerous human pathogen. As one of the most commonly isolated pathogens in nosocomial infections S. aureus is adept causing infections in high-risk patients, particularly post-operative surgical site infections (SSI). The situation is exacerbated by the emergence of methicillin-resistant S. aureus (MRSA), leading to increased mortality rates, hospitalisation periods and a significant financial burden for health services; with an estimate annual medical costs to be around $5 billion in the US alone 1.

Patients most at risk to MRSA SSI are already colonised; latest estimates suggest ~20% of the population are persistent carriers, ~80% intermittent carriers. It has been shown that eradicating MRSA from its most common niche, the anterior nares, radically reduces colonisation elsewhere and consequently SSI rates plummet. Early detection is therefore crucial to patient outcomes. Allied to this, the prognosis of minor to moderate infections improves with early and effective treatment.

Numerous detection methods are currently employed to address this problem, with significant pitfalls. Bacterial culture and metabolic tests are standard protocols for bacterial identification in use by most hospitals. This process might take days to identify the bacterium—an unacceptable delay in critical situations such as sepsis. To combat the delay, several ultra-sensitive detection methods based on nucleic acid amplification have been introduced, capable of detecting low numbers of bacterial cells within several hours 2. However, these technologies require prior isolation of bacterial DNA, reagent preparation and expensive instruments for nucleic acid amplification. High costs and complex procedures limit the widespread use of these technologies.

Antibody-based immunoassays are also well established. Ultrasensitive detection by this approach is limited by antibody characteristics; they are proteins and cannot be amplified. This limitation was circumvented by the development of immuno-PCR, wherein the antibody is cross-linked with a DNA “barcode” for PCR amplification 3. Although this technology is sensitive, the conjugation and purification of antibody-DNA complexes is still a major challenge.

Aptamers targeting S. aureus

Using aptamers in new platform technologies offers a potential alternative to the aforementioned methods, addressing many of the drawbacks.

Aptamers are short nucleic acids that exhibit high affinity and specificity to their targets without the high-costs and ethical concerns of animal husbandry associated with antibody production. Researchers can freely distribute aptamers and develop assays without proprietary issues increasing the per-sample cost of immuno-affinity assays. A highly sensitive, selective dual-aptamer-based immunosensor for the detection of S. aureus was recently reported. The assay incorporated a biotinylated primary anti-S.aureus aptamer that serves as a capture probe, and a secondary anti-S.aureus aptamer conjugated to silver nanoparticles (Apt-AgNP) that reports the detection of the target. The electrochemical immunosensor showed an extended dynamic range from 10 to 106 cfu ml-1 with a low detection limit of 1.0 cfu ml-1 4.

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Figure 1. Aptamer S.aureus and aptamer-AgNP sandwich complex is formed on the Magnetic Bead surface and the electrochemical signal of AgNPs read-out as a simple and sensitive detection method 4.

Huang et al. has also reported aptamers that bind to S. aureus enterotoxin A (SEA) with 48.57 ± 6.52 nM affinity and high selectivity. This aptamer was then used successfully in a fluorescent bioassay detecting SEA in a food sample with a detection limit of 8.7 × 10−3 μg ml−1 3.

Aptamers can therefore be used as new molecular recognition elements to facilitate innovative biosensors for highly sensitive and specific S. aureus detections. This new platform technology may have potential for development as a rapid and sensitive multiplex detection system for common pathogens in clinical settings such as intensive care units. Taken together, the versatility of aptamers in this setting makes them an appealing choice of affinity reagents for future development of point-of-care pathogen testing.

To learn more about the Aptamer Group’s Diagnostic arm, please click the following link Aptadx.