In this study, the newly isolated Streptomyces phages Vanseggelen and Verabelle have been fully characterized and sequenced. These results increase our insights on phage-host dynamics in Streptomyces and expand available, sequenced and characterized Streptomyces phages. TEM images showed that both phages have a long, flexible non-contractile tail with an icosahedral capsid head typical for siphoviruses. Both phages belong to the class Caudoviricites, which was confirmed by whole genome sequencing. Since the majority of the sequenced Streptomyces phages have a long non-contractile tail, this tail morphotype is possibly necessary for effectively penetrating the thick peptidoglycan layer of Gram-positive bacteria. The host range analysis showed that both phages can form lysis zones with a turbid morphology with some host strains, indicating that Vanseggelen and Verabelle are likely able to switch from a lytic to a lysogenic lifecycle. Indeed, since all other phages belonging to the genus Camvirus are temperate phages and whole genome sequencing revealed that both genomes contain an integrase protein, we can conclude that Vanseggelen and Verabelle are temperate phages as well18.
Of all sequenced Streptomyces phages available in the Actinobacteriophage database8, we have calculated that the average genome length is 73,556 bp with a G + C content of ~ 61%, which is relatively high, and could be explained by the relatively high G + C-content of their bacterial hosts. Both Vanseggelen and Verabelle have a double-stranded DNA with a genome length of 48,720 bp and 48,126 bp, which is a bit smaller than average. Both phages were able to infect several different Streptomyces strains, but could not infect K. virifidaciens. Kitasatospora is the sister genus of Streptomyces that also belongs to the phylum Actinobacteria. Their morphology and lifecycle are similar to Streptomyces bacteria, but Kitasatospora is usually identified by the fact that they cannot be infected by Streptomyces phages19,20. It would be interesting to isolate and characterize phages on Kitasatospora species in the future to dissect differences in infectivity between Streptomyces and Kitasatospora phages.
Streptomyces phage Verabelle showed a slightly broader host range infectivity than Vanseggelen, which could be explained by potential phage specific defense mechanisms. Despite the high degree of nucleotide similarity between the genomes of Vanseggelen and Verabelle, there were sequence variations that resulted in differences at the protein level, which could also account for the differences in host range.
The one-step growth curve of Vanseggelen and Verabelle showed a long latent period of 140 minutes and a relatively low burst size, even when compared to other Streptomyces phages.5,21 The low burst size could be the result of the morphological complexity of Streptomyces21. The meaning of MOI loses its significance once spores have germinated and a mycelium is formed as multiple phages are able to attach, but the mycelium would still be counted as one colony forming unit13,22.
Temperature plays a fundamental role in phage attachment, penetration, multiplication, and the length of the latent period, and is therefore a crucial factor for phage viability23. Both phages remain stable at temperatures ranging from 25 to 45°C while phage viability decreases at higher temperatures. These results are consistent with a previous study that shows that phages most likely lose their DNA from the capsid head between temperatures of 50 and 60°C due to the tail complex breaking of the capsid head24. This results in the marked loss of infectivity and structural changes in the capsid proteins. It is suspected that denaturation of DNA and proteins within the capsid head only occurs at temperatures above 80°C 25. Another important factor for phage viability is the acidity of the environment in which the phage persists. Streptomyces strains can grow well between a pH range of 5.5–11.5 24. However, both Vanseggelen and Verabelle remain stable at pH values from 4.0 to 10.0. Although the exact effect of acidity on the capsid head or tail has not been clearly elucidated yet, a previous study showed that extreme pH values, such as highly acidic or highly alkaline conditions, can result in the denaturation or structural changes to phage capsid head proteins26.
The Streptomyces phages described in this study can help to discover new insights in the interactions between Streptomyces bacteria and bacteriophages. Such studies become increasingly important in order to keep exploiting Streptomyces’ antimicrobial potential in various industries like biotechnology, agriculture and medicine.