[Basic knowledge of phages and their therapeutic application].

Phages (bacteriophages) are viruses that specifically infect and kill bacteria. They are very abundant in nature, playing a highly relevant role in microbial ecosystems. In medicine, they are investigated as a potential alternative or supplement to antibiotics and can be used to treat wound, urinary tract and lung infections, for example. Single phages or so-called "phage cocktails" are applied.This overview article on basic knowledge of phages sheds light on well-known keywords from classical knowledge of phage biology and on state-of-the-art research focuses. Mechanisms of phage activity are presented as a basis for therapeutic application. Particularly, the phage-host interaction, lysis mechanisms, phage morphologies and specific methods for visualisation are discussed. Being part of the human microbiome, phages contribute to immune defence, especially in the mucosa. Temperate phages that are able to reside in bacterial genomes as prophages and therefore not suitable for therapy use as well as the CrAss phages (Crassvirales) and Lak megaphages discovered in recent years are also introduced. Further topics are bacterial phage defence, phage resistance and phage-antibiotic synergies. An outlook on future research is given, emphasising the importance of a coordinated collection of scientific results.Phages should not replace antibiotics, but they can even improve their efficiency. Currently, the licensing processes for phage therapy are still challenging. However, trust in phage preparations must be based on quality, which has to be guaranteed by harmonised standards.