No abstract

We report here the draft genome sequence of a lethal pathogen of farmed salmonids, Piscirickettsia salmonis strain AUSTRAL-005. This virulent strain was isolated in 2008 from Oncorhynchus mykiss farms, and multiple genes involved in pathogenicity, environmental adaptation, and metabolic pathways were identified.

Piscirickettsia salmonis is a fastidious intracellular pathogen responsible for high mortality rates in farmed salmonids, with serious economic consequences for the Chilean aquaculture industry. Oxytetracycline and florfenicol are the most frequently used antibiotics against P. salmonis, but routine use could contribute to drug resistance. This study identified differentiated florfenicol susceptibilities in two P. salmonis strains, LF-89 and AUSTRAL-005. The less susceptible isolate, AUSTRAL-005, also showed a high ethidium bromide efflux rate, indicating a higher activity of general efflux pump genes than LF-89. The P. salmonis genome presented resistance nodulation division (RND) family members, a family containing typical multidrug resistance-related efflux pumps in Gram-negative bacteria. Additionally, efflux pump acrAB genes were overexpressed in AUSTRAL-005 following exposure to the tolerated maximal concentration of florfenicol, in contrast to LF-89. These results indicate that tolerated maximum concentrations of florfenicol can modulate RND gene expression and increase efflux pump activity. We propose that the acrAB efflux pump is essential for P. salmonis survival at critical florfenicol concentrations and for the generation of antibiotic-resistant bacterial strains.

Piscirickettsia salmonis is the predominant bacterial pathogen affecting the Chilean salmonid industry. This bacterium is the etiological agent of piscirickettsiosis, a significant fish disease. Membrane vesicles (MVs) released by P. salmonis deliver several virulence factors to host cells. To improve on existing knowledge for the pathogenicity-associated functions of P. salmonis MVs, we studied the proteome of purified MVs from the P. salmonis LF-89 type strain using multidimensional protein identification technology. Initially, the cytotoxicity of different MV concentration purified from P. salmonis LF-89 was confirmed in an in vivo adult zebrafish infection model. The cumulative mortality of zebrafish injected with MVs showed a dose-dependent pattern. Analyses identified 452 proteins of different subcellular origins; most of them were associated with the cytoplasmic compartment and were mainly related to key functions for pathogen survival. Interestingly, previously unidentified putative virulence-related proteins were identified in P. salmonis MVs, such as outer membrane porin F and hemolysin. Additionally, five amino acid sequences corresponding to the Bordetella pertussis toxin subunit 1 and two amino acid sequences corresponding to the heat-labile enterotoxin alpha chain of Escherichia coli were located in the P. salmonis MV proteome. Curiously, these putative toxins were located in a plasmid region of P. salmonis LF-89. Based on the identified proteins, we propose that the protein composition of P. salmonis LF-89 MVs could reflect total protein characteristics of this P. salmonis type strain.

The allotetraploid Tympanoctomys barrerae has a broad, patchy distribution around salt flats in western Argentina. To gain insights into its phylogenetic relationships, phylogeographical patterns, and origin, seven populations of T. barrerae and its allied taxa were studied through a 1075-bp fragment of cytochrome b and cytochrome oxidase I. Matrilineal phylogenetic relationships were explored with maximum likelihood and Bayesian approaches. The intraspecific genealogy was inferred by median-joining networks. The populational structure was assessed by molecular variance, and the demographic history through coalescence. The tree topology depicted sister-group relationship between Octomys mimax to the pair T. barrerae-Pipanacoctomys aureus, suggesting P. aureus belongs to the maternal lineage that gave rise to T. barrerae. High degrees of intrapopulational variation and the several instances of interpopulational polyphyly suggest range shifts and secondary contact. Consistent with the phylogenetic results, the network analysis revealed two haplotypic lineages depicting genetic admixtures unrelated to the current geographical distribution, and a deep split with the southernmost lineage of Chubut. The origin of T barrerae was estimated at approximately 2.52 Mya, whereas the divergence estimate for Chubut coincides with the end of largest Patagonian glaciation, at 1.47 Mya. Apparently, this population remained isolated during a northward Pleistocenic range shift. The historical demographic patterns of the lineages of T. barrerae fit with a contraction-expansion model coincident with Quaternary cycling events.