Bacteria naturally live in a multispecies community. Cell-to-cell communication is therefore crucial for bacteria to
adapt, survive, and regulate virulence. Indole has been recently reported as an intercellular signal that regulates diverse bacterial
physiologies such as biofilm formation, antibiotic tolerance, and virulence factor production. The role of indole on bacterial
virulence was mainly studied in indole-producing bacteria. Therefore, this study aimed to study the effect of indole on pathogenic
non-indole-producing bacteria including Klebsiella pneumoniae, Proteus mirabilis, and Enterobacter cloacae. Bacteria were
cultured in a physiological concentration of indole (1 mM), and then growth, biofilm formation, autoaggregation, and motility
were evaluated. Indole could increase biofilm formation and autoaggregation of K. pneumoniae, whereas biofilm formation and
autoaggregation decreased in E. cloacae and P. mirabilis. Growth of K. pneumoniae and E. cloacae was slightly affected by
exposure to indole. This finding indicated that indole influences the virulence of pathogenic non-indole-producing bacteria in
different ways. Further studies are still required for a better understanding of the biological functions of indole signaling.