2022

Runtuvuori-Salmela, Anniina

Antibiotic resistance is one of the gravest threats to global health and food production. Resistant strains can cause bacterial infections that were previously treated easily with antibiotics to become lethal. Therefore, there is a need for alternatives to antibiotics, such as bacteriophages, or phages, which are viruses of bacteria. In this thesis, I studied the use of these natural enemies of bacteria against Flavobacterium columnare. F. columnare is a globally emerging bacterium that causes high mortality among fish and large economic losses at freshwater fish farms during warm-water periods unless treated with antibiotics. At fish farms, the spread of antibiotic-resistant strains and antibiotics in the environment is a risk. Phage therapy has the potential to reduce the use of antibiotics against F. columnare. A large collection of new phages that may be used against the pathogen were collected. The most promising phages from this collection were selected so as to study their resistance effects on bacterial strains and to test the different administration routes by which phage therapy can decrease the mortality that columnaris disease causes in rainbow trout. It was seen that F. columnare strain adherence and biofilm formation are lower and, consequently, virulence is weaker as a trade-off for phage resistance. It emerged that the optimal means of controlling F. columnare infections is to bathe fish in phage solution after the first columnaris symptoms appear. Phage bathing before bacterium infection and the use of immobilised phages on plastic sheets slow the progression of bacterial infections. This thesis reinforces the notion that phage therapy holds considerable promise for combatting bacterial infections, but more studies are needed to understand the interactions between different bacterial strains and these phages.