Smartphone-based, sensitive µPAD detection of urinary tract infection and gonorrhea

The presence of bacteria in urine can be used to monitor the onset or prognosis of urinary tract infection (UTI) and some sexually-transmitted diseases (STDs), such as gonorrhea. Typically, bacteria's presence in urine is confirmed by culturing samples overnight on agar plates, followed by a microscopic examination. Additionally, the presence of Escherichia coli in a urine sample can be indirectly confirmed through assaying for nitrite (generated by reducing nitrate in urine), however this is not sufficiently specific and sensitive. Species/strains identification of bacteria in a urine sample provides insight to appropriate antibiotic treatment options. In this work, a microfluidic paper analytical device (µPAD) was designed and fabricated for evaluating UTI (E. coli) and STD (Neisseria gonorrhoeae) from human urine samples. Anti-E. coli or anti-N. gonorrhoeae antibodies were conjugated to submicron particles then pre-loaded and dried in the center of each paper microfluidic channel. Human urine samples (undiluted) spiked with E. coli or N. gonorrhoeae were incubated for 5 min with 1% Tween 80. The bacteria-spiked urine samples were then introduced to the inlet of paper microfluidic channel, which flowed through the channel by capillary force. Data confirms that proteins were not filtered by μPAD, which is essential for this assay. Urobilin, the component responsible for the yellow appearance of urine and green fluorescence emission, was filtered by μPAD, resulting in significantly minimized false-positive signals. This filtration was simultaneously made during the μPAD assay and no pretreatment/purification step was necessary. Antibody-conjugated particles were immunoagglutinated at the center of the paper channel. The extent of immunoagglutination was quantified by angle-specific Mie scatter under ambient lighting conditions, utilizing a smartphone camera as a detector. The total μPAD assay time was less than 30s. The detection limit was 10 CFU/mL for both E. coli and N. gonorrhoeae, while commercially available gonorrhea rapid kit showed a detection limit of 10(6) CFU/mL. A commercially available nitrite assay test strip also had a detection limit of 10(6) CFU/mL, but this method is not antibody-based and thus not sufficiently specific. By optimizing the particle concentration, we were also able to extend the linear range of the assay up to 10(7) CFU/mL. The proposed prototype will serve as a low-cost, point-of-care, sensitive urinalysis biosensor to monitor UTI and gonorrhea from human urine.

The highly abundant urinary metabolite urobilin interferes with the bicinchoninic acid assay

Estimation of total protein concentration is an essential step in any protein- or peptide-centric analysis pipeline. This study demonstrates that urobilin, a breakdown product of heme and a major constituent of urine, interferes considerably with the bicinchoninic acid (BCA) assay. This interference is probably due to the propensity of urobilin to reduce cupric ions (Cu(2+)) to cuprous ions (Cu(1+)), thus mimicking the reduction of copper by proteins, which the assay was designed to do. In addition, it is demonstrated that the Bradford assay is more resistant to the influence of urobilin and other small molecules. As such, urobilin has a strong confounding effect on the estimate of total protein concentrations obtained by BCA assay and thus this assay should not be used for urinary protein quantification. It is recommended that the Bradford assay be used instead.