Research on Legionella pneumophila, the causative agent of Legionnaires' disease, has been hampered due to the lack of selectable markers for genetic manipulation. We report the construction of a mutant strain of L. pneumophila lacking loxA, a chromosomally encoded β-lactamase, that has enhanced sensitivity to ampicillin. Also described are a method for converting Legionella strains to ampicillin sensitivity and conditions for utilizing bla as a selectable marker
