Disposable amperometric magnetoimmunosensors for the specific detection of Streptococcus pneumoniae

Disposable amperometric magnetoimmunosensors, based on the use of functionalized magnetic beads and gold screen-printed electrodes, have been developed for the selective detection and quantification of Streptococcus pneumoniae. A specific antibody prepared against a serotype 37 S. pneumoniae strain, selected by flow cytometry among seven anticapsular or antisomatic antibodies, was linked to Protein A-modified magnetic beads and incubated with bacteria. The same antibody, conjugated with horseradish peroxidase, was attached to the bacteria and the resulting modified magnetic beads were captured by a magnetic field on the surface of tetrathiafulvalene-modified gold screen-printed electrodes. The amperometric response obtained at -0.15 V vs. the silver pseudoreference electrode of the Au/SPE after the addition of H(2)O(2) was used as transduction signal. Different assay formats were examined and the experimental variables optimized. The limits of detection achieved, without pre-concentration or pre-enrichment steps, were 1.5×10(4) cfu mL(-1) (colony forming unit) and 6.3×10(5) cfu mL(-1) for S. pneumoniae strains Dawn (serotype 37) and R6 (non-encapsulated), respectively. The developed methodology shows a good selectivity against closely related streptococci and its usefulness for the analysis of inoculated urine samples has been demonstrated. The total analysis time of 3.5 h from sampling to measurement, the possibility to prepare up to 30 sensors per day and the use of small amounts of test solution for S. pneumoniae identification, constitute important advantages that make the developed methodology a promising alternative for clinical diagnosis.