Capsule and O-antigen from an extraintestinal isolate of Escherichia coli modulate cytokine levels in rat macrophages in vitro and in a rat model of pneumonia

Effect of spleen surgeries on Escherichia coli distribution on the mononuclear phagocytic system

OBJECTIVE

To avoid asplenic state, many approaches preserving the spleen have been proposed in the literature: splenorraphy, partial splenectomy with or without preservation of hilar vessels and splenic tissue auto-implants. Subtotal splenectomy, preserving the upper spleen pole nourished only by splenogastric vessels is an alternative when the splenic pedicle must be ligated. The purpose of this study was to evaluate the influence of partial, subtotal and total splenectomies on the distribution of Escherichia coli in the mononuclear phagocytic system.

METHOD

Thirty-two rats divided into four groups were studied: sham operation (total spleen preservation), partial splenectomy, subtotal splenectomy and total splenectomy. Five weeks after surgeries, an aliquot of E. coli marked with tecnetium-99m was injected intravenously. The animals were killed 20min later and the spleen, lungs and liver were removed in order to determine the distribution of labeled bacteria.

RESULTS

The amount of E. coli in the splenic tissue was greater in the intact spleen group than in the partial or subtotal splenectomy groups. Phagocytosis through the spleen did not differ between the partial and subtotal splenectomy groups. The amount of bacteria in the lungs was greater in the partial than in the subtotal splenectomy group. The distribution of labeled bacteria was greater in the liver of animals submitted to subtotal splenectomy than in the other groups.

CONCLUSION

The upper splenic pole, supplied only by splenogastric vessels, has the ability to remove live bacteria from the blood stream, showing that effective blood clearance occurs even without vascularization through the splenic pedicle. Thus, the distribution of E. coli through the mononuclear phagocytic system shows different behavior depending on the type of splenectomy to which the animals are submitted.

Rat pneumonia and soft-tissue infection models for the study of Acinetobacter baumannii biology

Acinetobacter baumannii is a bacterial pathogen of increasing medical importance. Little is known about its mechanisms of pathogenesis, and safe reliable agents with predictable activity against A. baumannii are presently nonexistent. The availability of relevant animal infection models will facilitate the study of Acinetobacter biology. In this report we tested the hypothesis that the rat pneumonia and soft-tissue infection models that our laboratory had previously used for studies of extraintestinal pathogenic Escherichia coli were clinically relevant for A. baumannii. Advantages of these models over previously described models were that the animals were not rendered neutropenic and they did not receive porcine mucin with bacterial challenge. Using the A. baumannii model pathogen 307-0294 as the challenge pathogen, the pneumonia model demonstrated all of the features of infection that are critical for a clinically relevant model: namely, bacterial growth/clearance, an ensuing host inflammatory response, acute lung injury, and, following progressive bacterial proliferation, death due to respiratory failure. We were also able to demonstrate growth of 307-0294 in the soft-tissue infection model. Next we tested the hypothesis that the soft-tissue infection model could be used to discriminate between the inherent differences in virulence of various A. baumannii clinical isolates. The ability of A. baumannii to grow and/or be cleared in this model was dependent on the challenge strain. We also hypothesized that complement is an important host factor in protecting against A. baumannii infection in vivo. In support of this hypothesis was the observation that the serum sensitivity of various A. baumannii clinical isolates in vitro roughly paralleled their growth/clearance in the soft-tissue infection model in vivo. Lastly we hypothesized that the soft-tissue infection model would serve as an efficient screening mechanism for identifying gene essentiality for drug discovery. Random mutants of 307-0294 were initially screened for lack of growth in human ascites in vitro. Selected mutants were subsequently used for challenge in the soft-tissue infection model to determine if the disrupted gene was essential for growth in vivo. Using this approach, we have been able to successfully identify a number of genes essential for the growth of 307-0294 in vivo. In summary, these models are clinically relevant and can be used to study the innate virulence of various Acinetobacter clinical isolates and to assess potential virulence factors, vaccine candidates, and drug targets in vivo and can be used for pharmacokinetic and chemotherapeutic investigations.