Screening of Lactic Acid Bacteria for Their Calcium Enrichment Capacity and Structural Characteristics of Calcium-Enriched Bacteria

To investigate the potential application of lactic acid bacteria (LAB) as a novel calcium carrier, 20 LAB strains isolated from traditional fermented dairy products were evaluated for growth curves and characteristics in MRS medium and calcium-rich MRS (Ca-MRS) medium. Calcium content was determined using atomic absorption spectroscopy (AAS) to select strains with high calcium enrichment capacity. Structural characteristics of calcium-enriched bacteria were analyzed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Results showed that the tested strains reached the late logarithmic growth phase 2–4 h later when cultured in Ca-MRS medium than in MRS medium, with the optical density at 600 nm (OD₆₀₀) value slightly decreasing upon reaching the stationary phase. Among the 20 strains, Lactobacillus casei KLDS1.0375 exhibited the highest calcium enrichment capacity, reaching 43.94 mg/g. Structural analysis indicated that bacteria bound calcium primarily through surface functional groups such as amino groups, amide bonds, hydroxyl groups, and C–H bonds. After calcium enrichment, the bacterial cells exhibited an amorphous state, with rougher surfaces, and enhanced intracellular calcium fluorescence intensity; the calcium content reached 2.08%. Meanwhile, the thermal stability improved. This study has elucidated the calcium enrichment capacity of L. casei KLDS1.0375 and characterized its cellular structure following calcium accumulation, laying the foundation for the development and application of lactic acid bacteria as a novel calcium carrier.