Fibrinogen Depletion Attenuates Staphyloccocus aureus Infection by Preventing Density-Dependent Virulence Gene Up-Regulation

Abstract

Staphylococcus aureus undergoes a density-dependent conversion in phenotype from tissue-adhering to tissue-damaging and phagocyte-evading that is mediated in part by the quorum-sensing operon, agr, and its effector, RNAIII. Contributions of host factors to this mechanism for regulating virulence have not been studied. We hypothesized that fibrinogen, as a component of the inflammatory response, could create spatially constrained microenvironments around bacteria that increase density independently of bacterial numbers and thus potentiate quorum-sensing-dependent virulence gene expression. Here we show that transient fibrinogen depletion significantly reduces the bacterial burden and the consequential morbidity and mortality during experimental infection with wild-type S. aureus, but not with bacteria that lack expression of the quorum-sensing operon, agr. In addition, it inhibits in vivo activation of the promoter for the agr effector, RNAIII, and downstream targets of RNAIII, including α hemolysin and capsule production. Moreover, both in vitro and in vivo, the mechanism for promoting this phenotypic switch in virulence involves clumping of the bacteria, demonstrating that S. aureus responds to fibrinogen-mediated bacterial clumping by enhancing density-dependent virulence gene expression. These data demonstrate that down-modulation of specific inflammatory components of the host that augment bacterial quorum sensing can be a strategy for enhancing host defense against infection.

Inactivation of a bacterial virulence pheromone by phagocyte-derived oxidants: New role for the NADPH oxidase in host defense

“…Pouches created by the injection of air (7) were infected with 100 l of early exponential phase WT or agr Ϫ bacteria. After 4-28 h, the pouch was lavaged (5 ml) and sonicated, and the kidneys (a measure of blood stream metastasis) were homogenized and sonicated before colony-forming unit (cfu) evaluation (7). The intracellular viable cfu (25) in the This paper was submitted directly (Track II) to the PNAS office.…”

“…Early exponential phase, nonfluorescent bacteria with a mean channel of fluorescence (MCF) of 10-20 by flow cytometry (FACSCalibur, Becton Dickinson) were prepared as described (7). Culture supernatants from WT and agr Ϫ bacteria were prepared as described (7).…”

“…AIP group I WT RN6390, its agr deleted mutant RN6911, and AIP group I methicillin-resistant S. aureus (MRSA) COL strains containing plasmids encoding the promoter for RNAIII or ␣ hemolysin (hla) driving expression of GFP were generated as described (24). Early exponential phase, nonfluorescent bacteria with a mean channel of fluorescence (MCF) of 10-20 by flow cytometry (FACSCalibur, Becton Dickinson) were prepared as described (7). Culture supernatants from WT and agr Ϫ bacteria were prepared as described (7).…”

“…Staphylococcus aureus is a medically important bacterial pathogen that uses quorum sensing to regulate virulence in several experimental models of infection (4,(7)(8)(9). The agr operon, which is responsible in part for this regulation, combines secretion of an autoinducing thiolactone peptide (AIP) with a two-component regulatory pathway to generate a regulatory RNA transcript, RNAIII, that is the effector of the operon (10)(11)(12).…”

Stimulus-Dependent Impairment of the Neutrophil Oxidative Burst Response in Lactoferrin-Deficient Mice

“…To compare the antimicrobial response of WT and LFKO mice to S. aureus challenge, both an airpouch 55,56 and an intraperitoneal 50 model of bacterial infection were performed. For the airpouch study, a subcutaneous pouch was created in anesthetized WT and LFKO mice on day 0 using 5 ml of sterile air followed by reinflation on day 3 with 2.5 ml of sterile air.…”