The Type II Protein Secretion System of Legionella pneumophila Is Important for Growth in Iron-Rich Media and Survival in Tap Water at Low Temperatures

Abstract

The type II protein secretion system of Legionella pneumophila is important for bacterial survival in protozoa, macrophages, and the lungs of A/J mice. Presently, L. pneumophila is known to express both type II and type IV secretion systems. Type II secretion (lsp) mutants of L. pneumophila grow normally in bacteriologic media at 30 to 37°C. The low-temperature defect of lsp mutants of L. pneumophila serogroup 1 strain 130b was partially due to the presence of 0.25 g/liter ferric pyrophosphate in the growth media, since bacterial growth on agar plates at 25°C improved significantly when that supplement was omitted. The inhibitory effect of iron was specific to the lsp mutant and to low-temperature growth, since the growth of wild type at 25°C did not change with variations in the amount of supplemental iron, and the growth of the lsp mutant was not negatively influenced by iron supplementation when incubated at 37°C. In the environment, L. pneumophila is found in freshwater and man-made water systems. In sum, the results presented indicate that the L. pneumophila type II secretion system is not only important for growth at 35 to 37°C in host cells but is also critical for extracellular growth and survival at lower temperatures, including iron-rich bacteriological media and environmental water samples. Thus, type II secretion appears to play a central and multifaceted role in the natural history of Legionnaires’ disease.