Novel lysozyme–mannooligosaccharide conjugate with improved antimicrobial activity: preparation and characterization

Lysozyme is a well-known natural antimicrobial agent commonly used in the food industry. However, the lack of antimicrobial activity against Gram-negative bacteria limits its application. In order to extend the antimicrobial spectrum of lysozyme, a lysozyme–mannooligosaccharide (MOS) conjugate was prepared in this study via the Maillard reaction under optimal conditions, i.e., the lysozyme–MOS ratio of 1:7.5, pH 7.5 and reaction duration of 6 days. The prepared conjugate was subjected to a series of assays, including the free amino group content assay, intrinsic fluorescence spectrometry, surface hydrophobicity assay and FT-IR spectrometry. During conjugation, 55.76% of the free amino groups on lysozyme were consumed, suggesting better reactivity of MOS. Both intrinsic fluorescence spectra and surface hydrophobicity index indicated that more hydrophobic residues embedded in lysozyme would be exposed during conjugation, but the conjugation of MOS lowered both parameters, making the lysozyme–MOS conjugate more hydrophilic and soluble. Furthermore, the antimicrobial activity demonstrated that the lysozyme–MOS conjugate possessed improved activity against Gram-negative bacteria, including Escherichia coli and Salmonella enterica, with almost 50% reduction of log CFU, compared to the native lysozyme. The extended antimicrobial spectrum of lysozyme will broaden its applications as a preservative and provide an effective alternative to existing antibiotics in the food industry.