Ingestion of Staphylococcus aureus by bovine endothelial cells results in time- and inoculum-dependent damage to endothelial cell monolayers

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Abstract
Cultured endothelial cells phagocytize Staphylococcus aureus, but the resultant effects are unknown. Monolayers of cultured bovine endothelial cells with or without [3H]adenine label were exposed to 100, 10, or 1 S. aureus organism per endothelial cell for 3.5 h. Lysostaphin was then applied to all cultures to destroy extracellular but not phagocytized S. aureus. In cultures treated for only 20 min with lysostaphin, S. aureua multiplied exponentially after a 9- to 12-h lag period. In cultures treated continously with lysostaphin, numbers of S. aureus remained constant or decreased. These results indicate that S. aureus became extracellular and multiplied but did not multiply intracellularly. In parallel experiments, the release of 3H-adenine from prelabled endothelial cell monolayers was assayed to indicate cytotoxicity. Results indicated that the loss of 3H-adenine from endothelial cell monolayers depended on the following: (i) the size of the S. aureus inoculum, (ii) the strain of S. aureus, and (iii) the length of time after exposure by S. aureus, S. aureus endocarditis and persistent septicemia could arise, at least in part, from ingestion of S. aureus by host endothelium. The intracellular location would afford S. aureus protection from host defenses and antibiotics. Eventual damage to endothelial cells could expose collagen, thus resulting in platelet adherence and vegetation formation. Intracellular S. aureus would be continuously released into the circulation, possibly accounting for the persistant bacteremia that is found in S. aureus endovascular infections.