Neutrophils have long been regarded as essential for host defense against Staphylococcus aureus infection. However, survival of the pathogen inside various cells, including phagocytes, has been proposed as a mechanism for persistence of this microorganism in certain infections. Therefore, we investigated whether survival of the pathogen inside polymorphonuclear neutrophils (PMN) contributes to the pathogenesis of S. aureus infection. Our data demonstrate that PMN isolated from the site of infection contain viable intracellular organisms and that these infected PMN are sufficient to establish infection in a naive animal. In addition, we show that limiting, but not ablating, PMN migration into the site of infection enhances host defense and that repletion of PMN, as well as promoting PMN influx by CXC chemokine administration, leads to decreased survival of the mice and an increased bacterial burden. Moreover, a global regulator mutant of S. aureus (sar−) that lacks the expression of several virulence factors is less able to survive and/or avoid clearance in the presence of PMN. These data suggest that the ability of S. aureus to exploit the inflammatory response of the host by surviving inside PMN is a virulence mechanism for this pathogen and that modulation of the inflammatory response is sufficient to significantly alter morbidity and mortality induced by S. aureus infection.

The growth of Staphylococcus epidermidis sensu stricto and Staphylococcus saprophyticus on Memphis agar yielded up to 6 morphotypes with each strain. With S. epidermidis, one morphotype produced slime (rho) but became non-slime-producing (epsilon) at a high frequency. The slime-producing rho variants were methicillin-resistant and more virulent than methicillin-susceptible epsilon variants in an endocarditis model. With S. saprophyticus, phase variation was of higher frequency. Nitrosoguanidine mutagenesis produced a stable blue epsilon form that was more virulent than the parent in a mouse model of urinary tract infection. Mutants with the blue epsilon phenotype differed from gold epsilon parents in a variety of phenotypic properties, including increased resistance to oxacillin. These staphylococcal species have a high frequency of phase variation: Phase variants differ in antibiotic resistance and virulence, which is only partially correlated with suggested virulence factors such as slime production.

SummaryInfection with gram-negative and gram-positive bacteria remains a leading cause of death in patients with systemic lupus erythematosis (SLE), even in the absence of immunosuppresive therapy. To elucidate the mechanisms that underly the increased risk of infection observed in patients with systemic autoimmunity, we have investigated host defense against bacterial infection in a murine model of autoimmunity, the MRL/Mp-lpr/lpr (MRL/Ipr) mouse. Our previous study implicated transforming growth factor ~ (TGF-/$) in a novel acquired defect in neutrophil function in MRL/lpr but not congenic MRL/Mp-+/+ (MR.L/n) mice (Gresham, H.D., C.J. Ray, and F.K. O'Sullivan. 1991.j. Immunol. 146:3911.) We hypothesized from these observations that MRL/lpr mice would have defects in host defense against bacterial infection and that they would have constitutively higher local and systemic levels of active TGF-/~ which would be responsible, at least in part, for the defect in host defense. We show in this paper that spontaneous elaboration of active TGF-/$ adversely affects host defense against both gram-negative and gram-positive bacterial infection in MRL/1pr mice. Our data indicate that MRL/lpr mice, as compared with congenic MRL/n mice, exhibit decreased survival in response to bacterial infection, that polymorphonuclear leukocytes (PMN) from MR1/lpr mice fail to migrate to the site of infection during the initial stages of infection, that MR.L/lpr mice have a significantly increased bacterial burden at the site of infection and at other tissue sites, and that this increased bacterial growth occurs at a time (>20 h after infection) when PMN influx is greatly enhanced in MRL/lpr mice. Most intriguingly, the alteration in PMN extravasation during the initial stages of infection and failure to restrict bacterial growth in vivo could be duplicated in MRL/n mice with a parenteral injection of active TGF-/51 at the time of bacterial challenge. Moreover, these alterations in host defense, including survival in response to lethal infection, could be ameliorated in MRL/lpr mice by the parenteral administration of a monoclonal antibody that neutralizes the activity of TGF-fi. These data indicate that elaboration of TGF-fi as a result of autoimmune phenomenon suppresses host defense against bacterial infection and that such a mechanism could be responsible for the increased risk of bacterial infection observed in patients with autoimmune diseases.

The virulence of the Staphylococcus aureus strains that differed only in capsule expression was compared in a rat model of catheter-induced experimental endocarditis. The ID50 of all the strains was low (less than 3 x 10(3) cfu of S. aureus), suggesting that this model may be more sensitive than other animal models to differences in bacterial virulence. Compared with the wild-type strains that expressed type 5 or type 8 capsular polysaccharides, mutant strains devoid of capsule had significantly lower ID50 values. In contrast, a mutant that produced scant amounts of the type 5 polysaccharide had an ID50 similar to that of the parental type 5 isolate. As the bacterial inoculum was increased, each of the S. aureus strains reached final concentrations of 10(10)-10(11) cfu/g of vegetation; however, the nonencapsulated mutants colonized the left-sided vegetations at lower inocula than did the wild-type strains. This study indicates that microcapsule expression attenuates bacterial virulence in a rat model of catheter-induced endocarditis.

Inactivation of fibronectin (Fn) binding by insertional mutagenesis of Streptococcus sanguis with Tn916 reduces virulence of this bacterium in the rat model of infective endocarditis (IE). Transconjugants were screened for Fn adherence using an ELISA adherence test. One transconjugant had a decreased adherence to immobilized Fn. Southern hybridization demonstrated that the insertion occurred only once in this mutant. The parent strain and mutant strain JL113 were used as challenge strains in a rat endocarditis model. These experiments demonstrated that the mutant had a reduced ability (P < 0.05) to produce TE. Spontaneous excision of Tn916 from JL113 produced strains identical to both the parental and mutant phenotypes. One strain that retained the mutant phenotype and one (JLR-15) that regained the parental phenotype for Fn binding were tested for their ability to produce IE. These strains demonstrated that the ability to bind Fn and to produce IE were correlated after Tn916 excision. The reduced virulence of the mutant suggested that adherence of S. sanguis to immobilized Fn plays an important role in the production of IE. (J. Clin. Invest. 1990. 86:7-13.)