Staphylococcusaureus (S. aureus) isa bacterium responsible for several life-threatening diseases such as osteomyelitis, septicemia, pneumonia, endocarditis and toxic shock syndrome. The infection vector is usually food and this makes the rapid and highly sensitive detection of these gram-positive bacteria in food samples vital to prevent contamination. The most used method for detection is culture in solid medium and counting of colonies, since it is inexpensive and easy to perform.
However, this method takes several hours or even days and it is not useful for large scale and rapid screening of samples in the field. There are other methods used nowadays that rely on the detection of nucleic acids. These methods include polymerase chain reaction, fluorescence in situ hybridization and surface plasmon resonance sensor. These methods are highly specific and sensitive but require extensive sample processing to isolate the nucleic acids.
Magnetics beads introduce a new approach for the techniques to detectS. aureus since they can be coupled with molecules that bind specifically to these bacteria and can be easily isolated by applying magnetic fields. This allows high sensitivity and specificity while making the protocol simpler. The work from Yang and colleagues, used the magnetic beads with an antibiotic, vancomycin. This is possible since vancomycin has the ability to bindwiththeD-alanyl-D-alaninedipeptide that is present inthecellwallofS. aureus. This allows the magnetic bead-vancomycin conjugate to detect and concentrate S. aureus and is a cheaper and more stable method of detection than the previously used ones. However, it is not specific since vancomycin targets all gram-positive bacteria and not only S. aureus. Therefore, the authors developed a double recognition mechanism to increase specificity. Vancomycin magnetic beads conjugates are first applied to bind and concentrate S. aureus, then alkaline phosphatase coupled with a rabbit IgG antibody is applied to provide chemiluminescent detection specific of the target bacteria. This method is highly specific, rapid, and sensitive, improving upon the existing methods, all thanks to the application of magnetic beads technology.
Dual-recognition detection of Staphylococcus aureus using vancomycin-functionalized magnetic beads as concentration carriers.Yang, S., Ouyang, H., Su, X., Gao, H., Kong, W., Wang, M., Fu, Z. (2016). Biosensors and Bioelectronics, 78, 174–180. doi:10.1016/j.bios.2015.11.041
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