Salty virus-host interactions: infection mechanisms of haloarchaeal viruses HRTV-7 and HRTV-10

ABSTRACT Archaea can be infected by morphologically diverse viruses, and the study of these viruses is important for understanding viral evolution in general. Archaeal tailed viruses make up the largest group of isolated archaeal viruses and the major class of isolated viruses infecting haloarchaea. Yet, little is known about the infection strategies of these viruses. In this study, we focus on archaeal myoviruses and investigate their infection cycles. HRTV-7 and HRTV-10 are both capable of infecting multiple Halorubrum strains, including Halorubrum salinarum B2-2, which is well suited for light microscopy. Virus lifecycle experiments combined with plaque assays and time-lapse microscopy showed that both viruses have a predominantly lytic life cycle, with complete lysis occurring between 9 and 12 h post-infection. Cells swell considerably just prior to lysis, indicating that the cell envelope becomes compromised. Whole-genome sequencing of virus-insensitive strains suggests that HRTV-10 likely uses the S-layer, the main cell envelope component of these archaea, as a receptor. Interestingly, we observed full-length integration of HRTV-7 in virus-resistant cells, as well as parts of the viral genome within the host genome, suggesting viral resistance based on superinfection exclusion. Together, these findings contribute to our understanding of archaeal virus-host interactions and provide insight into their infection strategies. IMPORTANCE Viruses of archaea are important to understand the diversity and evolution of all viruses. However, they are little studied. Archaea are ubiquitous microorganisms, of which some thrive at extreme conditions, such as high salt concentrations. Salt-loving haloarchaea are excellent models to study archaeal virus-host interactions due to the high availability of viral isolates and suitability for microscopy. Bacteria and archaea are both infected by tailed viruses; however, in archaea, their infection mechanisms are largely unknown. We used two tailed archaeal viruses infecting the same host to compare their infection strategies. Both viruses induce cell lysis of the host, which is preceded by cell bloating. Interestingly, one of the viruses can integrate into the genome of the host, which renders the strain resistant, suggesting a mechanism of superinfection exclusion. This work provides insight into archaeal virus-host interactions, thereby contributing to the exploration of the unusual archaeal virosphere.