Red cabbage extract-mediated colorimetric sensor for swift, sensitive and economic detection of urease-positive bacteria by naked eye and Smartphone platform.

The bacterial pathogens have caused various serious infectious diseases in the human body, and even some threats to human life by leading to deaths. Enterobacteriaceae species especially urease positive ones, Proteus mirabilis (P. mirabilis) and Klebsiella pneumoniae (K. pneumoniae), show resistance to antibiotics and cause respiratory and urinary tract infections. We have developed natural indicator-incorporated colorimetric urease tests with a naked eye and smartphone readout to rapidly, sensitively and economically detect P. mirabilis and K. pneumoniae. We utilized anthocyanin found as a predominant component in red cabbage (Brassica oleracea) extract as a natural pH indicator instead of toxic and synthetic indicators. As a mechanistic explanation for the detection of P. mirabilis and K. pneumoniae, urease enzymes secreted from the P. mirabilis and K. pneumoniae hydrolyze urea to produce ammonia (NH3), which increases the pH value of the reaction environment and leads to deprotonation from anthocyanins. The changes in the molecular structure and electronic structure of anthocyanins are responsible for revealing many different colors. We demonstrated how some reaction parameters including the concentration of the bacteria (colony-forming unit, CFU), the concentration of anthocyanin in the tests, initial color and pH values (pHs) of the tests influence their detection performance. We further developed a 3D-printed smartphone platform with smartphone based digital image processing software to improve the detection limit and shorten the detection time. We claim that natural indicator-incorporated rapid urease tests providing colorimetric readout evaluated by the human eye and smartphone imaging processing has great potential in practical use and they can be implemented in clinics.

Preparation of natural indicator incorporated media and its logical use as a colorimetric biosensor for rapid and sensitive detection of Methicillin-resistant Staphylococcus aureus.

Herein, we report a rational design in preparation of natural indicator incorporated media used as a rapid, selective, sensitive and economic colorimetric biosensor for detection of Methicillin-resistant Staphylococcus aureus (MRSA). Anthocyanins obtained from red cabbage (Brassica oleracea) extract (RCE) are major components and acted as pH responsive indicators in the RCE media. The RCE media was tested for eight gram-positive, four gram-negative bacterial and one model fungal pathogens. We experimentally revealed how salt concentration and antibiotic (cefoxitin) in the RCE media influence the growth of pathogens and especially MRSA. Benefiting pH dependent color change function of anthocyanins is indispensable point of the RCE media in sensing of MRSA growth. The potential MRSA colorimetric sensing mechanism of anthocyanins relies on both protonation and decrease in electron density on structures of anthocyanins by acidic organic volatile compounds produced during growth of MRSA. The protonated anthocyanins with low electron density changed original purple color of the RCE media to pink color. We demonstrated that detection of MRSA was achieved by spectrophotometrically (one single cell) and visually with naked eyes (100 cells) in 2nd hrs and 6th hrs of incubation, respectively. We optimized concentrations of cefoxitin (from 1 µg/mL to 20 µg/mL) against MRSA and Methicillin-sensitive Staphylococcus aureus (MSSA) bacterial cell (3 McFarland) suspensions, then growth of MRSA was visually and clearly detected in the presence of 4 µg/mL cefoxitin between 90 min and 3 h. We persistently offer that the proposed RCE media can be a well alternative to currently used phenotypic methods in clinics owing to its much rapid, sensitive, selective and economic properties.