The Cytopathic Effect

Cell cultures are the main method for “virus isolation” and are considered the gold standard even in the face of improving technology:

“With the recent advances in technology, cell culture is considered a gold standard for virus isolation.”

A typical cell culture consists of taking a sample from a sick person (called the isolate even though nothing is isolated), adding it to a culture containing either animal or cancer cells, mixing in toxic additives such as antibiotics, fetal bovine serum, DMEM “nutrients,” etc., and then letting this concoction sit for a week or so while checking for what is called cytopathic effects.

“Changes in monolayer cells (e.g., swelling, shrinking, syncytium formation) indicate the presence of viruses. These changes in cell culture are defined as the cytopathic effect (CPE), which is due to the presence of the virus [10].”

This effect is the goal of the culture. Without it, “infectious” or viable “virus” is not considered to be within the sample.

Viruses, by definition, are invisible to our eyes. They are nanostructures that can only become visible using electron microscopy, a technique that is time-consuming and not accessible for everyone. As a consequence, a virologist has to find alternatives to be able to work and study its object of interest. A practical way of “seeing” and indirectly measuring a viral infection is by looking at the damage a virus causes to a cell. This suffering or damage is known as cytopathic effect (CPE) and its measurement is widely used in virology labs all over the world.”

Looking at the definition of Cytopathic Effects (CPE), one can see that this measurement is only intended for the IDENTIFICATION OF “VIRUSES:”

Cytopathic Effect (CPE), structural changes in a host cell resulting from viral infection. CPE occurs when the infecting virus causes lysis (dissolution) of the host cell or when the cell dies without lysis because of its inability to reproduce.”

So if CPE is the method used for identifying “viruses” in cell cultures and the only way to determine if the “viruses” are “viable” and “infectious,” what does it mean if there are other possible causes of CPE in cell cultures that are not caused by “viruses?”

According to a manual on Cell Culture Contamination:

“Since cytopathic viruses usually destroy the cultures they infect, they tend to be self-limiting. Thus, when cultures self-destruct for no apparent reason and no evidence of common biological contaminants can be found, cryptic viruses are often blamed. (See Figures 3a and 3b.) They are perfect culprits, unseen and undetectable; guilty without direct evidence. This is unfortunate, since the real cause of this culture destruction may be something else, possibly mycoplasma or a chemical contaminant, and as a result
will go undetected to become a more serious problem.”

It would appear as if it is known that there are other factors that can cause CPE in cell cultures such as mycoplasma or chemical contaminants yet unseen “viruses” are often incorrectly blamed.

Let’s see how many other CPE causing contaminants we can find:



Some mycoplasmas have been shown to produce severe cytopathic effects (CPE) characterized by stunted, abnormal growth and rounded, degenerated cells, apparently due to the promotion or inhibition of apoptosis [56].”


“Mycoplasmal attachment to eukaryotic cells may sometimes lead to a pronounced cytopathic effect. Attachment permits the mycoplasma contaminant to release noxious enzymatic and cytolytic metabolites directly onto the tissue cell membrane. Some mycoplasmas selectively colonize defined areas of the cell culture. This results in microcolony formation producing microlesions and small foci of necrosis, e.g., M. pulmonis, or form plaques, e.g., M. gallisepticum, in an agar overlay system [5]. Microcolonization suggests that mycoplasma-specific receptors are localized in defined areas of the cell monolayer. However, other fermenting mycoplasmas, e.g., M. hyorhinis, attach to every cell and destroy the entire monolayer, producing a generalized cytopathic effect. With HeLa cells infected by the invasive M. penetrans, the most pronounced effect was the vacuolation of the host cells [22].”


“Mycoplasmas may alter the progress of viral infections in cell cultures [83, 84]. As mycoplasmas may also cause virus-like CPE, many investigators have mistaken cytolytic mycoplasmas for viruses. Like viruses, mycoplasmas are filterable, hemadsorbant, hemagglutinant, resistant to certain antibiotics, able to induce chromosomal aberrations, and sensitive to detergents, ether and chloroform; thus the first established mycoplasma pathogens of humans (M. pneumoniae), animals (M. mycoides) or plants (Spiroplasma spp.) were believed to be viruses.”

Isolated Dentilisin

Cytopathic effects of isolated dentilisin, including membrane blebbing, vacuolization, inhibition of motility, loss of epithelial cell contacts and release of a cytosolic enzyme have been demonstrated in multi-layer epithelial cell culture systems.

E. Coli

Cytopathic effects induced by E. Coli live bacteria or protein extracts on epithelial cells at three-day post-infection. For CNF and CDT, the cytopathic effect is only observable with bacterial lysates. In contrast, for colibactin and CIF, a contact between bacteria and host cells is required. Colibactin, CDT and CIF induced cytopathic effects as seen by enlarged nuclei and cell distension (megalocytosis), while CNF induced multinucleation and enlargement of HeLa cells.”

Both parasitic and free living amoeba occasionally identified as cell culture contaminants. They can cause CPE resembling viral damage and completely destroy a culture within 10 days.”


A. Castellani Trophozoites and/or Cysts

“In this study, we observed the cytopathic effect, in vitro cytotoxicity, and secretion pattern of cytokines in human corneal epithelial cells (HCECs) induced by A. Castellani Trophozoites and/or cysts. In vitro cytotoxicity assay revealed the highest cytotoxicity to HCECs in the co-culture system with amoeba cysts.”

Naegleria Fowleri

Naegleria Fowleri, strain HB-1, caused a destructive cytopathic effect (CPE) in secondary mouse-embryo (ME) cells. No evidence was found to suggest that cell-free cytotoxic factors secreted by the amoebae play a part in ME-cell destruction. In culture systems designed for the study of cytopathic factors, mammalian-cell damage seemed to occur only as a result of direct contact with active amoeba.


T. Vaginalis

“Thus, to establish the possible outcome from the interaction of T. vaginalis with lung cells, the cytopathic effects of the parasites were evaluated using monolayer cultures of the human lung alveolar basal carcinoma epithelial cell line A549.

These observations indicate that T. Vaginalis causes cytopathic effects on A549 cell. To date, this is the first report showing a possible interaction of T. vaginalis with the lung cells using A549 monolayer cultures.”

Cryptosporidium Parvum

Unexpectedly, direct cytopathic effects were noted in infected monolayers, with widespread programmed cell death (i.e., apoptosis) of biliary epithelial cells as assessed both morphologically and biochemically beginning within hours after exposure to the organism. The novel finding of specific cytopathic invasion of biliary epithelial by C. Parvum may be relevant to the pathogenesis and possible therapy of the secondary sclerosing cholangitis seen in AIDS patients with biliary cryptosporidiosis.”

“The cell culture model for biliary cryptosporidiosis described in this report provides a useful method for investigating the cytopathic effects of C. Parvum, and may yield information regarding the in vivo consequences of C. parvum infection.”


Antibiotics and Antifungals:

“The toxicity of drug mixtures has not been thoroughly studied. We therefore investigated cytopathic effects on primary cultures of human corneal cells of six topical antimicrobials singly and in combinations of any two, to determine the combined toxicity ranking and the interaction between duration of exposure and concentration.”

“An in vitro system was chosen to isolate drug toxicity from cytopathic effects of a microbial infection. Human corneal epithelial cell cultures were exposed to antimicrobial drugs for a period commensurate with clinical use. They were exposed to fixed and to sequentially decreasing concentrations of test substance, the latter mimicking a tapering off regime.

Cytopathic effects depended on the drug(s), concentration and duration of exposure. Rankings of single drugs and combinations were not correlated. Exposure to diminishing drug concentrations did not always improve the outcome.”

Just off a cursory look, it would seem that there are various other factors besides “viruses” which can bring about the CPE assumed to be caused by them such as:

  • Bacteria
  • Parasites
  • Amoebas
  • Chemical Contaminants

Obviously, this means that the CPE observed in cell cultures is NOT SPECIFIC TO “VIRUSES” and can therefore not be used as indirect evidence of their existence nor as a measure of their viability or “infectious” potential.



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