Ultrastructure of cardiac bacterial vegetations on native valves with emphasis on alterations in bacterial morphology following antibiotic treatment.

Bacterial vegetations involving the aortic valves of six patients were studied by scanning and transmission electron microscopy. The microorganisms isolated were Staphylococcus aureus--2, coagulase-negative staphylococcus--1, Streptococcus fecalis--2 and Streptococcus MG--1. The surface of the vegetations was usually amorphous. However, in areas where the surface of the vegetation was broken, myriads of microorganisms were seen. Transmission electron microscopy revealed bacteria embedded in an electron dense matrix in all vegetations despite the fact that they were negative on culture. Cell wall rupture was common. Thinning, and in some instances thickening, of the cell wall was also observed. The Streptococcus MG cells showed abnormal division with daughter cells being unable to separate. It is likely that the altered morphology of these bacterial cells is due to antibiotic treatment, however studies of an animal model of endocarditis are needed to dissect out the various possible contributions to these changes: host defenses; bacterial malnutrition in the depths of the vegetation; and antibiotic effect.