A method for treating a skin disorder e.g. acne e.g. acne vulgaris in a subject is provided. The method comprises administering e.g. applying e.g. topically administering ammonia oxidizing bacteria e.g. a preparation comprising ammonia oxidizing bacteria to a surface of the subject. Preparations comprising ammonia oxidizing bacteria for treating such skin disorder e.g. acne e.g. acne vulgaris in a subject are also provided.