Fc-MBL-modified Fe3O4 magnetic bead enrichment and fixation in Gram stain for rapid detection of low-concentration bacteria

Functional bacterial enrichment magnetic beads (Fe3O4@SiO2@Fc-MBL) and Gram staining were combined for the fast diagnosis of infecting bacteria in meningitis. Fe3O4@SiO2@Fc-MBL has excellent microbial binding ability and can be used for bacterial enrichment from cerebrospinal fluid (CSF). The enriched bacteria are recognized by Gram stain at very low concentrations (10 CFU·mL−1). The feasibility of this method was verified by five common bacteria in meningitis infection (Gram-positive: Staphylococcus epidermidis, Staphylococcus haemolyticus, and Staphylococcus capitis; Gram-negative: Klebsiella pneumoniae and Escherichia coli). The extraction efficiency of Fc-MBL-modified Fe3O4 magnetic beads was approximately 90% in artificial CSF for the selected bacteria, with the exception of E. coli (~ 60%). The bacteria were successfully recognized by Gram staining and microscopic observation. Fe3O4@SiO2@Fc-MBL acts by capturing and fixing the bacteria in a magnetic field throughout the experiment. Compared with traditional CSF Gram staining, this new method avoids interference by inflammatory cells and red blood cells during microscopic examination. Furthermore, the sensitivity of this method is much better than the centrifugation smear method. The whole process can be accomplished within 30 min. This novel method may have potential as a clinical tool for analysis of bacteria in the CSF.Graphical abstract