Transposition of bacteriophage Mu in the Legionnaires disease bacterium

Rapid adaptations of Legionella pneumophila to the human host

Legionella pneumophila are host-adapted bacteria that infect and reproduce primarily in amoeboid protists. Using similar infection mechanisms, they infect human macrophages, and cause Legionnaires' disease, an atypical pneumonia, and the milder Pontiac fever. We hypothesized that, despite the similarities in infection mechanisms, the hosts are different enough that there exist high-selective value mutations that would dramatically increase the fitness of Legionella inside the human host. By comparing a large number of isolates from independent infections, we identified two genes, mutated in three unrelated patients, despite the short duration of the incubation period (2-14 days). One is a gene coding for an outer membrane protein (OMP) belonging to the OmpP1/FadL family. The other is a gene coding for an EAL-domain-containing protein involved in cyclic-di-GMP regulation, which in turn modulates flagellar activity. The clinical strain, carrying the mutated EAL-domain-containing homologue, grows faster in macrophages than the wild-type strain, and thus appears to be better adapted to the human host. As human-to-human transmission is very rare, fixation of these mutations into the population and spread into the environment is unlikely. Therefore, parallel evolution - here mutations in the same genes observed in independent human infections - could point to adaptations to the accidental human host. These results suggest that despite the ability of L. pneumophila to infect, replicate in and exit from macrophages, its human-specific adaptations are unlikely to be fixed in the population.

Dynamics and impact of homologous recombination on the evolution of Legionella pneumophila

Legionella pneumophila is an environmental bacterium and the causative agent of Legionnaires' disease. Previous genomic studies have shown that recombination accounts for a high proportion (>96%) of diversity within several major disease-associated sequence types (STs) of L. pneumophila. This suggests that recombination represents a potentially important force shaping adaptation and virulence. Despite this, little is known about the biological effects of recombination in L. pneumophila, particularly with regards to homologous recombination (whereby genes are replaced with alternative allelic variants). Using newly available population genomic data, we have disentangled events arising from homologous and non-homologous recombination in six major disease-associated STs of L. pneumophila (subsp. pneumophila), and subsequently performed a detailed characterisation of the dynamics and impact of homologous recombination. We identified genomic "hotspots" of homologous recombination that include regions containing outer membrane proteins, the lipopolysaccharide (LPS) region and Dot/Icm effectors, which provide interesting clues to the selection pressures faced by L. pneumophila. Inference of the origin of the recombined regions showed that isolates have most frequently imported DNA from isolates belonging to their own clade, but also occasionally from other major clades of the same subspecies. This supports the hypothesis that the possibility for horizontal exchange of new adaptations between major clades of the subspecies may have been a critical factor in the recent emergence of several clinically important STs from diverse genomic backgrounds. However, acquisition of recombined regions from another subspecies, L. pneumophila subsp. fraseri, was rarely observed, suggesting the existence of a recombination barrier and/or the possibility of ongoing speciation between the two subspecies. Finally, we suggest that multi-fragment recombination may occur in L. pneumophila, whereby multiple non-contiguous segments that originate from the same molecule of donor DNA are imported into a recipient genome during a single episode of recombination.

The amoebae plate test implicates a paralogue of lpxB in the interaction of Legionella pneumophila with Acanthamoeba castellanii

Legionella pneumophila is a bacterial parasite of freshwater amoebae which also grows in alveolar macrophages and thus causes the potentially fatal pneumonia Legionnaires' disease. Intracellular growth within amoebae and macrophages is mechanistically similar and requires the Icm/Dot type IV secretion system. This paper reports the development of an assay, the amoebae plate test (APT), to analyse growth of L. pneumophila wild-type and icm/dot mutant strains spotted on agar plates in the presence of Acanthamoeba castellanii. In the APT, wild-type L. pneumophila formed robust colonies even at high dilutions, icmT, -R, -P or dotB mutants failed to grow, and icmS or -G mutants were partially growth defective. The icmS or icmG mutant strains were used to screen an L. pneumophila chromosomal library for genes that suppress the growth defect in the presence of the amoebae. An icmS suppressor plasmid was isolated that harboured the icmS and flanking icm genes, indicating that this plasmid complements the intracellular growth defect of the mutant. In contrast, different icmG suppressor plasmids rendered the icmG mutant more cytotoxic for A. castellanii without enhancing intracellular multiplication in amoebae or RAW264.7 macrophages. Deletion of individual genes in the suppressor plasmids inserts identified lcs (Legionella cytotoxic suppressor) -A, -B, -C and -D as being required for enhanced cytotoxicity of an icmG mutant strain. The corresponding proteins show sequence similarity to hydrolases, NlpD-related metalloproteases, lipid A disaccharide synthases and ABC transporters, respectively. Overexpression of LcsC, a putative paralogue of the lipid A disaccharide synthase LpxB, increased cytotoxicity of an icmG mutant but not that of other icm/dot or rpoS mutant strains against A. castellanii. Based on sequence comparison and chromosomal location, lcsB and lcsC probably encode enzymes involved in cell wall maintenance and peptidoglycan metabolism. The APT established here may prove useful to identify other bacterial factors relevant for interactions with amoeba hosts.

Molecular cloning and characterization of a locus responsible for O acetylation of the O polysaccharide of Legionella pneumophila serogroup 1 lipopolysaccharide

Complementation experiments, Tn5 mutagenesis, and DNA sequencing were used to identify a locus (lag-1) that participates in acetylation of Legionella pneumophila serogroup 1 lipopolysaccharide. Nuclear magnetic resonance analyses of lipopolysaccharides from mutant and complemented strains suggest that lag-1 is responsible for O acetylation of serogroup 1 O polysaccharide.

Random mutagenesis of Legionella pneumophila with mini-Tn10

To more effectively study the genetic basis of Legionnaires' disease, we characterized a system for mini-Tn10 mutagenesis in Legionella pneumophila. The mini-transposons were first electroporated into Legionella on counterselectable vectors expressing altered target site transposases. Then, by simultaneously selecting for the kanamycin-resistance gene within the transposon and counterselecting against the maintenance of the plasmid, we directly and readily isolated strains bearing single chromosomal insertions. Southern hybridization analysis further demonstrated that the insertions were randomly distributed throughout the Legionella genome. The mini-Tn10 insertions were stable during extracellular and intracellular growth, and did not alter the infectivity of L. pneumophila. Thus, this mutagenesis system offers an easy, one-step approach toward isolating large populations of random mutants which can be screened for defects in virulence.

Mutagenesis of Legionella pneumophila using Tn903 dlllacZ: identification of a growth-phase-regulated pigmentation gene

Study of the molecular basis for Legionella pneumophila pathogenicity would be facilitated with an efficient mutagen that can not only mark genomic mutations, but can also be used to reflect gene expression during macrophage infection. A derivative of Tn903, Tn903dlllacZ, is shown to transpose with high efficiency in L. pneumophila. Tn903dlllacZ encodes resistance to kanamycin (KmR) and carries a 5' truncated 'lacZ gene that can form translational fusions to L. pneumophila genes upon transposition. The cis-acting Tn903 transposase is supplied outside Tn903dlllacZ, and hence chromosomally integrated copies are stable. KmR LacZ+ insertion mutants of L. pneumophila were isolated and shown by DNA hybridization to carry a single Tn903dlllacZ inserted within their chromosomes at various locations. One particular KmR LacZ+ mutant, AB1156, does not produce the brown pigment (Pig-) characteristic of Legionella species. Tn903dlllacZ is responsible for this phenotype since reintroduction of the transposon-linked mutation into a wild-type background results in a Pig- phenotype. L. pneumophila pigment production is normally observed in stationary-phase growth of cells in culture, and beta-galactosidase activity measured from the pig::lacZ fusion increased during the logarithmic-phase growth and peaked at the onset of stationary phase. Interestingly, pig::lacZ expression also increased during macrophage infection. The pigment itself, however, does not appear to be required for L. pneumophila to grow within or kill host macrophages.

PhoA gene fusions in Legionella pneumophila generated in vivo using a new transposon, MudphoA

To enable effective use of phoA gene fusions in Legionella pneumophila, we constructed MudphoA, a derivative of the mini-Mu phage Mu dII4041, which is capable of generating gene fusions to the Escherichia coli alkaline phosphatase gene (EC 3.1.3.1). Although an existing fusion-generating transposon, TnphoA, has been a useful tool for studying secreted proteins in other bacteria, this transposon and other Tn5 derivatives transpose inefficiently in Legionella pneumophila, necessitating the construction of a more effective vector for use in this pathogen. Using MudphoA we generated fusions to an E. coli gene encoding a periplasmic protein and to an L. pneumophila gene encoding an outer membrane protein; both sets of fusions resulted in alkaline phosphatase activity. We have begun to use MudphoA to mutate secreted proteins of L. pneumophila specifically, since this subset of bacterial proteins is most likely to be involved in host-bacterial interactions. This modified transposon may be useful for studies of other bacteria that support transposition of Mu, but not Tn5, derivatives.

Isolation of a Legionella pneumophila restriction mutant with increased ability to act as a recipient in heterospecific matings

The ability of Legionella pneumophila to act as a recipient of IncP and IncQ plasmids in matings with Escherichia coli varies widely from strain to strain. We found that the low efficiency of mating of the Philadelphia-1 strain is due to a type II restriction-modification system, and we isolated and characterized a Philadelphia-1 mutant that lacks the restriction enzyme activity.