Ampicillin tolerance of Legionella pneumophila for counter-selection of transconjugants in heterospecific matings with Escherichia coli donors.

Legionella pneumophila, the causative agent of Legionnaires' disease is sensitive to ampicillin. However, the slowly growing bacteria are not killed even by high doses of this antibiotic. This natural tolerance was used for counter-selection of trans-conjugants in heterospecific matings with Escherichia coli as donor. This approach is useful for gentic manipulations in Legionella, as it avoids the use of antibiotic-resistant variants, which have to be tested for full virulence before use.