Thin sectioning and eectron microscopy of SP50 bacteriophage adsorbed to Bacillus subtilis

Imaging Techniques for Detecting Prokaryotic Viruses in Environmental Samples

Viruses are the most abundant biological entities on Earth with an estimate of 1031 viral particles across all ecosystems. Prokaryotic viruses-bacteriophages and archaeal viruses-influence global biogeochemical cycles by shaping microbial communities through predation, through the effect of horizontal gene transfer on the host genome evolution, and through manipulating the host cellular metabolism. Imaging techniques have played an important role in understanding the biology and lifestyle of prokaryotic viruses. Specifically, structure-resolving microscopy methods, for example, transmission electron microscopy, are commonly used for understanding viral morphology, ultrastructure, and host interaction. These methods have been applied mostly to cultivated phage-host pairs. However, recent advances in environmental genomics have demonstrated that the majority of viruses remain uncultivated, and thus microscopically uncharacterized. Although light- and structure-resolving microscopy of viruses from environmental samples is possible, quite often the link between the visualization and the genomic information of uncultivated prokaryotic viruses is missing. In this minireview, we summarize the current state of the art of imaging techniques available for characterizing viruses in environmental samples and discuss potential links between viral imaging and environmental genomics for shedding light on the morphology of uncultivated viruses and their lifestyles in Earth's ecosystems.

The SPO1-related bacteriophages

A large and diverse group of bacteriophages has been termed 'SPO1-like viruses'. To date, molecular data and genome sequences are available for Bacillus phage SPO1 and eight related phages infecting members of other bacterial genera. Many additional bacteriophages have been described as SPO1-related, but very few data are available for most of them. We present an overview of putative 'SPO1-like viruses' and shall discuss the available data in view of the recently proposed expansion of this group of bacteriophages to the tentative subfamily Spounavirinae. Characteristics of SPO1-related phages include (a) the host organisms are Firmicutes; (b) members are strictly virulent myoviruses; (c) all phages feature common morphological properties; (d) the phage genome consists of a terminally redundant, non-permuted dsDNA molecule of 127-157 kb in size; and (e) phages share considerable amino acid homology. The number of phages isolated consistent with these parameters is large, suggesting a ubiquitous nature of this group of viruses.