Using gadolinium ions as affinity probes to selectively enrich and magnetically isolate bacteria from complex samples

Magnetic trapping has been employed in the development of analytical methods owing to its ease and simplicity in handling samples. Nevertheless, the generation of functional probes is usually time consuming. A new and simple affinity method that uses gadolinium ion (Gd³⁺), a magnetic ion, as affinity probe for magnetic tapping of pathogenic bacteria was demonstrated in the present study. Escherichia coli O157:H7, Staphylococcus aureus, and Acinetobacter baumannii were selected as model bacteria. The model bacteria were magnetically isolated after incubation in Tris buffer (pH 8) containing Gd³⁺ (0.1 M) under microwave heating (power: 180 W, 90 s × 3). The resultant Gd³⁺-bacterium conjugates possessed sufficient magnetism, resulting in magnetic aggregations by an external magnet (∼4,000 Gauss). For ease of magnetic isolation, the sample containing Gd³⁺-bacterium complexes was stirred by a small magnet. After 1 h, the magnet attached with precipitates, i.e., Gd³⁺-bacterium conjugates, was readily removed using a pair of tweezers. The bacteria in the resultant conjugates were characterized by matrix-assisted laser desorption/ionization mass spectrometry. The limits of detection of the current approach toward E. coli O157:H7, S. aureus, and A. baumannii in complex samples were ∼10⁴-10⁵ cells mL^-1.