Abstract: Extracts of nonfruiting (nonfructifying) nonphotosynthetic filamentous bacteria, notably the bacterium Vitreoscilla filiformis, are well suited for improving the structure of keratin materials, especially for (i) hardening the nails, increasing their thickness, stimulating their rate of growth and/or of regrowth, and increasing their resistance to breaking and/or to splitting; or (ii) increasing the elasticity and/or maintaining the shaping of the hair and/or of the eyelashes; or (iii) for shaping and/or styling the hair, or for the care of and/or for making up the eyelashes or the nails.