Chloroform-soluble material was extracted from two strains ofL. pneumophila serogroup 1 following growth in continuous culture. The purified material was identified as poly-3-hydroxybutyrate (PHB) by nuclear magnetic resonance spectroscopy and by gas chromatography-mass spectrometry. PHB yields of up to 16% of cell dry weight were extracted from culture samples. The PHB was located in electron-dense intracellular inclusions, which fluoresced bright yellow when stained with the lipophilic dye Nile red. A Nile red spectrofluorometric assay provided a more accurate and reliable determination of the PHB content. PHB accumulation increased threefold during iron-limited culture and was inversely related to the concentration of iron metabolized. Chemostat-grown cells survived in a culturable state for at least 600 days when incubated at 24°C in a low-nutrient tap water environment. Nile red spectrofluorometry and flow cytometry demonstrated that PHB reserves were utilized during starvation. PHB utilization, as revealed by the decline in mean cellular fluorescence and cell complexity, correlated with loss of culturability. Fluorescence microscopy provided visual evidence of PHB utilization, with a marked reduction in the number of Nile red-stained granules during starvation. Heat shock treatment failed to resuscitate nonculturable cells. This study demonstrates that L. pneumophila accumulates significant intracellular reserves of PHB, which promote its long-term survival under conditions of starvation.
A virulent strain of Legionella pneumophila serogroup 1, subgroup Pontiac, was grown in continuous culture at a constant growth rate under iron-replete and iron-limited conditions. Iron limitation was achieved by the removal of ferrous sulfate and hemin from the chemically defined medium. Residual contaminating iron, 0.45 M, was sufficient to support iron-limited growth. Typical iron-replete cultures metabolized 3.3 M iron. Serine provided the principal source of carbon and energy for both cultures, although iron-replete cultures also depleted a number of other amino acids. There was a 40% decrease in culture biomass under iron-restricted conditions. Iron limitation did not significantly affect carbohydrate metabolism, with the molar growth yield for carbon (Y carbon) comparable for both cultures. However, under iron-limited conditions a sixfold increase in Y iron correlated with a significant decrease in the iron content of the biomass, as the culture utilized the available iron more efficiently. Highly pleomorphic iron-replete cultures became uniform cultures of short fine rods when adapted to iron-deficient conditions. In addition to the morphological and physiological changes, iron limitation had a critical effect on culture virulence. The virulence of this strain was significantly (P < 0.05) reduced when the culture was subjected to iron-limited conditions. This phenomenon was reversible, with a significant increase in culture virulence upon reversion to iron-replete conditions. When compared in an in vitro macrophage assay, the number of culturable avirulent iron-limited cells located intracellularly after infection was significantly lower than for the virulent replete and control cultures. These results further support the role of environmental parameters in regulating the virulence of L. pneumophila.
In chemostat culture, the virulence of two strains of Legionella pneumophila was shown to be significantly (P < 0.05) reduced when the culture temperature was lowered from 37 to 24°C. This modulation was reversed by returning the temperature to 37°C, which resulted in a statistically significant (P < 0.05) increase in virulence.
The genus Legionella consists of 51 serogroups comprising 34 species. Biochemical reactions and cell wall fatty acid and quinone analyses may confirm that an isolate is a Legionella sp. and indicate to which species it belongs, but DNA hybridization studies have been necessary for a definitive identification. Recently, the commercially available BIOLOG identification system has offered a standardized, easily reproducible system of substrate metabolism by bacteria resuspended in multiwell plates. A tetrazolium dye acts as an electron acceptor during the oxidation of the wide range of substrates and forms an irreversible, highly colored formazan when reduced. The 95 substrate wells are read rapidly with a conventional plate reader, and the results are downloaded for comparison with a computer data base, allowing quick identification. The BIOLOG system's ability to test more diverse classes of substrates, including amino acids, peptides, carboxylic acids, and carbohydrates, was used in this study to establish a new data base and identify the asaccharolytic Legionella spp. In particular, Legionella pneumophila behaved as a microaerophile, and the fastest, most diverse metabolic activities occurred after the development of a low-oxygen incubation environment. Alternatively, bacteria could be successfully incubated in air when their concentration was double that recommended by the manufacturer. Similar results were obtained by using either Page's amoebal saline or distilled water as the resuspending and incubation medium. Type strains did not cross-identify with any of the strains already in the manufacturer's data base. The results indicate that this modified system has value in being able to identify Legionella isolates to the species level.
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