Long-term in vitro cultivation of Histomonas meleagridis coincides with the dominance of a very distinct phenotype of the parasite exhibiting increased tenacity and improved cell yields

Abstract

The majority of research on Histomonas meleagridis was performed in the first half of the last century, especially those on morphological aspects. In the present study identical monoxenic settings for cultures of the same H. meleagridis clonal strain in its virulent low passage and attenuated high passage form enabled a comparative analysis of parasite characteristics. For the first time, it could be shown that long-term in vitro cultivation led to a severe shift in cell morphology, with the occurrence of a very distinct phenotype expressing a flagellated and highly amoebic cell morphology. Furthermore, the attenuated parasites showed better growth rates and a higher tenacity when confronted with adverse conditions. During these experiments up to 100% of the parasites, both virulent and attenuated, assumed a completely rounded morphology elucidated by electron microscopy. The findings indicate that such previously reported cyst-like stages are a defence strategy of H. meleagridis, independent of the passage level in vitro and pathogenicity in vivo. In conclusion, long-term in vitro passaging of H. meleagridis led not only to an attenuation of the parasite, as previously demonstrated, but also to a shift in the parasite's phenotype regarding morphology, growth behaviour and a higher level of tenacity.

An Alliance of Gel-Based and Gel-Free Proteomic Techniques Displays Substantial Insight Into the Proteome of a Virulent and an Attenuated Histomonas meleagridis Strain

“…The overexpression of the above proteins in the gel images of the virulent H. meleagridis was accompanied by the absence of hydrogenosomal enzyme overexpression. In recent detailed investigations on H. meleagridis morphology, a genuine encystation process was not observed (Gruber et al, 2017 ). However, as it was the case for T. vaginalis , the flagellate was capable of defending itself by forming cyst-like stages under adverse in vitro conditions characterized by rounded morphology and double cell membrane (Zaragatzki et al, 2010 ; Gruber et al, 2017 ).…”

“…In recent detailed investigations on H. meleagridis morphology, a genuine encystation process was not observed (Gruber et al, 2017 ). However, as it was the case for T. vaginalis , the flagellate was capable of defending itself by forming cyst-like stages under adverse in vitro conditions characterized by rounded morphology and double cell membrane (Zaragatzki et al, 2010 ; Gruber et al, 2017 ). The upregulation of the above proteins in the virulent parasite, which is less adapted to the in vitro environment, indicates the existence of these defense mechanisms on a protein level.…”

Molecular characterization of Histomonas meleagridis exoproteome with emphasis on protease secretion and parasite-bacteria interaction

“…Attenuated H . meleagridis cells are well adapted to in vitro growth conditions in complete medium, showing higher cell numbers as compared to the virulent parasites [16]. It is therefore plausible that attenuated histomonads needed slightly longer to completely cease their growth.…”

“…Both, the virulent and the attenuated H . meleagridis cultures, were maintained in sterile T-75 cm 2 closed cap tissue culture flasks (Sarstedt, Wiener Neudorf, Austria) at 40°C for 50-56h representing the logarithmic phase of growth [16]. Following this, a protocol employing serum-free culture was executed.…”

Unravelling the Immunity of Poultry Against the Extracellular Protozoan Parasite Histomonas meleagridis Is a Cornerstone for Vaccine Development: A Review

“…More recently, the occurrence of a cyst-like stage was reported ( 27 ). Later on, this resistant stage of H. meleagridis was investigated in vitro and it could be observed independent of the passage level and pathogenicity in vivo indicating an early adaption to in vitro conditions ( 28 ).…”