Metagenomic analyses of single phages and phage cocktails show instances of contamination with temperate phages and bacterial DNA

Increasing antibiotic resistance has led to renewed attention being paid to bacteriophage therapy. Commercial phage cocktails are available but often their contents of the phages are not well defined. Some metagenomic studies have been done to retrospectively characterise these cocktails, but little is known about the replication cycle of the included phages, or about the possible bacterial DNA contamination. In this study, published metagenomic sequences were reanalysed using recent advances in viromics tools. Signs of temperate phage contigs were found in all cocktail metagenomes, as well as host DNA, which could poses a risk as it may lead to horizontal gene transfer of virulence factors to commensals and pathogens. This suggests the need to implement further quality measures before using phage cocktails therapeutically.