Light microscope through human heart tissue showing bacterial myocarditis. Myocarditis is an inflammation of the heart muscle (myocardium). It can affect the heart muscle reducing the heart's ability to pump, causing rapid or abnormal heart rhythms (arrhythmias). Bacterial myocarditis is an uncommon cause of infectious myocarditis, usually seen in the context of overwhelming sepsis or associated with specific bacterial infections, such as staphylococcus and streptococcus.