Legionnaires' Disease Bacterium (Legionella pneumophila) Multiplies Intracellularly in Human Monocytes

Abstract

A B S T R A C T We have studied the interaction between virulent egg yolk-grown Legionella pneumophila Philadelphia 1 and human blood monocytes in vitro. The leukocytes were cultured in antibiotic-free tissue culture medium supplemented with 15% autologous human serum.L. pneumophila multiplied several logs, as measured by colony-forming units, when incubated with monocytes or mononuclear cells; the mid-log phase doubling time was 2 h. The level to which L. pneumophila multiplied was proportional to the number of mononuclear cells in the culture. L. pneumophila multiplied only in the adherent fraction of the mononuclear cell population indicating that monocytes but not lymphocytes support growth of the bacteria. Peak growth ofL. pneumophila was correlated with destruction of the monocyte monolayer. By fluorescence microscopy using fluorescein conjugated rabbit anti-L. pneumophila antiserum, the number of monocytes containing L. pneumophila increased in parallel with bacterial growth in the culture. At the peak of infection, monocytes were packed full with organisms. By electron microscopy, L. pneumophila in such monocytes were found in membrane-bound cytoplasmic vacuoles studded with structures resembling host cell ribosomes.

Legionella pneumophila inhibits acidification of its phagosome in human monocytes.

“…pneumophila harvested in the same way and then flash-frozen and maintained at -80"C until use; under these conditions the bacteria maintained their viability (1). In all other experiments, freshly harvested L. pneumophila were used.…”

FliA expression analysis and influence of the regulatory proteins RpoN, FleQ and FliA on virulence and in vivo fitness in Legionella pneumophila

“…After inhalation of L. pneumophila by immunocompromised humans, bacteria survive and replicate within alveolar macrophages (Horwitz and Silverstein 1980). Within the host cell, the bacteria establish a specialized Legionella-containing vacuole (LCV) in which L. pneumophila starts to replicate (Horwitz 1983;Horwitz and Maxfield 1984;Sturgill-Koszycki and Swanson 2000).…”

The Response Regulator CpxR Directly Regulates Expression of SeveralLegionella pneumophila icm/dotComponents as Well as New Translocated Substrates

“…Legionella pneumophila, the causative agent of Legionnaires' disease, is an intracellular pathogen that multiplies in nature inside amoebae and in human macrophages during disease (24,47). The icm/dot type IV secretion system is the major virulence system known in L. pneumophila; this system consists of 26 Icm/Dot proteins that probably constitute the secretion complex itself, as well as accessory proteins, such as chaperones (for a review, see reference 49).…”