Evaluation of Intercellular Signaling in Legionella pneumophila

Abstract

The number of prokaryotes known to participate in intercellular signaling is growing. The objective of this research was to determine if Legionella species are able to communicate through the use of intercellular signals with other bacteria and with eukaryotic hosts. The evaluation of intercellular signaling in L. pneumophila was conducted in three phases: i) determine if Legionella species produce intercellular signals; ii) determine if L. pneumophila responds to signals produced by unrelated microorganisms, and iii) evaluate if L. pneumophila can respond to signals or other metabolites produced by its natural, protozoan hosts. Among bacteria, known intercellular signals include N-acyl homoserine lactones (AHSL), autoinducer-2 (AI-2), and quinolone autoinducers. In the evaluation of Legionella species’ ability to produce signaling metabolites, biological sensors that detect v AHSL, AI-2, and quinolone autoinducers were used. The data from each of the trials demonstrated conclusively that Legionella species do not produce AHSL, AI-2, or quinolone signals. In the evaluation of the ability of L. pneumophila to respond to interspecies or inter-kingdom signals, the pathogen was exposed to supernatants of signal producing microorganisms and that of protozoa known to support their intracellular growth. An evaluation of the protein profile of L. pneumophila following its exposure to these supernatants by two-dimensional electrophoresis (2-DE) revealed the expression of multiple proteins. Subsequent analysis by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) aided in the presumptive identification of many of these proteins. Of the 40 proteins that were identified, 36 were differentially expressed in L. pneumophila following exposure to the supernatants of signal producing bacteria. These proteins included global stress protein GspA and a 24kDa macrophage induced protein. Following exposure to protozoa, 36 proteins were identified as a part of the L. pneumophila response and include a Legionella long chain fatty acid transporter and cold shock domain protein CspA. In terms of its protein expression, the reaction demonstrated by L. pneumophila in the presence of unrelated prokaryotes and potential host protozoa represented a stress response and suggested that L. pneumophila can respond to these organisms via intercellular signaling.