The invention relates to methods, kits and compositions for reducing the level of or eliminating Bacteroides in situ. The invention encompasses methods of preventing myocarditis, treating myocarditis or dilated cardiomyopathy, or limiting progression of myocarditis toward dilated cardiomyopathy in a subject in need thereof, comprising reducing the amount of Bacteroides sp. in the subject. The invention further encompasses methods of diagnosis of a subject as having myocarditis or dilated cardiomyopathy. The invention also encompasses compositions preventing myocarditis, treating myocarditis or dilated cardiomyopathy, or limiting progression of myocarditis toward dilated cardiomyopathy in a subject in need thereof.