Visible morphological changes in cell cultures caused by viral infections are called cytopathic effects (CPE). The degree of visible changes to cells caused by viral infection varies with the type of virus, type of host cells, the multiplicity of infection (MOI), and other factors.
Some viruses cause very little or no CPE in the cells of their natural host, while others cause complete and rapid destruction of the cell monolayer after infection. Cell line supporting virus replication, the time required to produce CPE, and the microscopic appearance of the CPEs may be sufficiently characteristic to allow the provisional identification of an unknown virus.
Viruses have distinct CPEs, just as colonies of bacteria on agar plates have unique morphologies.
Some CPE can be readily observed in unfixed, unstained cells under the low power of the light microscope. Still, several types of CPE are distinguishable in living cultures, thus requiring fixation and staining of the cells. Cell cultures are stained with hematoxylin, a basic dye, and eosin, an acidic dye.
Recognizing CPE and using it as a diagnostic tool requires much experience examining both stained and unstained cultures of many cell types. Uninfected cells should always be run as a control to distinguish age-related changes in infected cells from the cytopathic effects.
Types of Cytopathic effects
Summary of Cytopathic effect(s) of common, clinically encountered viruses
|Nuclear shrinking (pyknosis), proliferation of membrane||Picornaviruses|
|The proliferation of nuclear membrane||Alphaviruses, herpesviruses|
|Vacuoles in cytoplasm||Polyomaviruses, papillomaviruses|
|Syncytium formation (cell fusion)||Paramyxoviruses, coronaviruses|
|Margination and breaking of chromosomes||Herpesviruses|
|Rounding up and detachment of cultured cells||Herpesviruses, rhabdoviruses, adenoviruses, picornaviruses|
|Virions in nucleus||Adenoviruses|
|Virions in the cytoplasm (Negri bodies)||Rabies virus|
|“Factories” in the cytoplasm (Guarnieri bodies)||Poxviruses|
|Clumps of ribosomes in virions||Arenaviruses|
|Clumps of chromatin in the nucleus||Herpesviruses|
It is the most severe form of CPE. All cells in the monolayer rapidly shrink, become dense (pyknosis), and detach from the glass within 72 hours. This CPE is typical of most enteroviruses.
It consists of detachment (death) of some but not all of the cells in the monolayer, which can be observed using the 20X objective. Some togaviruses (alphaviruses), some picornaviruses, and some paramyxoviruses may cause this type of CPE.
Instead of causing generalized cell monolayer destruction, some viruses produce localized areas (foci) of infection. Cells become enlarged, rounded, and refractile, eventually detaching from the growth surface, leaving cleared areas surrounded by rounded-up cells as the infection spreads concentrically. Focal degeneration is characteristic of the herpesviruses and poxviruses.
Swelling and clumping
Infected cells greatly enlarge and clump together in “grape-like” clusters. For example, adenoviruses.
Foamy degeneration (vacuolization)
Severa virus families produce large and/or numerous cytoplasmic vacuoles. Vacuolation is visible only after staining. Retroviruses, paramyxoviruses, and flaviviruses may cause vacuolization.
Cell fusion (syncytium or polykaryon formation)
It involves the fusion of the plasma membranes of four or more cells to produce an enlarged cell with four or more nuclei. Some paramyxoviruses; and herpesviruses may produce syncytia. Syncytia are much easier to observe after staining.
These are areas of altered staining in cells that cannot be seen in live cell cultures. Depending on the causative virus, these inclusions may be single or multiple, large or small, round or irregularly shaped, intranuclear or intracytoplasmic, eosinophilic (pink staining), or basophilic (blue-purple staining).
Before, virologists used to rely on cytopathic effects to identify viruses. The reliance has decreased substantially due to a rise in nucleic acid amplification tests such as polymerase chain reaction (PCR), which is faster and more specific.
References and further readings
- Suchman, E., & Blair, C. (2007, September 29). Cytopathic Effects of Viruses Protocols.
- Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.