Picasso's vision of licentiousness. Source.
Bacteria have been sexually promiscuous, swapping genes with gusto, for a very long time. More than 15% of E. coli's genome has arrived via horizontal gene transfer (HGT), with some 200 installments having turned up since it diverged from Salmonella 100 million years ago. And, as you are probably thinking, those 200 are but the tip of the HGT iceberg, the small fraction of transfers that were useful enough to be not only maintained by the recipient but also to spread through the population. But how much of this gene transport is the work of viruses?
Virus-mediated transduction is but one of the three major mechanisms that bacteria use to pick up genes, the other two being conjugation and transformation. The rates of transduction as conventionally measured for freshwater and marine ecosystems have been highly variable but always low, ranging from 10-11 to 10-5 transduced bacteria per phage (i.e., per plaque-forming unit or PFU). However, there being such huge numbers of phages, even these low rates lead to large numbers of transferred genes. Some 1014 bacteria in Tampa Bay and 1013 in the Mediterranean basin garner genes this way each year. Because they thought that these measured rates are way low, a group of Japanese researchers developed a new methodology for measuring the frequency of phage-mediated gene transfer.