This bacillus is so named because it mutates almost as soon as it is sub-cultured from a non-lactose to a lactose fermenter and is of interest mainly from a bacteriological point of view as the Bacillus Mutabile is an intermediary form between the B. Coli and the true non-lactose fermenting type. Its associated remedy is Pulsatilla and the nosode Mutabile (Bach) is likely to be of value in treatment where there is alteration of symptoms, e. g. where skin eruption alternates with asthmatic symptoms.