Au@Fe₃O₄ Nanoparticle-Based Colorimetric Aptasensor for Noninvasive Screening of Colorectal Cancer via Detection of <i>Parvimonas micra</i>

Colorectal cancer (CRC) is a common malignancy requiring early screening to improve patient outcomes. Current screening methods such as colonoscopy and fecal occult blood tests have several limitations including high cost, poor specificity, invasiveness, and inconvenience. Recent research has identified specific bacterial communities associated with CRC, notably <i>Parvimonas micra</i> (<i>P. micra</i>), which serves as a biomarker for early screening and diagnosis owing to its accumulation in the malignant tissues and feces of CRC patients. Herein, we employed the whole-bacterium systematic evolution of ligands by the exponential enrichment (SELEX) method to isolate high-affinity aptamers against <i>P. micra</i> using 17 selection cycles. These aptamers were subsequently bound to Au@Fe₃O₄ nanoparticles, and the interaction of <i>P. micra</i> and aptamers inhibited the peroxidase-like activity of Au@Fe₃O₄ nanoparticles, thereby blocking the 3,3',5,5'-tetramethylbenzidine (TMB) chromogenic reaction and resulting in a measurable reduction in absorbance. This colorimetric detection strategy demonstrated a linear response across a range of 10⁰-10⁸ CFU/mL for <i>P. micra</i> with a limit of detection of 11 CFU/mL. Using a colorimetric aptasensor, we assessed the abundance of <i>P. micra</i> in clinical fecal samples and found significantly higher levels in the feces of CRC patients as compared to that of healthy individuals, which was consistent with the quantitative polymerase chain reaction results. This study therefore represents the first successful identification of an aptamer with high affinity and specificity for <i>P. micra</i>, leading to the development of a highly specific and sensitive aptasensor for its detection. The presented approach has a significant potential for CRC screening and diagnosis.