Chapter 6: Viruses & Acellular Infectious Agents

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome to the Deep Dive.

Today, we are taking a rapid descent into the world of cellular infectious agents.

Yeah, things like viruses, viroids, and prions.

Exactly.

These entities that, well, they challenge our whole definition of life really, but they shape global health, ecosystems,

even what's on our dinner plates.

And speaking of dinner, here's a fact to maybe hook you, viruses are actually FDA approved, deliberately sprayed onto things like cold cuts, you know, ready to eat meats, as a food additive.

Sounds completely wild, doesn't it?

But it's very real and it's actually incredibly precise microbial warfare.

Warfare against what?

The target is listeria monocytogens.

It's this nasty little bacterium, a gram positive raw.

I've heard of that one, causes listeriosis.

Exactly.

And the problem is, it's tough.

It tolerates cold salt acid, perfect for processed foods stored in the fridge.

Okay.

Now, if you're a healthy adult, you might just get mild symptoms.

But for vulnerable people, especially newborns or pregnant women, it can cause miscarriage, stillbirth, meningitis.

The fatality rate is around 15%.

Pretty serious stuff.

Wow.

So these viruses are like microscopic assassins targeting just listeria.

Precisely.

They're called bacteriophages or just phages for short.

And the key thing is their specificity.

They are hyper specialized.

They only recognize and attack those listeria bacterial cells, your human cells, totally ignored.

So they're safe for us to eat them.

Completely safe in that context.

And it's a great example actually showing that viruses aren't just bad news, not disease agents.

They can be tools.

They're vital parts of ecosystems, even model organisms for research.

Okay, let's unpack this properly then.

Our mission today is to go beyond just definitions and really get into the mechanics.

You know, how their weird minimal structure leads to these really complex life cycles and interactions with hosts all the way from bacteria up to us.

Right.

And the starting point for virology, the study of viruses, is acknowledging they are a cellular.

It's fundamental.

Meaning no cells.

No cells.

They lack the whole basic toolkit for life as we know it.

No ribosomes to make proteins, no cytoplasm, no way to make their own energy, their own ATP.

They absolutely have to hijack a host cell to reproduce their obligate intracellular parasites.

And they infect basically everything.

Everything we know of.

All cell types.

Bacteria, animals, fungi, pro -tausts.

Nothing's off limits.

And the complete infectious particle, that's the virion.

That's the term.

Yeah, the virion.

And they range hugely in size.

Some are tiny, like 20 nanometers, way too small for a regular microscope.

Others, like Mimivirus, are enormous, actually visible with a light microscope.

Whoa.

But even the tiny ones, their structure is a marvel of efficiency.

At minimum, a virion is just a nucleocapsid.

That's the nucleic acid genome, DNA or RNA, wrapped up in a protein coat, the capsid.

And that capsid, that's where the clever design comes in, right?

It's made of repeating protein subunits.

Called protomers, yeah.

They often self -assemble, which is incredibly efficient.

Think about tobacco mosaic virus, TMV.

It's whole coat protein, coated by only about 474 nucleotides.

That's tiny.

They have to squeeze maximum function out of a very small genome.

Makes sense.

So how does that efficiency translate into shape?

Well, you get two main types of symmetry, generally speaking.

First, there are helical capsids.

Imagine hollow tubes made of protein.

Like TMV.

Exactly like TMV, which is quite rigid.

Or they can be flexible, like influenza virus.

The length of the tube is basically set by the length of the nucleic acid inside.

Okay, helical.

What's the other main type?

The really cool looking ones, icosahedral capsids.

And icosahedron is the most efficient way to enclose a space using identical subunits.

It's a geometric shape with 20 triangular faces and 12 corners, or vertices.

Right, like a 20 -sided die.

Pretty much.

And instead of just protomers lining up, they often group together into larger units called capsimers.

These can be made of five protomers, pentamers, usually found at the vertices, or six protomers, hexamers, making up the flat faces and edges.

It's very elegant construction.

But not all viruses fit those neat categories.

No, definitely not.

You get viruses with complex symmetry.

Pox viruses, for example, are quite large and sort of brick shaped.

And then there are the T -even bacteriophages.

The ones that look like little lunar landers.

Huh.

Yeah, that's a good description.

They have what's called binal symmetry.

It's a combination.

An icosahedral head structure joined to a distinct helical tail structure, often with a base plate and tail fibers for attachment.

Real nanomachines.

Okay, so that's the protein code, the capsid.

But some have another layer, right?

An envelope.

Correct.

You distinguish between non -enveloped or naked viruses and enveloped viruses.

The enveloped ones have this extralipid membrane layer covering the nucleocapsid.

Where does that membrane come from?

Usually it's stolen, basically.

Derived from the host cell's membranes, often the plasma membrane or maybe the nuclear membrane, as the virus exits the cell.

So it's like camouflage?

Kind of.

But the envelope itself isn't the main player.

What's critical are the viral proteins embedded in that These often stick out as spikes or peplomers.

And those are the important bits for getting in.

Absolutely.

They're often the keys the virus uses to unlock the host cell.

Think about influenza again.

It has hemagglutinin spikes for attaching to cells and then different spikes, neuraminidase that are needed later on to help the new viruses get released.

Got it.

So, capsid structure, envelopes, spikes.

Now, what about the core?

The genome?

You said it gets weird It really does.

Because unlike all cellular life, you, me, bacteria, trees, which universally uses double -stranded DNA,

dsDNA, as its genetic blueprint,

viruses throw out the rulebook.

How so?

They use all four possibilities.

Some have dsDNA, yes, that's common for phages, but others use single -stranded DNA, ssDNA.

Then you've got RNA viruses, which can be single -stranded RNA, ssRNA, that's most plant viruses, or even double -stranded RNA.

Whoa!

Yes, RNA, that's unusual.

And it doesn't stop there.

Some viruses, like influenza, have segmented genome.

Meaning the genome isn't one long molecule.

Right.

It's split into multiple separate pieces.

Influenza has like seven or eight different RNA molecules that all make up its complete genome.

Why do that?

It gives them a massive evolutionary advantage, actually.

It allows for reassortment.

If two different strains of influenza infect the same cell,

yeah,

during assembly, the new virus particles can grab a random mix of segments from both parent viruses.

This segment swapping can create brand new combinations of genes very quickly.

It's how we get major antigenic shifts that can lead to flu pandemics.

Okay, that genetic diversity is wild, and it must mean they have really different ways of actually functioning, of replicating.

Completely different strategies are needed, which brings us nicely to the viral life cycle.

How did scientists first figure this out?

It seems complicated.

It was a landmark experiment,

really.

Back in 1939, Delbruck and Ellis did the one -step growth curve experiment using T4 phage and E.

coli.

One step.

Yeah, they infected bacteria all at once and tracked the number of infectious viruses over time.

What they saw wasn't a smooth, steady increase.

Instead, there was this initial period where nothing seemed to happen in the latent period.

And within that latent period, there's an even earlier phase called the eclipse period.

During the eclipse, the number of infectious viruses actually drops to near zero.

Why?

Where did they go?

They hadn't gone anywhere.

They were inside the bacteria, but they had uncoated, released their genome, and were basically hidden, busy directing the cell to make new virus parts.

You only see the big burst of new infectious virions released much later.

Ah, so the latent period covers all the internal steps before release.

Exactly.

And those steps are generally considered universal, though the details very wildly.

There are five key stages.

One, attachment or adsorption.

Two, entry and the crucial uncoating step.

Three, synthesis of viral components.

Four, assembly of new virions.

And finally, five, release.

Let's walk through those.

Attachment must be pretty specific.

Oh, absolutely critical.

It's usually a lock and key mechanism.

Specific viral proteins or lipids, the ligands, bind to specific receptor molecules on the host cell surface.

And the virus targets receptors the cell actually needs.

Yes, that's the devious part.

Viruses evolve to use receptors that are essential for the host cell's normal function.

The cell can't easily just get rid of the receptor to avoid infection.

And this binding dictates tropism.

Tropism, meaning the type of cell or tissue the virus infects.

Precisely.

It's preference.

Polivirus is a classic example.

Its receptor is mainly found on cells in the nasopharynx, the gut, and specific motor neurons in the spinal cord, the anterior horn cells.

And that's why it causes paralysis and gastrointestinal issues.

Exactly.

That receptor distribution defines where the disease manifests.

But there's a big exception we should mention.

Plant viruses.

Right.

You said they do things differently.

They generally don't rely on specific receptors because they have to get through that really tough plant cell wall first.

So they often need some kind of mechanical damage, maybe an insect bite or abrasion to gain entry.

Okay.

So once attached, how do they get in, especially into animal cells without walls?

For eukaryotic cells, there are three common ways.

Enveloped viruses often use fusion.

Merging membranes.

Yeah.

The viral envelope literally fuses with the host cell's plasma membrane and it just dumps the nucleocapsid straight into the cytoplasm.

Seems efficient.

What else?

A very common route used by both naked and some enveloped viruses is endocytosis.

The cell is tricked into engulfing the virus particle in a vesicle.

Like eating it?

Kind of.

And then the uncoating, the release of the genome often happens inside that vesicle, that endosome.

Sometimes the acidic environment inside the endosome triggers the uncoating process.

Okay.

Fusion or endocytosis?

You said three ways.

The third is less common, but some viruses might be able to directly inject their nucleic acid across the membrane, though that's more typical for phages injecting through bacterial walls.

Right.

So the genome is inside.

Now comes the takeover.

The synthesis stage.

And this is where that genome diversity we talked about really dictates the strategy.

How so?

Well, DNA viruses, especially DSDNA ones, often have it relatively easy.

They can frequently use the host cell's own DNA polymerase and RNA polymerase enzymes to replicate their DNA and transcribe their genes into mRNA.

Makes sense.

The machinery is already there.

But the RNA viruses have a big problem.

Host cells simply don't have enzymes that can make RNA copies from an RNA template or DNA from an RNA template in the case of retroviruses.

So what do they do?

They have to bring their own.

Exactly.

They must either carry the necessary enzymes inside their virion, like an RNA dependent RNA polymerase, or their genome must contain the gene for that enzyme so it gets made immediately after entry.

That's a crucial difference.

Huge.

And regardless of genome type, synthesis is tightly regulated.

You get early, middle, and late genes expressed in sequence.

Early proteins often shut down host functions and take over the cell machinery.

Late proteins are typically the structural components, like capsid proteins needed for assembly.

And they have ways to protect this whole process.

Often, yes.

To shield the replication from host defenses,

many viruses reorganize host cell membranes to create specialized compartments, sometimes called viral replication complexes or viral plasms.

Basically, little shielded virus factories inside the cell.

Wow.

Okay, so lots of parts are made.

Then comes assembly.

Is that straightforward?

Not always.

For simple viruses, maybe.

But think about those complex T4 phages again.

Assembly is like a production line.

Separate lines for head, tail.

Exactly.

Head components assemble, tail components assemble, base plate components assemble.

Then they all get put together in a precise order.

And getting the DNA inside the head requires energy.

There's an enzyme complex called the pacosome that acts like a motor, using ATP to literally stuff the long DNA molecule into the preformed pro -head.

That sounds energy intensive.

It is.

It shows the commitment required.

Okay, so now we have fully assembled virions inside the cell.

Last step, release.

How do they get out?

Just burst the cell?

That's one way.

Common for bacteriophages and many non -enveloped viruses.

It's called lysis.

It requires viral proteins.

One is often holin, which pokes holes in the cell's plasma membrane.

Another is typically an enzyme like lysozyme to break down the bacterial cell wall, peptidoglycan.

Then boom, the cell bursts, releasing the progeny.

Messy.

What about enveloped viruses?

They usually get out via budding.

The nucleocapsid associates with a patch of host membrane, like the plasma membrane, that already has those viral spike proteins embedded in it.

Then it pushes outward, pinching off and taking that patch of membrane with it as its envelope.

Envelope formation and release happen together?

Simultaneously, yes.

And a key difference with budding is that it doesn't necessarily kill the host cell immediately.

The cell can often survive and continue to produce and release virus particles over a long period.

It becomes a persistent factory.

Though interestingly, some archaeoviruses have a totally unique release mechanism involving pyramid -like structures bursting from the cell surface.

Always exceptions.

Fascinating.

So this difference between immediate lysis and slower budding or even staying dormant, that leads into host interactions, right?

Different strategy.

Exactly.

Let's focus on bacteriophages first.

You have the purely destructive ones, the virulent phages like T4.

They only do the lytic cycle.

In fact, replicate and lie as the host.

Done.

The quick and deadly approach.

Right.

But then you have temperate phages, like the famous lambda phage.

They have a choice.

They can go lytic or they can enter lysogyny.

Lysogyny.

That's the dormant state.

Correct.

In lysogyny, the viral genome, now called a prophage, doesn't immediately replicate and destroy.

Instead, it usually either integrates itself into the host bacterium's chromosome or exists as a separate stable plasmid.

And just hangs out there.

It hangs out, yeah.

The host cell, now called a lysogyn, lives and reproduces normally and every time it divides, it copies the prophage DNA right along with its own.

The lysogyn also gains an advantage.

It becomes immune to further infection by the same type of phage.

Immunity to super infection.

Clever.

But it doesn't stay dormant forever.

Usually not.

If the host cell gets stressed, maybe DNA damage from UV light, or nutrient starvation that can trigger induction,

the prophage essentially wakes up, cuts itself out of the host DNA if necessary, and switches back to the lytic cycle, producing new phages and lysing the cell.

So why bother with lysogyny?

What's the advantage for the virus?

It's a survival strategy.

Imagine if host bacteria are scarce, or maybe they're dormant because there are no nutrients.

Lysis would be pointless.

Lysogyny lets the virus wait for better conditions.

Okay.

There's another interesting angle, too.

If there are tons of phages around, relative to bacteria,

a high multiplicity of infection, or MOI, going exoticalytic,

immediately could wipe out the entire host population too quickly.

Lysogyny provides a way to maintain the host population for future infection.

It's a strategic pause.

That makes sense.

And does this lysogyny ever affect the host bacteria itself, besides making it immune?

Or absolutely.

And this is medically important.

It's called lysogenic conversion.

The presence of the prophage actually changes the host's characteristics.

It's phenotype.

Ow.

Because the prophage genome might carry extra genes that the bacterium can now express.

The classic example is coronabacterium The bacteria that causes diphtheria.

Yes.

But it only causes the severe disease diphtheria if it's carrying a specific prophage.

Why?

Because the gene encoding the deadly diphtheria toxin is actually located on the phage genome, not the bacterial genome.

No phage, no toxin, no disease.

Wow.

So the phage makes the bacteria dangerous.

In that case, yes.

Okay.

Shifting to eukaryotic cells, like ours.

Virus infections there also show a range of outcomes.

You have cytocidal infections, which kill the host cell, often through lysis.

Similar to phages.

Similar outcome, yeah.

But you also get persistent infections.

These can be latent, where the virus is present but dormant, not actively replicating, think herpes viruses causing cold sores that flare up periodically.

Right.

Or they can be chronic, where the virus replicates slowly, and virions are released continuously over a long time, often via budding, without immediately killing the cell.

Hepatitis C can be like this.

Sometimes these persistent infections cause visible damage or changes in the cells, distinct from lysis, called cytopathic effects or CPEs.

And sometimes they cause cancer.

Unfortunately, yes.

About 10 to 20 % of human cancers are thought to be linked to viruses, which we call oncoviruses.

How does a virus trigger cancer?

Isn't cancer about uncontrolled cell growth?

Exactly.

Cancer fundamentally involves neoplasia, this abnormal, unregulated cell growth.

It starts with mutations in the cell's own genes that normally control division.

There are two main types you hear about.

Proto -oncogenes and tumor suppressors.

You got it.

Proto -oncogenes normally act like gas pedals, promoting cell division when needed.

Tumor suppressor genes are the brakes, acting as checkpoints to stop division if something's wrong, like DNA damage.

Like the famous P53 gene.

Exactly.

P53 is a critical tumor suppressor, often called the guardian of the genome.

It detects damage and can trigger programmed cell death, apoptosis, to eliminate faulty cells.

So how do viruses mess this up?

Different viruses have different strategies.

Many DNA oncoviruses, like human papillomavirus HPV, cause transformation by producing proteins that specifically target and inactivate the host's tumor suppressor proteins, like P53 and another one called RB.

They essentially cut the cell's allowing uncontrolled division.

Precisely.

Retroviruses, which are RNA viruses that convert their genome to DNA, have other tricks.

Some might actually carry their own versions of oncogenes, called viral oncogenes,

derived from host proto -oncogenes they picked up previously.

Or when their DNA integrates into the host genome, it might land right next to a host proto -oncogene, causing that gene to be constantly turned on, again flooring the gas pedal.

Okay, that's a scary link.

Let's switch gears slightly to the practical side.

How do scientists actually grow and count these things in the lab?

They need living cells, right?

Absolutely.

You can't grow them in sterile broth like bacteria.

Their cultivation requires providing the right living host cells.

So for animal viruses?

You might use inoculation into suitable live animals, though that's less common now for ethical reasons.

A classic method is using embryonated chicken eggs.

Different viruses replicate in different parts of the egg.

But the most common way today is tissue culture, or cell culture.

Growing host cells in dishes.

Exactly.

Growing animal or human cells as a layer, a monolayer, in plastic flasks or plates, and then infecting those cells.

For bacteriophages, it's easier.

You mix the phages with susceptible bacteria and spread them onto an agar plate.

And then the phages kill the bacteria.

Where a phage infects and destroys bacteria, it creates a clear zone in the otherwise cloudy lawn of bacterial growth.

That clear zone is called a plaque.

Each plaque theoretically starts from one infectious phage particle.

For plant viruses, you often have to mechanically inoculate the leaves, maybe by rubbing them with an abrasive and the virus preparation.

This often results in localized dead spots called necrotic lesions.

Okay, so you can grow them.

How do you count them?

How many are there?

Good question.

You can do direct counts using things like electron microscopy, but that counts all particles, infectious or not.

Or you can use molecular methods like qPCR to quantify the amount of viral nucleic acid.

But that doesn't tell you how many are actually infectious.

Right.

For infectivity, the gold standard is often the plaque assay we just mentioned.

You perform dilutions of your virus sample, plate them with bacteria, and count the number of plaques formed.

This gives you a titer in plaque -forming units, or PFU per milliliter.

So PFU represents infectious particles.

Essentially, yes.

For some animal viruses, you might use a hemagglutination assay, which relies on the ability of viruses like influenza to clump red blood cells together.

It's faster, but less precise than plaque assays.

And in animal studies, you often measure dosage effects.

Like the lethal dose 50, LD50, the dose needed to kill 50 % of test animals.

Or the infectious dose 50, ID50, the dose needed to infect 50%.

Okay, that covers viruses pretty well.

But you mentioned even simpler things at the start.

Yes, let's touch on the subviral agents.

First, viroids.

Simpler than a virus?

What does that even look like?

Dramatically simpler.

They are infectious agents, but only known to cause diseases in plants over 20 different diseases, like potato spindle tuber disease.

Structurally, a viroid is just a small, covalently closed circle of single -stranded RNA, SSRNA.

Usually only about 250 to 400 nucleotides long.

Just RNA.

No protein coat.

No capsid at all.

And here's the really mind -bending part.

The tiny RNA molecule does not include any proteins.

None.

Wait, if it doesn't make proteins, how does it replicate?

How does it do anything?

It's pure hijacking.

Viroids somehow trick the host plant cell's own DNA -dependent RNA polymerase, an enzyme normally used to make RNA from a DNA template into using the viroid's RNA as a template to make more baroroid RNA.

It forces the host enzyme to do something it's not supposed to do.

Exactly.

And how do they cause disease?

The leading hypothesis is that the replicating viroid RNA, which likely forms double -stranded regions, triggers the plant's own defense mechanism called RNA silencing.

This system normally targets and destroys foreign dsRNA, like from viruses.

But in this case, it might end up targeting the plant's own essential mRNAs by mistake because of similarity to the viroid sequence, so the host damages itself.

Wow.

Okay, viroids.

RNA only.

No protein.

What else?

Then there are satellites.

These are also nucleic, acid -based agents, can be RNA or DNA, but they do depend on another virus, a helper virus, being present in the same host cell in order to replicate.

So they're like parasites of viruses.

Kind of, yeah.

They can't complete their life cycle alone.

Now there's a distinction.

Satellite viruses actually encode their own capsid protein, even though they need the helper for replication enzymes.

But satellite RNAs, or DNAs, don't even encode a capsid.

They rely on getting packaged inside the helper virus's capsid.

Hepatitis D virus is a human example.

It's a satellite that requires hepatitis B virus as its helper.

Okay.

Dependent on a helper.

Getting simpler.

Is there anything even more basic?

One more step down.

To the simplest infectious agents we know.

Prions.

Prions.

Protein only, right.

This is the mad cow disease agent.

Exactly.

Prion stands for proteinaceous infectious particles that cause fatal neurodegenerative diseases in mammals.

Scrapian sheep, bovine spongiform encephalopathy, BSE,

or mad cow disease in cattle, Creutzfeldt -Jakob disease, CJD in humans,

and crucially, there's no effective treatment.

Just protein.

No DNA, no RNA at all.

None detected.

The infectious agent appears to be solely an abnormal form of a normal cellular protein.

A normal protein.

Yes.

There's a normal protein found on the surface of neurons and other cells called PRPC, prion protein cellular.

The prion disease is caused by an abnormally folded version of this exact same protein called PRPSC, prion protein scrapie.

So just a shape change makes it infectious?

How does it replicate if there's no genetic material?

That seems impossible.

It's not replication in the genetic sense.

It's structural conversion, like a template.

The theory is that when the abnormal PRPSC protein comes into contact with a normal PRPC protein.

Yeah.

It somehow induces or forces the normal PRPC to misfold and adopt the abnormal PRPSC shape.

It corrupts the normal ones.

That's the idea.

Yeah.

And this becomes a chain reaction.

The newly misfolded PRPSC can then go on to convert more normal PRPC.

This abnormal form tends to clump together, forming aggregates in the brain tissue, which leads to neuron damage and the characteristic spongy appearance of the brain in these diseases.

It's a propagating misfolding cascade.

That is fundamentally different and terrifying.

It really is.

It challenges our core concepts of infection and replication.

So if you look at the whole picture we've discussed, virology really spans this incredible range from complex DNA viruses that cause cancer down through RNA viruses with segmented genomes, simpler phages, then viroids, which are just RNA, and finally prions, which seem to be just protein.

It connects the chemical world to the biological world in a unique way.

Yeah.

You see how just a protein coat and some nucleic acid, or even less, can result in these incredibly diverse and sophisticated, sometimes devastating life cycles.

From that quick lytic burst to the patient waiting game of lysogeny, or the slow corruption of a prion.

It's all about structure, interaction, and regulation.

So thinking about all this, what does it mean for you, the listener, consider that idea of again, that incredible specificity where,

say, an Ebola virus has evolved to look like a dying cell fragment to trick its way inside, or rabies finds a receptor that's present across lots of different mammal species, allowing it to jump between them.

Masters of disguise and infiltration.

Exactly.

So if viruses and even prions are such masters of exploiting structure and specific interactions, maybe the next big breakthroughs in fighting them won't just vaccines or general antivirals, but drugs that target those very specific structural points we talked about.

Could you design something to destabilize the assembly of a phage head?

Or maybe even more challenging, something to prevent that initial non -genetic misfolding event of the prion protein?

That's certainly where a lot of research is focused, targeting those unique viral or prion -specific structural vulnerabilities.

Lost to think about there.

Well, thank you for joining us on this deep dive into the fascinating and often strange world of a Cellular Infectious Agents.

A pleasure.

We'll catch you next time.