Visible label-free detection of bacterial DNA using flocculation of sterically stabilised cationic latexes

The current gold standard diagnostic for bacterial infections is the use of culture, which can be time consuming and can take up to five days for results to be reported. There is therefore an unmet clinical need for a rapid and label free alternative. This paper demonstrates a method of detecting the presence of amplified DNA from bacterial samples using a sterically-stabilised, cationic polymer latex and widely available equipment, providing an accessible alternative DNA detection technique. If DNA is present in a sample, successful amplification by polymerase chain-reaction (PCR) results in the amplified DNA inducing flocculation of the polymer latex followed by rapid sedimentation. This results in a visible and obvious change from a milky-white dispersion to a precipitated latex with a colourless and transparent supernatant, thus giving a clear visual indication of the presence or absence of amplified DNA. Specifically, the response of four polymer latexes with different morphologies to the addition of amplified bacterial DNA was investigated. Cationic latexes flocculated rapidly whereas non-ionic and anionic latexes did not, as judged by eye, disc centrifuge photosedimentometry (DCP), and UV-visible spectrophotometry. The stability of several cationic latexes with different morphologies in typical PCR reagents was investigated. It was found that unwanted flocculation occurred for a latex with a non-ionic core and a cationic corona (poly[2-vinyl pyridine-b-benzyl methacrylate], prepared by polymerisation-induced self-assembly) whereas a ∼700 nm PEGMA-stabilised P2VP latex (non-ionic stabiliser, cationic core), prepared by emulsion polymerisation remained stable. The sensitivity and rate of sedimentation of the PEGMA-stabilised P2VP latex was demonstrated by varying the sequence length and concentration of amplified DNA from Pseudomonas aeruginosa using universal bacterial primers. DNA concentrations as low as 0.78 ng μl^-1 could readily be detected within 30 minutes from the addition of amplified DNA to the latex. Furthermore, the specificity of this method was demonstrated by showing a negative result occurs (no flocculation of the latex) when PCR product from a fungal (Candida albicans) sample using bacterial primers was added to the latex.