Clade 3 species include L. longbeachae that, along with Lp, accounts for over 95% of LD cases8. L. longbeachae was previously reported to contain an accessory gene, the tetracycline destructase, that was probably acquired by HGT from the soil, the natural habitat of this species49. The destructase confers tetracycline resistance to L. longbeachae, and has not been reported in Lp50 or any other Legionella species. While we found no evidence for the gene in any of the Lp1 or 16 other Lp serogroups, it was present in Clade 2 species L. jordanis and L. nautarum with 100% homology to the destructase in L. longbeachae. Since doxycycline is one of the first line drugs for treating legionellosis in the outpatient setting, presence of this gene may have led to the success of L. longbeachae in causing LD and to the success of the other species in causing human disease. L. jordanis has been association with human disease with a dozen or so recent cases reported5152. The acquisition of the destructase coupled with these reports may indicate a shift in virulence and the tip of the iceberg in terms of their potential for increasing the prevalence of human disease.