Terahertz Imaging Detects Oral Cariogenic Microbial Domains Characteristics

Dental caries, associated with plaque biofilm, is highly prevalent and significantly burdens public health. <i>Streptococcus mutans</i> is the main cariogenic bacteria that adheres to the tooth surface and forms an abundant extracellular polysaccharide matrix (EPS) as a cariogenic biofilm scaffold. <i>S. mutans</i> RNase III-encoding gene (<i>rnc</i>) and a putative chromosome segregation protein-encoding gene (<i>smc</i>) are potentially associated with EPS production. In addition, complex interactions between <i>S. mutans</i> and other oral microorganisms synergistically or antagonistically affect the cariogenicity. Commensal streptococci suppress the growth of cariogenic pathogens, whereas <i>Candida albicans</i> mediates the formation of cariogenic biofilm through aggregation and dual-species biofilm formation with <i>S. mutans</i>. However, label-free detection of cariogenic microbial interactions with the EPS matrix is still challenging during laboratory investigations. Herein, we hypothesized that the <i>S. mutans rnc-smc</i> operon affects EPS production and aimed to observe streptococci, <i>S. mutans</i>, and <i>S. mutans-C. albicans</i> using terahertz scanning near-field optical microscopy (THz s-SNOM). The light in the 0.1- to 0.3-THz frequency range interacted with the sample through a nano-probe tip by a point-by-point scanning process. Additional noise reduction of the original image was achieved by a dual kernel Gaussian filter. The monospecies of streptococci, <i>S. mutans smc/rnc</i> mutants, and the dual-species of <i>S. mutans-C. albicans</i> were scanned by THz s-SNOM. This technique provided terahertz near-field scanning images of <i>S. mutans smc/rnc</i> mutants, streptococci, and dual-species of <i>S. mutans-C. albicans</i>. Additional analysis of the original images potentially revealed the structures of the strains, such as cell diameters and cell wall thickness. In conclusion, the results suggested that the <i>S. mutans rnc-smc</i> operon regulates EPS production. Furthermore, this novel label-free detection of a THz near-field scanning technique had the potential to observe the morphologies of bacterial cells and EPS matrix.