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Staphylococcus aureus infections are often life threatening. Relatively little is known about the host response to these infections, in particular, the role played by cytokines. We established a mouse model of bacteremic S. aureus infection to correlate bacteriologic findings and pathologic changes with cytokine gene expression. Bacterial density in blood and tissue was highest at 1 h and minimal by 48 h. Despite the rapid clearance of bacteria, pathologic abnormalities and inflammatory cytokines were detected after clearance of the bacteria. The number of infiltrating inflammatory cells, as well as the size of inflammatory foci, increased with time. Interstitial accumulation of inflammatory cells and tissue damage, such as microabscesses, edema, and necrosis progressed following clearance of bacteria from the tissues. Levels of tumor necrosis factor and interleukin-1 protein in serum were detectable at 1 h and peaked at 4 h. Interleukin-6 protein expression showed different kinetics, with low levels detected at 1 h and increasing levels at 72 h postinfection. Tumor necrosis factor and the interleukins were expressed in inflammatory and noninflammatory cells in lung, liver, and heart tissues. Leukocytes in the infected tissues were highly reactive with antibodies to the three cytokines, suggesting that activated leukocytes are a major source of inflammatory cytokines after staphylococcal infection. Expression of interleukin-1 and interleukin-6 in tissue-specific cells and endothelial cells was also detected in infected tissues, indicating that cells other than leukocytes contribute to the elevated cytokine levels in this model. Once initiated, expression of inflammatory cytokines contributes to the pathogenesis of S. aureus disease.

Interleukin-8 (IL-8) plays a crucial role in the migration of neutrophils to bacterium-infected sites. This study investigated IL-8 induction in Staphylococcus aureus-infected endothelial cells (EC). We observed that S. aureus-infected EC were induced to express both IL-8 mRNA and protein, which can promote transmigration of neutrophils across an EC monolayer.

Colonization of human nasal mucosa with Staphylococcus aureus sets the stage for subsequent systemic infection. This study characterizes S. aureus adhesion to nasal mucosa in vitro and investigates the interaction of S. aureus with human nasal mucin. S. aureus binding to cell-associated and cell-free mucus was greater than to nonmucin-coated epithelial cells. Scanning electron microscopy of S. aureus incubated with human nasal mucosal tissue showed minimal binding to ciliated respiratory epithelium. In a solid-phase assay, S. aureus bound to purified human nasal mucin-coated wells significantly more than to bovine serum albumin-coated microtiter wells. Binding to mucin was saturable in a dose- and time-dependent fashion. Staphylococcal adherence to human nasal mucin was inhibited by bovine submaxillary mucin but not by fibrinogen. Pretreatment of mucin with periodate but not with pronase reduced adherence. Trypsin treatment of the bacteria significantly reduced adherence to mucin. 125I-labelled nasal mucin bound to two surface proteins (138 and 127 kDa) of lysostaphin-solubilized S. aureus. Binding to human nasal mucin occurs in part via specific adhesin-receptor interactions involving bacterial proteins and the carbohydrate moiety in mucin. These experiments suggest that S. aureus binding to mucin may be critical for colonization of the nasopharyngeal mucosa.

The ability of the vascular endothelium to elaborate cytokines in response to gram-positive sepsis has received limited attention. This study examined cytokine expression by human umbilical vein endothelial cells (EC) following infection with a gram-positive bacterial pathogen, Staphylococcus aureus. S. aureus infection of EC resulted in the production of interleukin-6 (IL-6) and IL-1 beta. For IL-6, message was detected at 3 h after infection, protein was present at 24 h, and both message and protein persisted for 72 h. IL-1 beta message was detected at 12 h, IL-1 beta protein was detected at 24 h, and both persisted for 72 h. Message for colony-stimulating factor 1 remained unaltered. UV-killed S. aureus also elicited IL-1 beta and IL-6 message and protein expression at 24 and 48 h. Twenty-one clinical isolates of S. aureus were tested, and all induced IL-6 release by 48 h. However, the laboratory strain 8325-4 did not induce cytokine expression at any time point and was internalized by EC 1,000-fold less than other strains were. Internalization of latex beads by EC did not induce IL-6 gene expression. Furthermore, cytochalasin D treatment of the EC prevented IL-1 and IL-6 induction by S. aureus but not by tumor necrosis factor alpha or lipopolysaccharide. These results indicate that S. aureus is a potent inducer of IL-1 and IL-6 in EC and that internalization of S. aureus by EC is necessary for their cytokine expression. Thus, our data suggest that the vascular endothelium may play an important role in the pathogenesis of septicemia caused by gram-positive organisms.

The adherence of Staphylococcus aureus to human endothelial cells is saturable in both dose- and time-dependent assays. Staphylococcal surface components which bound to endothelial cells in vitro were identified by using biotin-labeled, solubilized staphylococcal proteins. Four trypsin-sensitive components with molecular sizes of 30, 55 to 57, 70, and 85 kDa were recognized. These proteins did not label with the glycan detection system. When staphylococci were harvested during the exponential phase of growth, staphylococcal adherence to endothelial cells was significantly increased and increased expression of the S. aureus binding proteins was observed. Preincubation of endothelial cells with protein A did not reduce S. aureus adherence in an in vitro infection assay. Four S. aureus surface components whose expression is growth phase dependent adhere to human endothelial cells in vitro.

A 220-kDa glycoprotein from yeast extract causes a twofold decrease in S. aureus adherence to human endothelial cells in vitro. Medium constituents can have a significant effect on bacterial adherence interactions.

Differences in the ability of bacteria to adhere to normal valvular endothelium may account for the predominance of particular species as pathogens in acute endocarditis. An in vitro adherence assay was developed to simulate the host surface encountered in acute bacterial endocarditis by using confluent monolayers of human endothelial cells. Adherence of 32 gram-positive and -negative blood culture isolates to this surface was compared. All five Staphylococcus aureus strains tested were highly adherent to endothelial cells, as was one gram-negative strain (Serratia marcescens). The remaining gram-positive and -negative isolates, including four viridans streptococci, were relatively nonadherent. Transmission electron microscopy demonstrated attachment of Staphylococcus aureus and invagination of the underlying endothelial cell membrane at 1 h followed by engulfment of large numbers of bacteria after 3 h. The intracellular bacteria appeared to be contained within vacuoles. Preferential attachment of some strains of bacteria, in particular Staphylococcus aureus, to human endothelial cells occurred in vitro, suggesting that adherence is an important determinant of bacterial pathogenicity in acute endocarditis. Active uptake of bacteria by endothelial cells may help account for the virulence of Staphylococcus aureus in endovascular infections and for the ability of this organism to establish multiple metastatic foci of infection.

Alteration of human endothelial cells may increase their susceptibility to staphylococcal invasion and thus may contribute to the development of intravascular staphylococcal disease. Acidic fibroblast growth factor, a potent regulator of endothelial cell function, had a significant effect on Staphylococcus aureus infection of cultured human endothelial cells. Three of four S. aureus strains had diminished adherence to endothelial cells when the latter were grown in the presence of acidic fibroblast growth factor (P less than 0.05). The diminished adherence was time dependent, maximal at 72 h, and independent of the initial bacterial inoculum. A twofold enhancement of S. aureus adherence was observed when endothelial cells were pretreated with heparitinase. Adherence was unaffected by endothelial cell activation by interleukin-1 or endotoxin. Thus, acidic fibroblast growth factor exerted a protective effect, deterring S. aureus adherence to cultured endothelial cells. Endothelial cell heparan sulfate was also directly involved in the adherence process. Subtle modulations of endothelial cells can significantly affect the ability of S. aureus to adhere to and then infect these cells. Similar alterations may contribute to the ability of S. aureus to infect endovascular tissue in vivo.