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.