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Legionella pneumophila is a gram‐negative bacteria found in natural and anthropogenic aquatic environments such as evaporative cooling towers, where it reproduces as an intracellular parasite of cohabiting protozoa. If L. pneumophila is aerosolized and inhaled by a susceptible person, bacteria may colonize their alveolar macrophages causing the opportunistic pneumonia Legionnaires' disease. L. pneumophila utilizes an elaborate regulatory network to control virulence processes such as the Dot/Icm Type IV secretion system and effector repertoire, responding to changing nutritional cues as their host becomes depleted. The bacteria subsequently differentiate to a transmissive state that can survive in the environment until a replacement host is encountered and colonized. In this review, we discuss the lifecycle of L. pneumophila and the molecular regulatory network that senses nutritional depletion via the stringent response, a link to stationary phase‐like metabolic changes via alternative sigma factors, and two‐component systems that are homologous to stress sensors in other pathogens, to regulate differentiation between the intracellular replicative phase and more transmissible states. Together, we highlight how this prototypic intracellular pathogen offers enormous potential in understanding how molecular mechanisms enable intracellular parasitism and pathogenicity. [ABSTRACT FROM AUTHOR]

Previous work from our laboratory has shown that cosmid clone pFV100, containing a 26 kb insert, is able to restore O-antigen synthesis in serotype 05 rough mutants of Pseudomonas aeruginosa. Mobilization of pFV100 into two P. aeruginosa semi-rough (SR) mutants, AK1401 and rd7513, resulted in O-antigen expression, indicating that pFV100 may contain an 0-polymerase (rfc) gene. pFV.TK6, a subclone of pFV100 that contains a 5.6kb chromosomal insert. was able to complement O-antigen expression in these SR mutants. Mutagenesis of pFV.TK6 using Tn 1000 exposed a 1.5 kb region that was essential for complementing O-antigen expression in AK1401. A 20 kb Xhol-Hindlll fragment, containing this region, was cloned into vector pUCP26 and the resulting plasmid called pFV.TK8. In Southern analysis of the 20 P. aeruginosa serotypes using a probe generated from the 1.5 kb Xhol fragment of pFV.TK8, the rfc probe hybridized to a common fragment of the cross-reactive 02-05-016-018-020 serogroup, suggesting that these serotypes may share a common O-polymerase gene. In functional studies of the rfc gene, the PA01 (serotype 05) chromosomal rfc was mutated using a gene-replacement strategy. These knockout mutants expressed the SR lipopolysaccharide (LPS) phenotype, which indicated that they were no longer producing a functional 0-polymerase enzyme. Nucleotide sequence analysis of the insert DNA of pFV.TK8 revealed one open reading frame (ORF), designated ORF48.9, which could code for a 48.9 kDa protein. In comparisons of the P. aeruginosa rfc nucleotide and amino acid sequences with DNA and protein databases, no significant homology was found. However, the deduced structure of the P. aeruginosa Rfc protein indicated that it is very hydro-phobic and contains 11 putative membrane-spanning domains. Therefore, the predicted structure is similar to that of other reported Rfc... [ABSTRACT FROM AUTHOR]