Functional Nucleic Acids as Bacterial Biosensors: A Focus on <i>Clostridioides difficile</i> Infection

Abstract Clostridioides difficile (CD) is a Gram‐positive, anaerobic, and spore‐forming bacillus that colonizes the human gut and causes a range of diseases, such as pseudomembranous colitis and antibiotic‐associated diarrhea, that are generally known as CD infection (CDI). Rapid and accurate detection of CDI with high sensitivity and specificity is crucial for patient treatment, infection control, and epidemiological monitoring. Current diagnostic methods for CDI have several limitations, such as high cost, long turnaround time, suboptimal sensitivity, and the need for specialized equipment. Hence, novel detection methods that can overcome these limitations are needed. Functional nucleic acids (FNAs) are a promising class of molecular recognition element (MRE) that can be incorporated into biosensors for detecting infectious pathogens. Several FNAs have been developed for detecting CD. In this review, an overview of CD, CDI, and current diagnostic methods for CDI and their drawbacks are provided. Furthermore, the design principles and working mechanisms of FNAs as well as their applications for the detection of pathogenic bacteria, including CD, are discussed. The potential for developing point‐of‐care paper sensors using currently available CD‐selective FNAs is also highlighted.