Structure-activity correlation experiments demonstrated that the fine structure of 3-oxo-C 12 -HSL, the HSL backbone, and side chain length are required for maximal activity. These data suggest that Pseudomonas 3-oxo-C 12 -HSL specifically promotes induction of apoptosis, which may be associated with 3-oxo-C 12 -HSL-induced cytotoxicity in macrophages and neutrophils. Our data suggest that the quorum-sensing molecule 3-oxo-C 12 -HSL has critical roles in the pathogenesis of P. aeruginosa infection, not only in the induction of bacterial virulence factors but also in the modulation of host responses.

We report that 2 g of azithromycin/ml inhibits the quorum-sensing circuitry of Pseudomonas aeruginosa strain PAO1. Addition of synthetic autoinducers partially restored the expression of the trancriptional activator-encoding genes lasR and rhlR but not that of the autoinducer synthase-encoding gene lasI. We propose that azithromycin interferes with the synthesis of autoinducers, by an unknown mechanism, leading to a reduction of virulence factor production.

The contribution of neutrophils to lethal sensitivity and cytokine balance governing T1 and T2 host responses was assessed in a murine model of Legionella pneumophila pneumonia. Neutrophil depletion by administration of granulocyte-specific mAb RB6-8C5 at 1 day before infection rendered mice ∼100-fold more susceptible to lethal pneumonia induced by L. pneumophila. However, this treatment did not alter early bacterial clearance, despite a substantial decrease in neutrophil influx at this time point. Cytokine profiles in the lungs of control mice demonstrated strong T1 responses, characterized by an increase of IFN-γ and IL-12. In contrast, neutrophil-depleted mice exhibited significantly lower levels of IFN-γ and IL-12, and elevation of T2 cytokines, IL-4 and IL-10. Immunohistochemistry of bronchoalveolar lavage cells demonstrated the presence of IL-12 in neutrophils, but not alveolar macrophages. Moreover, IL-12 was detected in lavage cell lysates by ELISA, which was paralleled to neutrophil number. However, intratracheal administration of recombinant murine IL-12 did not restore resistance, whereas reconstitution of IFN-γ drastically improved bacterial clearance and survival in neutrophil-depleted mice. Taken together, these data demonstrated that neutrophils play crucial roles in primary L. pneumophila infection, not via direct killing but more immunomodulatory effects. Our results suggest that the early recruitment of neutrophils may contribute to T1 polarization in a murine model of L. pneumophila pneumonia.

The accumulation of certain species of bacteria in the intestine is involved in both tissue homeostasis and immune-mediated pathologies. The host mechanisms involved in controlling intestinal colonization with commensal bacteria are poorly understood. We observed that under specific pathogen-free or germ-free conditions, intragastric administration of Pseudomonas aeruginosa, E. coli, Staphylococcus aureus, or Lactobacillus gasseri resulted in increased colonization of the small intestine and bacterial translocation in mice lacking Cd1d, an MHC class I-like molecule, compared with WT mice. In contrast, activation of Cd1d-restricted T cells (NKT cells) with α-galactosylceramide caused diminished intestinal colonization with the same bacterial strains. We also found prominent differences in the composition of intestinal microbiota, including increased adherent bacteria, in Cd1d -/-mice in comparison to WT mice under specific pathogen-free conditions. Germ-free Cd1d -/-mice exhibited a defect in Paneth cell granule ultrastructure and ability to degranulate after bacterial colonization. In vitro, NKT cells were shown to induce the release of lysozyme from intestinal crypts. Together, these data support a role for Cd1d in regulating intestinal colonization through mechanisms that include the control of Paneth cell function.

We evaluated the efficacy of bacteriophage (phage) therapy by using a murine model of gut-derived sepsis caused by Pseudomonas aeruginosa that closely resembles the clinical pathophysiology of septicemia in humans. Oral administration of a newly isolated lytic phage strain (KPP10) significantly protected mice against mortality (survival rates, 66.7% for the phage-treated group versus 0% for the saline-treated control group; P < 0.01). Mice treated with phage also had lower numbers of viable P. aeruginosa cells in their blood, liver, and spleen. The levels of inflammatory cytokines (tumor necrosis factor alpha TNF-␣, interleukin-1␤ [IL-1␤], and IL-6) in blood and liver were significantly lower in phage-treated mice than in phage-untreated mice. The number of viable P. aeruginosa cells in fecal matter in the gastrointestinal tract was significantly lower in phage-treated mice than in the saline-treated control mice. We also studied the efficacy of phage treatment for intraperitoneal infection caused by P. aeruginosa and found that phage treatment significantly improved the survival of mice, but only under limited experimental conditions. In conclusion, our findings suggest that oral administration of phage may be effective against gut-derived sepsis caused by P. aeruginosa.