Micro sheet # 5 - Haneen Thnebat

بسم الله الرحمن الرحيم
Medical Virology for Dentistry Students

Microbiology lecture#5
Dr.Asem shehabi
14/2/2011
introduction
In this lecture we will start a new topic "virology", an important topic because human beings not affected only by bacteria but also by viruses, parasites, fungi…etc.
Viruses affect human, animals, plants, and even microorganisms such bacteria! Bacterial viruses known as: bacteriophage.
World of viruses composed of millions of species, about only100 major groups of viruses can cause symptoms& asymptomatic infections in humans.
Some types adapted to humans "only cause infection in humans" ,other adapted to animals, another to plants and so on….
Plant viruses, insects viruses rarely associated with human infection "lack mechanism to affect human tissue", while according animal viruses, it has been improved in last century that some of the human viruses originated from animal .These viruses affect animals, undergo mutations& recombination of nucleic acid (DNA,RNA),later on managed to transmitted from animal to human. Such as Tibian? virus ,birds virus ,influenza virus which common nowadays in some countries., swine virus "influenza 2l5nazer" which is a combination of animal human virus.
Studying main structures of viruses is necessary to understand mechanism (attachment, penetration to specific tissue) of infection, process of multiplication.
Genome
In contrast to bacteria and other eukaryotics"yeast, fungi", Virus genome composed normally of one single type of nucleic acid either DNA or RNA.
In prokaryotics,eukaryotics cells have only double stranded DNA(no single stranded ),while in viruses we have single stranded(DNA or RNA) and double stranded.
In relation to RNA ,it can be classified as positive or negative single stranded RNA.
Positive single stranded RNA means already presence of mRNA which serves as template to produce t-RNA. "Further explanations": if we said that this is a positive single stranded RNA virus means it has the genetics apparatus that allow it to replicate within infected tissue easily only by produce ( t-RNA ,polymeraze and other enzymes) to form the necessary DNA, and later to form the components need in replication& production e of viruses.
Single (+) RNA virus undergo translation directly.
Single (-)RNA virus undergo transcription then translation.
The following scheme may help you:
Viruses RNA virus strands

DNA virus


-viruses are smaller than bacteria and other types of prokaryotic cells.
-size ranges between (100-400) µm, so the largest virus is 10X smaller than the smallest type of bacteria!
-viruses, unlike bacteria and fungi, can't be recognized by light microscope, recognize only by electron microscope.
*electron microscope help in study of:
1-General morphological structures.
2-recognize types of viruses (wither associated with envelope or not ,associated with capside or not).capside :coat surrounded nucleic acid of virus.
Virus morphology
Any virus in general might be composed of only single type of nucleic acid surrounded by few numbers of polypeptides in form of capsomeres that together form capside. Refer to slide please: the capside green. Genome red.
Capside: numerous, identical, repeating subunits of polypeptides.
Each group of viruses have the same type of polypeptides,same morphology of capsomeres(weither small or larg…).
Precense of nucliec acid and capsomeres are sufficient to have what we called virion.
Virion:
-infectious particle of the virus.
-independent from presence of nucliocapsid or envelope.
-once virus attach and penetrate the host tissue ,it will lose capside and envelope outside the cell, so only the nucleic acid (as infectious particle) will penetrate into the cytoplasm.
So as conclusion: any virus to be infectious it must carry certain specific structures: 1) genome "Nucleic acid". 2) Surrounding capside.

Importance of capside:
• Protection of genome.
• Attachment to mucosa of respiratory tract, skin or other site of infection.
Attachment is an essential, primarily process to be able to produce infection. By attachment the first interaction occur ,virus will manage to enter the infected cell, start replication, produce virus particles.
Attachment achieved by certain receptors (receptors in relation to virus and receptors in relation to infected tissue).These receptors coincidence with each other "like key and lock".
In general ,viruses can't replicate by their own mechanism(their own nucleic acid),they must be supplied with energy source and metabolic activity of host tissue, so viruses can't survive, replicate and produce new generation without being inside the infected cell.
Although viruses depend on metabolic activity of host tissue, once virus excrete from human respiratory tract or excrete with urine, feces, it might survive for short period of time, depend on virus type.
Generally speaking, viruses which composed of genome and capside only (non enveloped virus) consider more resistance to environmental factors than the enveloped virus. enveloped virus more susceptible to certain agents such alcohol which dissolve the lipids present in envelope.
Envelope:
 Composed of specific subunits
 Originate from cell membrane of the infected cell
 Lipid bilayer
 You can recognize a projected elements (like pins)which allow virus to attach to the receptors of infected tissue, these projected elements composed of glycoprotein .without these glycoprotein structures or antigens ,envelop viruses can't attach & penetrate cell membrane of infected tissue.

So until now attachment achieve by:1)capsomers2)envelope. So we classify viruses in general to enveloped, non-enveloped viruses.

Virus structure component:
Viral symmetry is important to understand:
 Different types of viruses.
 Importance of these viruses in related to disease.
 Process of infection.
 Why one virus affect mucosa of respiratory tract, other affect mucosa of urinary tract, another affect cd4? of blood.
 In field of medical industry, symmetry explain How to prevent access of viruses& disinfect our environment, by production of vaccine,anti-viral drugs that affect specific part of virus"similar to bacteria in which have drugs that affevt cell membrane or nucleic acid…etc.
We have 2 majors groups of symmetry:
 Cubic/lcosahedral
 Helical.

Refer to slide addressed by "main structures of viruses cubic/lcosahedral&helical symmetry".
-you can see in cubic we have at least 20 lateral equal triangles ,mean if we multiply by 3 we will have 60 faces,this related to specific type of virus .nucleic acid & polypeptides found inside these cubic.
-Helical symmetry: usually spherical virus, surrounded by capside or envelope according to type of envelope.
-As we said before each type have exactly the same capsomeres(surface of these type of viruses composed of same polypeptide &glycoprotein structures).
-Doctor commented in slide addressed by "icosahedral/cubic-helical symmetry viruses": same units surrounding nucleic acid, projected from whole tube" from internal part of tube".

Viroids&prions:
Smaller particles than viruses,not classified as viruses or microorganism .
Viroids:
 single nucleic acid(DNA or RNA).
 No capsomeres ,no capside, no envelope.
 Found mainly in relation to RNA.
 Recognized only in association with host cell, not outside the body.
 Might be originated from virus, in this case the virus, due to a certain reason, has lost the mechanism to produce necessary capside or envelope while the important structure (the genome) remained. This type of viroids may found as part of infected tissue, without clinical features of disease.
 In relation to human genetics, they recognize that about 5% of human genome carry certain viroids ,they thought that, in the past some of these viroids particles incorporated with our gene, as a result, our gene isn't 100%originated from human nucleic acid,and this incorporation may contribute in developing of malignancy in certain % of population. later on these viroids may be activated, produce new forms of tissue which can't be controlled by our genetic mechanism. That was one of the philosophical theory, has been improved in certain studies with evidences.
Prions:
• More important than viroids.
• Infectious protein particles.
• Any protein in our body is controlled by genetic mechanism "presence of nucleic acid"as you know(DNA RNA protein)
But this prions produce from unknown type of virus(unknown type of infectious particle)so origin still anonymous.
• Transmitted from human to human or animal to human or animal to animal cause certain type of disease.
• Manage to reside in brain tissues, producing damage, left spaces "wholes", result on "spongiform Encephalopathies" disease.
• According to the two scientist who discover that disease, it's called "Creulzfeldt-Jakob disease" ,characterized by sever brain damage ,may cause death of infected person over long period of time "duration between infection &end result is 10 years".
• In England in 18th and beginning of 19th(1986-1992).cows suffering from disease are unstable and aggressive,tend to stop eating ,drinking, thus dyeing within a couple of weeks of infection. Mad Cow Disease probably originated from scarpie ,a neurodegenerative disease also caused by prions which affects sheep and goats. Infected sheep and goats were killed (as they can't be treated)and used as feed for cows. The disease was thus transferred to cows. Subsequently, humans who ate the infected cattle also developed a form of spongiform encephalitis.
Virus infection of Host-1
Process of infection:
-In order the virus to be infectious ( cause symptomatic "such increase in temperature ,skin etching, respiratory tract infection"& asymptomatic infection)must undergo certain procedures:
1)firstly, certain specify between virus and host tissue, means that each virus affect specific site of body. Some types of virus may spread to blood stream -such intestinal virus that cause diarrhea –reach meninges cause meningitis such as (Poliomyelitis)virusفيروس شلل الأطفال (an enteroviruse virus). (Poliomyelitis)virus must begin as infection in respiratory tract or intestinal tract .Even start in respiratory tract it must reach intestinal tract in order to become mature .In absence of immunity(immunological response),maturation of virus in intestinal tract will completed, manage to reach blood stream cause meningitis.
-in relation to oral cavity: viruses that cause infection only to oral cavity mucosa, once it is swallowed ,it can't attach to intestinal mucosa& can't cause intestinal infection.

Productive infection
To produce productive infection ,viruses undergo attachment, adsorption ,penetration, multiplication within infected tissue.

These process not always accomplish! This explains why few viruses manage to cause infection, although we are exposing to large numbers of viruses.
Immunological response& body experience prevent virus at first stage(attachment &adsorption) from entering cytoplasm& nucleus of infected tissue)as long as the virus doesn't reach the cytoplasm ,there will be no replication).

if the virus satisfy to multiply in host tissue ,it usually will end up with 100,000 of new viruses, with different incubation period.

To be able to recognize disease, signs, symptoms of viral infection we must have at least hundred million or more of virus particles.

Most of viruses have short incubation period for multiplication "few hours" such as Influenza virus, other such Hepatitis virus, Herpes virus HSV (herpes simplex virus) need at least 2 days (higher incubation period) to produce sufficient amount of particles for diagnosis.
Abortive infection
A case, in which the virus manage to enter infected tissue,but our body" by means of antibodies" succeed to stop it at replication site, or at earlier stages "prevent adsorption to cytoplasm).in such cases the virus can't be infectious(can't accomplish the productive cycle).

So we have two types of infection: productive &abortive. In both, end result ,is the presence of clinical disease. With few or now of signs &symptoms associated with abortive infection.

Other types of viruses may manage to enter host cell but can't produce the necessary enzymes for maturation.

Example for abortive &productive virus:
2 persons has been exposed to same virus, one developed asymptomatic infection(abortive) while the other developed clinical features of infection(productive)!!Why?

Actually many factors contribute to these differences, but the most important is the human response (presence of humoral antibodies)which control most types of viruses such polio virus(the causative agent of poliomyelitis), other Enteroviruses,so by this control we will have Abortive infection.


Virus infection of Host-2
Latent infection:Viruses that succeed to enter host tissue and reach even the nucleus !but stop there,part of the viral nucleic acid integrated with genome of infected tissue and remain inert in latent stage which ranges between days to months to years.
Person with latent infection ,already infected with specific type of virus such HSV,HIV(Human immunodeficiency virus) ,without presence of features of disease, but later on ,under certain condition, virus will be stimulated by certain factors such stress or other underlying disease ,latent infection become active ,virus start to replicate in infected tissue.

Human Virus Pathogenesis:
Only 10% of all viruses infection(by all types of viruses)end with over disease or what's called clinical disease .The majority 90%associated with subclinical, asymptomatic infection,so clinical disease not always necessary to be associated with presence of virus "exact sentence by the doctor).

You might discover that you have been infected with Herpes 1 or 2 without any presence of signs in oral cavity or lips and this normal! But presence of these viral particles associated with decrease in the numbers of antibodies result in activation of the virus.

It has been improved that contact with many viruses result in production of active immunoglobulin, mean that presence of antibodies in subclinical infection,prevent replication of viruses in clinical stages.

Site of entery:
As we said before that each virus interested in certain site of our body,and these specify between virus and tissue depend on many factors, the most important is the specific receptors on envelope or capside and on tissue.These receptors explain why each virus interest in certain site of body.
Also it was mentioned before,that once virus excrete from our body(respiratory tract,urine,feces…)it can infect other. Incubation period outside the body relatively short due to absence of host organic material.Ex:
-if feces dry, virus will be killed.
-fresh saliva can be associated with virus for only 10 minutes.
-incubation period outside body depend on type of virus(enveloped or not).
Virus stability
-Most viruses are stable at low temperature approximately (0-4)C°.
-Most viruses survive at PH(5-9), consider a wide range of PH.
-temperature& PH have a strong relation with disease, spread of virus in nature.
-most viruses can be inactivated at temperature range between (50-60)C°.in contrast to bacteria "remember spoil forming bacteria that resist 100C° up to 2 hours),so viruses especially those affected humans are highly susceptible to temperature above 60C° and can be killed easily.

-only few viruses can survive under high temperature such Hepatitis virus.

Do the viruses inactivated by alcohol, formaldehyde , or ultraviolet??
Dr. actually didn't answer but he said that ,many dentist use ultraviolet autoclave to sterilize their instruments instead of autoclave ,which is not enough!!!

Important note: ultraviolet autoclave is not enough to kill all types of viruses, may kill 99% but not all types, for example, ultraviolet autoclave will not kill hepatitis B,C/HIV. So it's important to be sure that all your instruments and devices are 100% sterilized(free from microorganism)by choose the true autoclave not ultraviolet autoclave.


Done by
Haneen Thnebat