# Rapid and Simple Detection of Mycobacterium avium subsp. paratuberculosis Using a Lateral Flow Assay Based on CRISPR-Cas12a Combined with Recombinase Polymerase Amplification or Nested PCR

## TL;DR

This paper introduces a new rapid and sensitive method for detecting Mycobacterium avium subsp. paratuberculosis using CRISPR-Cas12a combined with amplification techniques.

## Contribution

The first integration of RPA or nested PCR with CRISPR-Cas12a for MAP detection is presented.

## Key findings

- The RPA–CRISPR-Cas12a and nested PCR–CRISPR-Cas12a assays achieved high sensitivity with detection limits of 1 × 10−10 μg/μL and 1 × 10−14 μg/μL, respectively.
- The methods showed perfect agreement with reference assays on clinical samples and MAP cultures.
- Flexible detection options using qPCR, fluorescence, or lateral flow strips were validated.

## Abstract

Paratuberculosis (PTB), caused by Mycobacterium avium subsp. paratuberculosis (MAP), is a chronic intestinal disease in ruminants. PTB is difficult to diagnose, control, and eradicate, leading to substantial economic losses. Thus, sensitive and specific detection methods are urgently required. crRNA and primers targeting the MAP ATPase FtsK gene were designed for recombinase polymerase amplification (RPA) and nested PCR. Fecal DNA was amplified using RPA or nested PCR, purified with Tris-saturated phenol-chloroform-isoamyl alcohol, and detected via CRISPR-Cas12a. Moreover, signals were read using a qPCR instrument, fluorescence reader, or lateral flow strips. RPA–CRISPR-Cas12a and nested PCR–CRISPR-Cas12a assays were optimized and validated on 50 clinical samples and 7 MAP cultures. The limits of detection were 1 × 10−10 μg/μL for RPA–CRISPR-Cas12a and 1 × 10−14 μg/μL for nested PCR–CRISPR-Cas12a. Efficient cleavage of the ssDNA reporter occurred at DNA concentrations of ≥1 × 10−4 μg/μL, producing a strong fluorescent signal. All three detection methods showed perfect agreement with reference assays across both sample sets. This study presents the first integration of RPA or nested PCR with CRISPR-Cas12a for MAP detection, enabling rapid, specific, and highly sensitive diagnosis. Flexible detection options allow adaptation to available resources and bacterial loads, supporting practical use in PTB control.