Virulence conversion of Legionella pneumophila serogroup 1 by passage in guinea pigs and embryonated eggs

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Abstract
When virulent L. pneumophila is passaged on supplemented Mueller-Hinton agar, it remains virulent for guinea pigs and embryonated hen eggs for 2 passages. By the 5th passage the cultures become avirulent for guineas pigs. Flagella were not produced by L. pneumophila on the 1st passage on supplemented Mueller-Hinton agar. Twelve passages on charcoal-yeast extract agar did not result in the reduction of virulence or the loss of flagella of L. pneumophila. Growth in supplemented yeast extract broth or on Norit-A-filtered supplemented yeast extract agar also did not result in a reduction of the virulence of L. pneumophila. L. pneumophila did not produce flagella when grown on these 2 media. Thus, it appears that the production of flagella is not required for the virulence of L. pneumophila when administered by the i.p. route of infection. A virulent flagellate form of L. pneumophila was recovered by passing an avirulent form 6 times in guinea pigs. When avirulent L. pneumophila was passaged 12 times in embryonated eggs, a nonflagellated form of the bacterium was recovered which had an increased virulence for guinea pigs and embryonated eggs. Virulent forms were not recovered by passage of avirulent forms on commonly used laboratory media. Evidently, a suitable host is required for the selection of the virulent form of L. pneumophila from avirulent cultures.