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We initiated a worldwide collaborative study, including 455 episodes of bacteremia, to elucidate the clinical patterns of Klebsiella pneumoniae. Historically, community-acquired pneumonia has been consistently associated with K. pneumoniae. Only four cases of community-acquired bacteremic K. pneumoniae pneumonia were seen in the 2-year study period in the United States, Argentina, Europe, or Australia; none were in alcoholics. In contrast, 53 cases of bacteremic K. pneumoniae pneumonia were observed in South Africa and Taiwan, where an association with alcoholism persisted (p=0.007). Twenty-five cases of a distinctive syndrome consisting of K. pneumoniae bacteremia in conjunction with community-acquired liver abscess, meningitis, or endophthalmitis were observed. A distinctive form of K. pneumoniae infection, often causing liver abscess, was identified, almost exclusively in Taiwan.

Flt3 ligand (Flt3L) plays a critical role in the proliferation, differentiation and survival of haematopoietic progenitor cells. Its potential use in a clinical setting has been suggested. Here, we report that mice administered Flt3L displayed a nine-fold increase in size of their hepatic non-parenchymal cell population and an approximate 365-fold increase in number of mature dendritic cells within their livers. Such mice exhibited an elevated resistance to secondary infections with Listeria monocytogenes, an intracellular bacterial pathogen. More than 2.0 log(10)fewer listeriae were recovered in the livers of Flt3L-treated, than untreated, mice on day 2 following secondary challenge. Importantly, Flt3L-pretreated mice immunized with an avirulent (listeriolysin O-negative) strain of Listeria harbored significantly fewer ( approximately 1.5 log(10)) organisms in their spleens and livers than did control mice immunized with listeriolysin O-negative listeriae and challenged with a lethal dose of bacteria. The latter finding supports a potential role for Flt3L in strategies to develop vaccines to intracellular pathogens., (Copyright 2001 Academic Press.)

The uptake of Listeria monocytogenes by a variety of cell types in vitro is facilitated by the protein products of the inlAB (internalin) operon expressed by the organism. In the case of mouse hepatocytes, the extent to which inlAB expression influenced the uptake of Listeria in vitro was markedly dependent upon the ratio of bacteria to cells. At a ratio of 100:1, greater than 40-fold fewer transposon-induced inl4B mutant listeriae entered hepatocytes compared to the isogenic wild-type control; the difference was only fourfold, however, in cultures inoculated at a 1:1 ratio. Similarly, the uptake of in-frame inlB or inlAB deletion mutants differed only fourfold from the uptake of wild-type or inlA mutant Listeria at a 1:1 multiplicity of infection. Mutations affecting inlB or inlAB, on the other hand, resulted in a marked decrease in the capacity of Listeria to proliferate within mouse hepatocytes in vivo and in vitro. Electron micrographs of Listeria-infected hepatocytes demonstrated the impaired capacity of inlB mutants to escape from endocytic vacuoles and to enter the cytoplasm where proliferation occurs. These findings indicate that the protein product of inlB exerts a significant effect on the intracellular replication of Listeria.

Listeria monocytogenes injected intravenously into mice is taken up in the liver, where hepatocytes serve as the principal site of intracellular replication. The factors effecting entry of L. monocytogenes into hepatic cells remain to be determined. Others have shown that the protein products of the inlAB (internalin) operon are required for maximum entry of L. monocytogenes into a number of cell lines in vitro. Likewise, we report here that expression of the inlAB operon was required for maximum uptake of L. monocytogenes by primary cultures of mouse hepatocytes. Uptake of an inlAB mutant strain of L. monocytogenes was approximately 10-fold less than that of the isogenic wild-type control. In contrast, inlAB expression was not a factor in (i) clearance of L. monocytogenes injected intravenously into mice and taken up in the liver, (ii) the distribution of L. monocytogenes among hepatocytes and nonparenchymal cells in the liver, or (iii) internalization of L. monocytogenes by hepatic cells in vivo. These latter findings suggest that infection of hepatic cells by L. monocytogenes in vivo does not require the protein products of the inlAB operon.

The critical role of the liver in the resolution of systemic bacterial infections is well documented. In the case of Listeria monocytogenes, approximately 60% of bacteria inoculated i.v. into mice are recovered in the liver at 10 min after infection. Here we report that the Listeria recovered at 10 min were distributed equally among the hepatocyte and nonparenchymal liver cell populations. The majority (>/= 75%) of these organisms were bound extracellularly as judged by their sensitivity to gentamicin. In contrast, >/= 93% of Listeria recovered in the liver at 6 h were located within hepatocytes. The listerial burden of the liver decreased 0.5 to 1.0 log, between 10 min and 6 h after infection. This decrease correlated with a sevenfold increase in the percentage of neutrophils that constituted the nonparenchymal cell population. Mice rendered neutrophil deficient by pretreatment with anti-granulocyte (RB6-8C5) mAb exhibited a significant increase (>300%) rather than a decrease in liver Listeria and a marked increase in hepatocyte damage. Similarly, neutrophil-deficient mice exhibited a reduced capacity to eliminate Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus that were cleared by the liver and bound extracellularly to hepatocytes and nonparenchymal cells. These findings document the crucial role of immigrating neutrophils in nonspecific host defenses to systemic bacterial infections expressed within the liver.

Using analogues of arginine to inhibit nitric oxide (NO.) production, investigators have demonstrated the intermediary role of NO. in a variety of physiological events including the antimicrobial activity exhibited by macrophages in vitro. In an effort to establish the effector function of NO. in the antimicrobial activity expressed by macrophages in vivo, several groups report treating infected animals with relatively high concentrations of these same analogues. In the present study, we found that the arginine analogues NG-monomethyl-L-arginine, N omega-nitro-L-arginine methyl ester, aminoguanidine, and L-canavanine at concentrations > or = 10 mM significantly inhibited both the antigen-specific and -nonspecific proliferation of T lymphocytes in culture. These findings indicate that in vivo experiments demonstrating the suppressive effect of arginine analogues on host defenses are subject to alternative interpretations that do not directly involve the microbicidal activity of macrophages.