Virus cultivation in the virology laboratory is fundamental to virological research, serving key roles in diagnosis, basic studies, and vaccine or drug development. It enables the isolation and identification of disease-causing viruses from animal samples by observing cytopathic effects (CPE) and confirming results with molecular or immunological methods such as RT-PCR.<\/p>\n\n\n\n
In basic research, virus culture enables investigation of viral infection, replication, and host cell responses, providing insights into disease mechanisms and preventive strategies. It is also essential for producing large quantities of virus for use in attenuation or inactivation during vaccine production, such as for rabies, influenza, and African horse sickness.<\/p>\n\n\n\n
Moreover, in vitro virus culture provides a safe and efficient system for evaluating antiviral compounds, prototype vaccines, and neutralizing antibodies, supporting ongoing efforts in disease control and surveillance.<\/p>\n\n\n\n
Before using propagated viruses for various applications, an essential step is to accurately quantify the virus. Several quantification methods are available, and the choice depends on the intended purpose of the virus. Generally, these methods can be classified into two main categories:<\/p>\n\n\n\n
1. Infectious Virus Quantification Methods<\/strong> \u2014 These techniques measure only infectious (viable) viruses capable of replicating and causing infection in host cells. Common examples include the Plaque Assay, Tissue Culture Infectious Dose 50% (TCID50) Assay, Focus Forming Assay (FFA), and Endpoint Dilution (Quantitative Infection) Assay.<\/p>\n\n\n\n 2. Total Virus Quantification Methods<\/strong> \u2014 These approaches determine the total number of viral particles or their components, such as viral proteins or nucleic acids, without distinguishing between infectious and inactivated viruses. Representative methods include the Hemagglutination (HA) Assay, Enzyme-Linked Immunosorbent Assay (ELISA), Quantitative Polymerase Chain Reaction (qPCR or RT-qPCR), Transmission Electron Microscopy (TEM), and Flow Cytometry.<\/p>\n\n\n\n Among these methods, infectious virus quantification is particularly important when the goal is to measure the number of viruses capable of causing infection. Within this group, the Plaque Assay is widely regarded as the gold standard, providing accurate and reproducible measurements of infectious viral particles. Its reliability and ability to quantify viable viruses make it an essential tool in virology research.<\/p>\n\n\n\n Virus quantification by the plaque assay<\/strong> is a highly reliable method and is considered the gold standard for determining the concentration of viable viruses, as it allows accurate identification and quantification of infectious viral particles. The principle of the plaque assay is to measure the number of viruses capable of causing infection and cell death.<\/p>\n\n\n\n In this method, a sample of the virus is first inoculated onto a prepared monolayer of cultured cells. After the virus infects the cells, an overlay medium or substrate is applied to restrict viral spread. This setup enables progeny viruses originating from each initially infected particle to replicate locally, forming clear zones known as plaques.<\/p>\n\n\n\n Each plaque represents an area where cells have been destroyed by viral infection and can be observed as a visible hole or a transparent region on the cell monolayer. The shape of plaques may be circular or irregular with indistinct edges, depending on the virus type and the host cells used. Finally, the number of plaques can be counted to determine the viral concentration, with each plaque corresponding to a single infectious viral particle, defined as one plaque-forming unit (PFU). Examples of plaques produced by different viruses in various cell types are shown in Figure 1<\/em>.<\/p>\n\n\nPlaque Assay<\/h2>\n\n\n\n