<p><i>Salmonella typhimurium</i>, a leading cause of global foodborne illness, demands rapid, on-site detection to combat its widespread contamination in food supplies—where delayed diagnosis contributes to thousands of annual fatalities. Here, we present a novel colorimetric biosensing platform that exploits the intrinsic antimicrobial properties of silver ions (Ag⁺) to enable instrument-free, visual quantification of <i>Salmonella typhimurium</i>. The sensor relies on the controlled growth of gold@silver core–shell nanorods (Au@AgNRs), whose localized surface plasmon resonance (LSPR) peak undergoes a vivid, concentration-dependent color shift as silver shell deposition modulates their aspect ratio. In the absence of bacteria, Ag⁺ ions are reduced by ascorbic acid (AA) and uniformly coat AuNRs, inducing a blue shift. However, when <i>Salmonella typhimurium</i> is present, bacterial cells sequester Ag⁺ through interactions with surface biomolecules, inhibiting complete shell formation and resulting in a measurable red shift in the LSPR peak accompanied by visually discernible color transitions from red to magenta, purple, violet, dark blue, and teal. The system achieves a linear dynamic range of 18.75 × 10⁶ to 112.5 × 10⁶ CFU mL⁻¹ and a limit of detection (LOD) of 2.0 × 10⁶ CFU mL⁻¹. Prior to detection, target bacteria are isolated from complex matrices using anti-Salmonella aptamer-conjugated magnetic nanoparticles (MNPs), ensuring specificity and minimizing matrix interference. Successful validation in spiked chicken bouillon samples (recovery: 93.3%, RSD: 3.5%) confirms practical utility under real-world conditions. This strategy uniquely merges microbiology with nanomaterials science, offering a low-cost, rapid, and visually interpretable solution for decentralized pathogen screening.</p>