Effects of temperature on Paramoeba perurans growth in culture and the associated microbial community.

Population growth, in vitro, of three Paramoeba perurans cultures, one polyclonal (G) and two clonal (B8, CE6, derived from G), previously shown to differ in virulence (B8 > G > CE6), was compared at 10 and 15 °C. B8 showed a significantly higher increase in attached and in suspended amoebae over time at 15 and 10 °C, respectively. CE6 and G also had significantly higher numbers of suspended amoebae at 10 °C compared with 15 °C at experiment termination. However, in contrast to B8, numbers of attached amoebae were significantly higher at 10 °C in CE6 but showed a similar trend in G at the end of the experiment. Numbers of both suspended and attached amoebae were lower in B8 compared with CE6 and G. Significant differences in bacterial community composition and/or relative abundances were found, between cultures, between temperatures and between the same culture with and without amoebae, based on 16S rRNA Illumina MiSeq sequencing. Bacterial diversity was lower in B8 and CE6 compared with G, possibly reflecting selection during clonal isolation. The results indicate that polyclonal P. perurans populations may contain amoebae displaying different growth dynamics. Further studies are required to determine if these differences are linked to differences seen in the bacterial communities.

Aeromonas salmonicida type III secretion system-effectors-mediated immune suppression in rainbow trout (Oncorhynchus mykiss).

Aeromonas salmonicida subsp. salmonicida, the etiologic agent of furunculosis, is a major pathogen in aquaculture. Together with other pathogens, it is characterized by the presence of a type 3 secretion system (T3SS). The T3SS is the main virulence mechanism of A. salmonicida. It is used by the bacterium to secrete and translocate several toxins and effector proteins into the host cell. Some of these factors have a detrimental impact on the integrity of the cell cytoskeleton, likely contributing to impair phagocytosis. Furthermore, it has been suggested that effectors of the T3SS are able to modulate the host's immune response. Here we present the first partial characterization of the immune response in rainbow trout (Oncorhynchus mykiss) infected with distinct strains of A. salmonicida either carrying (i) a fully functional T3SS or (ii) a functionally impaired T3SS or (iii) devoid of T3SS ("cured" strain). Infection with an A. salmonicida strain either carrying a fully functional or a secretion-impaired T3SS was associated with a strong and persistent immune suppression. However, the infection appeared to be fatal only in the presence of a fully functional T3SS. In contrast, the absence of T3SS was neither associated with immune suppression nor fish death. These findings suggest that the T3SS and T3SS-delivered effector molecules and toxins of A. salmonicida do not only impair the host cells' cytoskeleton thus damaging cell physiology and phagocytosis, but also heavily affect the transcription of critical immune mediators including the shut-down of important warning signals to recognize infection and induce immune defense.