AimsSkin colonization of Staphylococcus spp. critically affects the severity of dermatitis in humans and animals. We examined different types of fatty acid salts for their antibacterial activity against Staphylococcus spp. when used in ultrapure soft water (UPSW). We also evaluated their therapeutic effect on a spontaneous canine model of dermatitis. Methods and ResultsUPSW, in which Ca++ and Mg++ were replaced with Na+, was generated using a water softener with cation-exchange resin. Staphylococcus aureus (Staph.aureus), Staphylococcus intermedius (Staph. intermedius), and Staphylococcus pseudintermedius (Staph.pseudintermedius) were incubated with various fatty acid salts in distilled water (DW) or UPSW and the number of bacteria was counted. Among the fatty acids, oleic acid salt and linoleic acid (LA) salt reduced the number of these bacteria. Also, UPSW enhanced the antibacterial effect of LA on Staph. spp. In spontaneously developed itchy dermatitis in companion dogs, shampoo treatment with liquid soap containing 10% LA in UPSW improved skin conditions. ConclusionsLA salt showed antibacterial activity against Staph. spp. Treatment with soap containing LA with UPSW reduced clinical conditions in dogs with dermatitis. Significance and Impact of the StudyBecause colonization of Staph. spp. on the skin exacerbates dermatitis, the use of LA-containing soap in UPSW may reduce unpleasant clinical symptoms of the skin.