A morphological comparison of treatment with different beta-lactam antibiotics on experimental staphylococcal endocarditis and aortitis.

The effect of treatment of staphylococcal endocarditis and aortitis with five different beta-lactam antibiotics (ceftazidime, cephaloridine, cefotaxime, methicillin and flucloxacillin) was evaluated by light and electron microscopy. It was found that therapy with all five antibiotics produced similar morphological changes. At 3 and 8 h, the bacterial colonies showed zonal changes with the bacteria furthest from the lumen exhibiting less severe damage while the outer region consisted largely of lysed cells. However, in the outer zone a few apparently viable, thick-walled persistent bacteria were observed. At 24 and 48 h, many colonies consisted of large masses of lysed bacteria with only a few thick-walled persistent bacteria. In all cases, therapy was associated with an increased host inflammatory cell response resulting in invasion of leucocytes through the aortic wall or vegetation towards and engulfing the colonies. However, even at 48 h the inflammatory cells had not reached all the deep-seated colonies. It would appear that all the antibiotics reached bactericidal concentrations within the lesions. However, the eradication of the few 'persistent' bacteria was delayed by the inability of the inflammatory cells to reach all the colonies.