• antibiotic treatment
• ciliates
• endosymbiont
• FISH
• growth curve
• Holospora

Protists often establish symbiotic interactions with intracellular bacteria, called endosymbionts. One of the most studied unicellular hosts for bacterial endosymbiosis is the ciliate Paramecium. The term ‘symbiosis’ is used according to the definition of de Bary and comprises the living together of two different species and thus includes mutualism as well as parasitism and commensalism. Paramecium is nowadays serving as ‘model cell’ for molecular and cell biologists and for biotechnological processes.
The aim of this thesis was to improve our understanding of the effects of intracellular bacteria on ciliate cells. First, strategies to visualize potential bacterial parasites were established. Then different strains of the tested host species, Paramecium biaurelia, were analysed for differences in their susceptibility towards an infectious bacterium. The endosymbiont used for this study was Holospora caryophila, a Gram-negative bacterium that belongs to the Alphaproteobacteria. Last, the actual fitness reductions caused by the pathogen were determined.

Fluorescence in situ hybridizations were performed as a screening method to detect intracellular bacteria inside the eukaryotic host cell. This approach uses fluorescently labelled rRNA-targeting oligonucleotide probes that bind only to the corresponding 16S rRNA. Subsequently, the bacteria are visualized using an epifluorescence microscope.
Infectious H. caryophila were isolated from three P. biaurelia strains chronically infected since years. The isolated bacteria were used to perform infection experiments. Eight naïve strains of P. biaurelia were infected, but did not show the same prevalence of infection. Differences in susceptibility could be attributed to the genotype of the host strain.
The two factors which were used for fitness determination were growth rate and carrying capacity. Therefore growth curves for different P. biaurelia lines (chronically infected, cured, naïve and experimentally infected) were reported. The three with H. caryophila infected P. biaurelia strains were treated with antibiotics to establish corresponding cured lines. Furthermore some of the above mentioned experimentally infected lines and their naïve counterpart were included in the growth curve experiments.
In most cases, growth rate and carrying capacity of infected lines was reduced compared to bacteria-free paramecia. Thus the bacterial parasite tends to reduce the fitness of its host. Interestingly, the observed effect was more often statistically significant in carrying capacity rather than during the exponential growth. Furthermore it was less pronounced in chronically infected lines.

In conclusion, before a ciliate is used for biotechnological applications, it should be controlled for the presence of intracellular bacteria. In the case this test results positive, a treatment to remove them should be considered. If that is not feasible, the fitness effect of the parasite during the growth condition of interest should be analyzed. Finally it should be considered that different strains can differ strongly in their susceptibility and impairment towards bacterial infections.