.................................................................................................................................... 5 Riassunto ................................................................................................................................... 8 1 General introduction ....................................................................................................... 11 1.1 The bacterial family Midichloriaceae ..............................................................................11 1.2 Midichloriaceae in ticks and transmission to vertebrate hosts ....................................... 14 1.2.1 Ticks and transmission of bacteria: a brief general description .................................... 14 1.2.2 Midichloria mitochondrii in Ixodes ricinus ................................................................... 18 1.2.3 Midichloriaceae in other tick species ............................................................................ 20 1.2.4 Transmission of Midichloriaceae from ticks to vertebrate hosts .................................. 22 1.3 Involvement of a bacterium of the family Midichloriaceae in rainbow trout Oncorhynchus mykiss affected by red mark syndrome .............................................................. 24 2 Purpose of the PhD project ............................................................................................ 29 3 Performed researches ...................................................................................................... 31 3.1 Molecular evidence for a bacterium of the family Midichloriaceae (order Rickettsiales) in skin and organs of the rainbow trout Oncorhynchus mykiss (Walbaum) affected by red mark syndrome .................................................................................................................................... 31 3.1.1 AIM OF THE STUDY .................................................................................................. 31 3.1.2 MATERIALS AND METHODS ................................................................................... 31 3.1.3 RESULTS AND DISCUSSION .................................................................................... 32 3.2 Antibiotic treatment of the hard tick Ixodes ricinus: influence on Midichloria mitochondrii following blood meal ............................................................................................ 37 3.2.1 AIM OF THE STUDY .................................................................................................. 37 3.2.2 MATERIALS AND METHODS ................................................................................... 38 3.2.3 RESULTS ...................................................................................................................... 41 3.2.4 DISCUSSION ............................................................................................................... 45 3.3 Molecular screening for Midichloria in hard and soft ticks reveals variable prevalence levels and bacterial loads in different tick species ...................................................................... 48 3.3.1 AIM OF THE STUDY .................................................................................................. 48 3.3.2 MATERIALS AND METHODS ................................................................................... 48 3.3.3 RESULTS ...................................................................................................................... 52 3.3.4 DISCUSSION ............................................................................................................... 56 3.3.5 SUPPORTING INFORMATION .................................................................................. 60