7 Viruses and Other Acellular Agents
Explore viral structure and classification, how viruses infect and reproduce in host cells, and the methods used to cultivate and detect them.
What Makes a Virus
Viruses are acellular infectious agents: they are not made of cells and cannot reproduce independently. A complete infectious particle outside a cell is a . It contains a genome made of either DNA or RNA, but not both as the genome of a single virus.
Viruses lack ribosomes and the full metabolic machinery needed to make proteins or generate energy on their own. They must use susceptible host cells to produce viral components and new particles.
Viral Structure and Envelopes
A viral is a protein shell made from repeating subunits that encloses the genome. Together, the genome and form the nucleocapsid. Capsids commonly have helical or icosahedral symmetry, while some viruses have more complex structures.
Many animal viruses also have an envelope, a lipid membrane acquired from a host-cell membrane. Viral proteins on the envelope can help the virus attach to host cells. Because the envelope contains lipids, enveloped viruses are generally more sensitive than non-enveloped viruses to drying, heat, detergents, and solvents that disrupt lipids.
Two Ways to Classify Viruses
Virus classification can describe viruses from complementary perspectives. The ICTV taxonomy organizes viruses into official hierarchical groups using evidence such as genome sequence and organization, structure, replication strategy, and . Because classification changes as evidence accumulates, current names should be checked against the current ICTV taxonomy.
The groups viruses by genome type and by how they produce messenger RNA (mRNA), which host ribosomes can translate into protein. Its seven groups are:
Group I: double-stranded DNA.
Group II: single-stranded DNA.
Group III: double-stranded RNA.
Group IV: positive-sense single-stranded RNA.
Group V: negative-sense single-stranded RNA.
Group VI: positive-sense RNA with reverse transcription.
Group VII: double-stranded DNA with reverse transcription.
Positive-sense RNA can function as mRNA. Negative-sense RNA must first be copied into a complementary positive-sense strand. In short, ICTV taxonomy emphasizes formal naming and evolutionary relationships, while the emphasizes the route from viral genome to mRNA.
Replication, , and Persistence
A typical viral replication cycle includes attachment, entry, uncoating, genome expression and replication, assembly, and release. The steps differ among viruses: bacteriophages commonly inject their genomes into bacteria, whereas animal viruses may enter through membrane fusion or endocytosis. New particles can leave by budding or exocytosis, or through lysis of the host cell.
Some bacteriophages follow a , rapidly producing progeny and lysing the host. Others can become , maintaining their genome in the host—sometimes integrated into its chromosome—and copying it as the host divides. Under certain conditions, a virus can switch to productive replication. Animal viruses can also establish latency, persisting with little or no production of infectious particles before reactivating.
A virus’s is the set of host species and cell types it can infect. Surface attachment proteins and host-cell receptors help determine which cells a virus can enter, but attachment alone is not sufficient: the cell must also provide compatible machinery and conditions for replication. The tendency to infect particular tissues is called tissue tropism.
Takeaway: Successful infection depends on both reaching a compatible cell and being able to replicate inside it.
Cultivation and Detection
Viruses depend on living cells, so they do not grow on ordinary cell-free nutrient agar. They can be cultivated in susceptible cell cultures or embryonated eggs; specialized research may also use whole animals. Bacteriophages can be grown with susceptible bacteria, where areas of cell destruction called plaques appear in a bacterial lawn. A assay estimates infectious virus as -forming units per volume of sample. In animal-cell cultures, infection may cause a visible .
Different tests detect different evidence of infection:
Nucleic acid tests, such as PCR and other amplification methods, detect targeted viral genetic material. A positive result does not by itself prove that intact, infectious virus is present.
Antigen tests detect viral proteins.
Serology detects host antibodies and can help assess prior exposure or an immune response.
Culture demonstrates replication in susceptible cells and can provide infectious-virus isolates, but it may be slower and is not suitable or available for every virus.
Test choice depends on the question being asked, the specimen, timing, and the virus. For potentially infectious specimens, appropriate containment, protective practices, and handling are essential; specimen type, transport conditions, and timing can affect test reliability.