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International audience; Adaptation to the host environment and exploitation of host cell functions are critical to the success of intracellular pathogens. Here, insight to these virulence mechanisms was obtained for the first time from the transcriptional program of the human pathogen Legionella pneumophila during infection of its natural host, Acanthamoeba castellanii. The biphasic life cycle of L. pneumophila was reflected by a major shift in gene expression from replicative to transmissive phase, concerning nearly half of the genes predicted in the genome. However, three different L. pneumophila strains showed similar in vivo gene expression patterns, indicating that common regulatory mechanisms govern the Legionella life cycle, despite the plasticity of its genome. During the replicative phase, in addition to components of aerobic metabolism and amino acid catabolism, the Entner-Doudoroff pathway, a NADPH producing mechanism used for sugar and/or gluconate assimilation, was expressed, suggesting for the first time that intracellular L. pneumophila may also scavenge host carbohydrates as nutrients and not only proteins. Identification of genes only upregulated in vivo but not in vitro, may explain higher virulence of in vivo grown L. pneumophila. Late in the life cycle, L. pneumophila upregulates genes predicted to promote transmission and manipulation of a new host cell, therewith priming it for the next attack. These including substrates of the Dot/Icm secretion system, other factors associated previously with invasion and virulence, the motility and the type IV pilus machineries, and > 90 proteins not characterized so far. Analysis of a fliA (sigma28) deletion mutant identified genes coregulated with the flagellar regulon, including GGDEF/EAL regulators and factors that promote host cell entry and survival.

Summary The parasite Entamoeba histolytica colonizes the large bowel where it may persist as an asymptomatic luminal gut infection, which changes to virulence. Parasite invasion of the intestine leads to dysentery and spreads to the liver, where amoebae form abscesses. We took advantage of changes in viru- lence that occurs after long-term in vitro culture of E. histolytica strains. Using microarrays, we con- cluded that virulence correlates with upregulation of key genes involved in stress response, including molecular chaperones, ssp1 and peroxiredoxin; as well as the induction of unknown genes encoding lysine-rich proteins. Seven of these were retained with respect to their lysine content higher than 25%. Among them, we found KERP1, formerly identified as associated to parasite surface and involved in the parasite adherence to host cells. Experimentally induced liver abscesses, using molecular beacons and protein analysis, allowed us to draw a parallel between the intricate upregulation of kerp1 gene expression during abscess development and the increased abundance of KERP1 in virulent trophozoi- tes. Following its characterization as a marker for the progression of infection, KERP1 was also seen to be a virulence marker as trophozoites affected in kerp1 expression by an antisense strategy were unable to form liver abscesses.