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Low-level iron-dependent mutants of Listeria monocytogenes and their virulence in macrophages

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Listeria monocytogenes is an opportunistic intracellular pathogen capable of growth within phagocytic cells that requires iron for growth and virulence expression. In the presence of an appropriate concentration of tropolone, an iron-chelating agent, growth of L. monocytogenes is completely inhibited. However, this inhibition can be relieved by addition of dopamine, norepinephrine, or ferric citrate. By selection on streptonigrin medium supplemented with tropolone and norepinephrine, we have obtained two spontaneous mutants, Lm-8 and Lm-15, with the same iron dependence but lower iron dependence than the wild-type Lm-B38. The association between iron requirement and virulence of the two mutants and the wild type was studied in the J774 macrophage cell line. One hour after phagocytosis by the J774 macrophage cell line, the two mutants and the parental strain displayed no difference in the number of phagocytosed bacteria. Twenty-four hours after phagocytosis, the number of bacteria within the surviving macrophages was identical for the wild strain and the two clones. However, only 40% of macrophage cells infected with Lm-8 and 90% of those infected with Lm-15 were alive after 24 h in comparison with macrophage cells infected with the parental strain Lm-B38. These data demonstrate that there is no direct correlation between iron requirement and virulence of L. monocytogenes in the J774 macrophage cell line.Key words: Listeria monocytogenes, iron, virulence, macrophages.

Listeria monocytogenes est un agent pathogène opportuniste capable de se multiplier dans les cellules phagocytaires et qui a besoin de fer pour sa croissance et pour l'expression de sa virulence. En présence de tropolone, un agent complexant le fer, la croissance de L. monocytogenes est complètement inhibée mais peut être restaurée par l'addition de dopamine, de noradrénaline ou de citrate ferrique. Par sélection sur un milieu à la streptonigrine, supplémenté en tropolone et en noradrénaline, nous avons obtenu deux clones Lm-8 et Lm-15 avec la même dépendance en fer mais moins dépendants en fer que la souche sauvage Lm-B38. La corrélation entre le besoin en fer et la virulence des deux clones est étudiée sur des macrophages. Une heure après avoir été phagocytés par les macrophages de la lignée J774, il n'y a pas de différence entre le nombre de bactéries phagocytées par les deux clones et par la souche parentale. Il en est de même au bout de 24 heures de phagocytose. Cependant seulement 40 % des macrophages infectés avec le clone Lm-8 étaient vivants au bout de 24 heures contre 90 % des macrophages infectés par le clone Lm-15. Ces résultats suggèrent qu'il n'y a pas de corrélation directe entre le besoin en fer et la virulence de L. monocytogenes dans les macrophages de la lignée J774.Mots clés : Listeria monocytogenes, fer, virulence, macrophages.

Document Type: Research Article

Publication date: February 1, 2003

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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