testing, its accuracy can be comparatively poor for certain organisms and antibiotics, so the disc diffusion test remains useful as a backup method. Genetic methods Genetic testing, such as via polymerase chain reaction (PCR), DNA microarray, DNA chips, and loop-mediated isothermal amplification, may be used to detect whether bacteria possess genes which confer antibiotic resistance. An example is the use of PCR to detect the mecA gene for beta-lactam resistant Staphylococcus aureus. Other examples include assays for testing vancomycin resistance genes vanA and vanB in Enteroccocus species, and antibiotic resistance in Pseudomonas aeruginosa, Klebsiella pneumoniae and Escherichia coli. These tests have the benefit of being direct and rapid, as compared with observable methods, and have a high likelihood of detecting a finding when there is one to detect. However, whether resistance genes are detected does not always match the resistance profile seen with phenotypic method. The tests are also expensive and require specifically trained personnel. Polymerase chain reaction is a method of identifying genes related to antibiotic susceptibility. In the PCR process, a bacterium's DNA is denatured and the two strands of the double helix separate. Primers specific to a sought-after gene are added to a solution containing the DNA, and a DNA polymerase is added alongside a mixture containing molecules that will be needed (for example, nucleotides and ions). If the relevant gene is present, every time this process runs, the quantity of the target gene will be doubled. After this process, the presence of the genes is demonstrated through a variety of methods including electrophoresis, southern blotting, and other DNA sequencing analysis methods. DNA microarrays and chips use the binding of complementary DNA to a target gene or nucleic acid sequence. The benefit of this is that multiple genes can be assessed simultaneously. MALDI-TOF Matrix-assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS) is another method of susceptibility testing. This is a form of time-of-flight mass spectrometry, in which the molecules of a bacterium are subject to matrix-assisted laser desorption. The ionised particles are then accelerated, and spectral peaks recorded, producing an expression profile, which is capable of differentiating specific bacterial strains after being compared to known profiles. This includes, in the context of antibiotic susceptibility testing, strains such as beta-lactamase producing E coli. MALDI-TOF is rapid and automated. There are limitations to testing in this format however; results may not match the results of phenotypic testing,