The invention relates to a the cosmetic use of a bacterial lysate pertaining to the Vitreoscilla sp. genus (particularly the Vitreoscilla filiformis species) in a complete fermentation medium, as an active agent for preventing and/or treating dandruff on the scalp, including dandruff on the scalp associated with a prevalence of the pathogenic microorganisms on the scalp and/or an imbalance of the ecoflora of the scalp.