Virus Replication

This chapter describes virus replication. Before the development of in vitro cell culture techniques, all viruses had to be propagated in their natural host. For bacterial viruses, this was a relatively simple process that permitted an earlier development of laboratory research methods than was possible with plant or animal viruses. For animal viruses, samples from affected animals were collected and used to infect other animals, initially of the same species. When consistent results were obtained, attempts were usually made to determine whether other species might also be susceptible. This chapter explains that the advent of in vitro animal cell culture brought research studies in line with those involving bacterial viruses, thereby reducing the risks associated with adventitious viruses in animal inoculation systems and enhancing diagnostic testing. Various in vitro cell culture systems are used since an artificial medium was developed to maintain cell viability outside the source species: organ cultures, explant cultures, primary cell cultures, and cell lines. Organ cultures maintain the three-dimensional structure of the tissue and are used for short-term experiments. A fundamental characteristic that separates viruses from other replicating entities is the manner in which new virus particles are synthesized. Viruses do not use binary fission; virus particles are assembled de novo from the various structural components synthesized as somewhat independent but synchronized events. The critical first step in the virus replication cycle is the binding of the virus particle to a host cell. This binding process may involve a series of interactions that define in part the host range of the virus and its tissue/organ specificity.