RESUMO
SUMMARYLegionella pneumophila is a Gram-negative environmental bacterium, which survives in planktonic form, colonizes biofilms, and infects protozoa. Upon inhalation of Legionella-contaminated aerosols, the opportunistic pathogen replicates within and destroys alveolar macrophages, thereby causing a severe pneumonia termed Legionnaires' disease. Gram-negative bacteria employ low molecular weight organic compounds as well as the inorganic gas nitric oxide (NO) for cell-cell communication. L. pneumophila produces, secretes, and detects the α-hydroxyketone compound Legionella autoinducer-1 (LAI-1, 3-hydroxypentadecane-4-one). LAI-1 is secreted by L. pneumophila in outer membrane vesicles and not only promotes communication among bacteria but also triggers responses from eukaryotic cells. L. pneumophila detects NO through three different receptors, and signaling through the volatile molecule translates into fluctuations of the intracellular second messenger cyclic-di-guanylate monophosphate. The LAI-1 and NO signaling pathways are linked via the pleiotropic transcription factor LvbR. In this review, we summarize current knowledge about inter-bacterial and inter-kingdom signaling through LAI-1 and NO by Legionella species.
RESUMO
To promote intracellular survival and infection, Legionella spp. translocate hundreds of effector proteins into eukaryotic host cells using a type IV b protein secretion system (T4bSS). T4bSS are well known to translocate soluble as well as transmembrane domain-containing effector proteins (TMD-effectors) but the mechanisms of secretion are still poorly understood. Herein we investigated the secretion of hydrophobic TMD-effectors, of which about 80 were previously reported to be encoded by L. pneumophila. A proteomic analysis of fractionated membranes revealed that TMD-effectors are targeted to and inserted into the bacterial inner membranes of L. pneumophila independent of the presence of a functional T4bSS. While the T4bSS chaperones IcmS and IcmW were critical for secretion of all tested TMD-effectors, they did not influence inner membrane targeting of these proteins. As for soluble effector proteins, translocation of TMD-effectors into host cells depended on a C-terminal secretion signal and this signal needed to be presented towards the cytoplasmic side of the inner membrane. A different secretion behavior of TMD- and soluble effectors and the need for small periplasmic loops within TMD-effectors provided strong evidence that TMD-effectors are secreted in a two-step secretion process: Initially, an inner membrane intermediate is formed, that is extracted towards the cytoplasmic side, possibly by the help of the type IV coupling protein complex and subsequently secreted into eukaryotic host cells by the T4bSS core complex. Overall, our study highlights the amazing versatility of T4bSS to secrete soluble and TMD-effectors from different subcellular locations of the bacterial cell.
RESUMO
Vinte e uma linhagens de Listeria monocytogenes amostradas no Brasil, no período de 1989-90, procedentes de material clínico e alimentos foram identificadas, sorotipadas e fagotipadas de acordo com métodos tradicionais. Estas linhagens foram, também, tipadas quanto ao perfil de macrorestriçäo de DNA, após clivagem com ApaI, SmaI e NotI e separadas por eletroforese em campo pulsado. Os resultados obtidos evidenciaram uma grande heterogeneidade entre as linhagens: enquanto a sorotipagem dividiu dois sorogrupos (1/2 e 4), fagotipagem definiu oito fagovares e, finalmente, perfil de macrorestriçäo de DNA permitiu distinguir doze diferentes grupos. Estes resultados enfatizam a necessidade de caracterizaçäo de L. monocytogenes com, pelo menos, três diferentes métodos para definir se isolados säo similares ou näo, especialmente por ocasiäo de investigaçöes epidemiológicas da transmissäo alimentar da listeriose humana