Long period fiber grating working in reflection mode as valuable biosensing platform for the detection of drug resistant bacteria

• A new label-free optical biosensor based on reflection-type long period grating (RT-LPG).
• RT-LPG sensitivity improvement in biological solutions by means an aPS overlay, enabling the modal transition phenomenon.
• A covalent bioreceptors (3-APBA) immobilization onto the biosensor surface by means of a secondary PMMA-co-MA overlay.
• RT-LPG biosensor capability to detect purified AmpC Betalactamases (BLs) in PBS with limits of detection of a few tens on nM.
• Confirmation of the effectiveness of the LPG biosensor in lysate samples, containing E.Coli bacteria over expressing AmpC BL.

We report here on a reflection-type long period fiber grating (RT-LPG) biosensor for the fast detection of class C (AmpC) β-lactamases (BLs), actually considered as one of the most important source of resistance to β-lactam antibiotics expressed by resistant bacteria. A standard LPG working in transmission configuration is first transformed in a more practical probe working in reflection mode and successively coated with a primary high refractive index (HRI) overlay of atactic polystyrene (aPS) in order to increase its surrounding refractive index sensitivity (SRI) in biological solutions. The aPS-coated RT-LPG is then coated by a secondary layer of poly(methylmethacrylate)-co-methacrylic acid (PMMA-co-MA) in order to provide the necessary surface functionalities to promote a stable covalent bioreceptors immobilization. The BLs detection has been performed by using the 3-aminophenylboronic acid (3-APBA) as biorecognition element, due to its excellent inhibition properties against class C BLs and specificity. Results here provided demonstrate that the proposed label free biosensor is capable of reliable detection of purified AmpC BLs in phosphate buffer solutions (PBS) with concentrations as low as one hundred nM, with a lowest limit of detection (LOD) of the order of a few tens of nM. The real effectiveness of the proposed biosensor has been also confirmed in lysate samples, which contain Escherichia coli bacteria overexpressing AmpC BLs.