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Lecture 4
Reaction to Physical & Chemical Agents
Heat & Cold
There is great variability in the heat stability of different viruses.
-
Icosahedral viruses tend to be stable, losing little infectivity after several hours at 37 °C.
-Enveloped viruses are much more heat-labile, rapidly dropping in titer at 37 °C.
-Viral infectivity is generally destroyed by heating at 50–60°C for 30 minutes, exceptions
(e.g., hepatitis B virus, polyomaviruses).
Viruses can be preserved by storage at subfreezing temperatures, and some may
withstand lyophilization and can thus be preserved in the dry state at 4 °C or even at
room temperature. Enveloped viruses tend to lose infectivity after prolonged storage even
at -90 °C and are particularly sensitive to repeated freezing and thawing.
Stabilization of Viruses by Salts
Many viruses can be stabilized by salts in concentrations of 1 mol/L. The mechanism by
which the salts stabilize viral preparations is not known. Viruses are preferentially
stabilized by certain salts. MgCl
2
, 1 mol/L, stabilizes picornaviruses and reoviruses;
MgSO
4
, 1 mol/L, stabilizes orthomyxoviruses and paramyxoviruses; and Na
2
SO
4
, 1
mol/L, stabilizes herpesviruses. They stabilize for weeks at ambient temperature.
PH
Viruses are usually stable between pH values of 5.0 and 9.0. Some viruses (e.g.,
enteroviruses) are resistant to acidic conditions. All viruses are destroyed by alkaline
conditions.
Radiation
Ultraviolet, x-ray, and high-energy particles inactivate viruses. The dose varies for
different viruses. Infectivity is the most radiosensitive property because replication
requires expression of the entire genetic contents.
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Photodynamic Inactivation
Viruses are penetrable to a varying degree by vital dyes such as toluidine blue, neutral
red, and proflavine. These dyes bind to the viral nucleic acid, and the virus then becomes
susceptible to inactivation by visible light.
Ether Susceptibility
Ether susceptibility can be used to distinguish viruses that possess an envelope from
those that do not as shown in Table 29–1.
Detergents
Nonionic detergents—e.g., Nonidet P40 and Triton X-100—solubilize lipid
constituents of viral membranes. The viral proteins in the envelope are released
(undenatured).
Anionic detergents, e.g. sodium dodecyl sulfate, also solubilize viral envelopes; in
addition, they disrupt capsids into separated polypeptides.
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Formaldehyde
Formaldehyde destroys viral infectivity by reacting with nucleic acid. Viruses with
single-stranded genomes are inactivated much more readily than those with double-
stranded genomes. Formaldehyde has minimal adverse effects on the antigenicity of
proteins and therefore has been used frequently in the production of inactivated viral
vaccines.
Antibiotics & Other Antibacterial Agents
Antibacterial antibiotics and sulfonamides have no effect on viruses. Some
antiviral drugs are available.
Larger concentrations of chlorine are required to destroy viruses than to kill
bacteria, especially in the presence of extraneous proteins. For example, the
chlorine treatment of stools adequate to inactivate typhoid bacilli is inadequate to
destroy poliomyelitis virus present in feces.
Alcohols, such as isopropanol and ethanol, are relatively ineffective against
certain viruses, especially picornaviruses.