Evaluation of the p45 Mobile Integrative Element and Its Role in Legionella pneumophila Virulence

Abstract

Legionella pneumophila are aqueous environmental bacilli that live within protozoal species and cause a potentially fatal form of pneumonia called Legionnaires’ disease. Not all L. pneumophila strains have the same capacity to cause disease in humans. The majority of strains that cause clinically relevant Legionnaires’ disease harbor the p45 mobile integrative genomic element. Contribution of the p45 element to L. pneumophila virulence and ability to withstand environmental stress were addressed in this study.

The L. pneumophila Philadelphia-1 (Phil-1) mobile integrative element, p45, was transferred into the attenuated strain Lp01 via conjugation, designating p45 an integrative conjugative element (ICE). The resulting trans-conjugate, Lp01⁺ᵖ⁴⁵, was compared with strains Phil-1 and Lp01 to assess p45 in virulence using a guinea pig model infected via aerosol. The p45 element partially recovered the loss of virulence in Lp01 compared to that of Phil-1 evident in morbidity, mortality, and bacterial burden in the lungs at the time of death. This phenotype was accompanied by enhanced expression of type II interferon in the lungs and spleens 48 hours after infection, independent of bacterial burden.

The p45 ICE was also evaluated for its contribution(s) to L. pneumophila’s capacity to withstand adverse environmental conditions. Results from these in vitro experiments revealed that p45 impacts sensitivity to sodium and ability of the bacteria to enter host cells. The ability to enter hosts and live in the presence of sodium contribute to L. pneumophila’s environmental fitness, and both are also associated with virulence. Sensitivity to sodium is utilized by researchers in the Legionella field as an in vitro proxy for virulence, where more sensitive bacteria tend to be more virulent. Thus, these results support that p45 ICE has a great impact on L. pneumophila virulence.