Medical Microbiology Techniques III (ML 302) Unit 2 PDF

MEDICAL MICROBIOLOGY TECHNIQUES III (ML 302) UNIT 2: Bacteriophages & review: features and classification of virus of medical importance  INTRODUCTION TO VIRUSES A virus is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host The word...

MEDICAL MICROBIOLOGY TECHNIQUES III (ML 302) UNIT 2: Bacteriophages & review: features and classification of virus of medical importance  INTRODUCTION TO VIRUSES A virus is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host The word virus is derived from Latin word venom which means poisonous fluid that causes infection They show living characteristics inside the host and non living characters outside the host They contain either DNA or RNA as genetic material They have different size and shape, and can cause disease in plants, animals and microorganisms  Not cellular Cannot carry on metabolic activities independently Contain either DNA or RNA, not both Lack ribosomes and enzymes necessary for CHARACTERISTICS OF protein synthesis VIRUSES Reproduce only within cells they infect STRUCTURE OF VIRUSES  GENERAL STRUCTURE OF VIRUSES Basic components of a virus Nucleic acid (genome) Capsid (protein coat) Envelope (in some viruses)  Viruses vary significantly in size, and their dimensions can be measured in nanometres (nm) Typically, the size of viruses ranges from about 20 nm to 300nm Small viruses are approximately 20 – 100 nm, e.g., poliovirus (22-30nm), adenovirus (70 – 90nm) Medium-sized viruses are approximately 100 – 200 nm. E.g. HIV is about 120 nm, Herpes simplex virus about 120 – 200 nm Large viruses are approximately 200 – 300 nm e.g. poxviruses about 200 – 300nm  The viral nucleic acid refers to the genetic material of a virus, which can either be DNA or RNA It is essential for the virus’s ability to replicate VIRAL and produce new viral particles once it infects NUCLEIC a host cell ACID Unlike cellular organisms, viruses can have a wide variety of nucleic acid types, making them unique and diverse in their replication strategies and life cycles  FUNCTION OF VIRAL NUCLEIC ACID The viral nucleic acid contains the genetic instructions necessary for building new viral particles. These instructions are encoded in the form of genes that specify the structure of viral proteins (capsid proteins, enzymes, glycoproteins) and regulatory elements needed for replication The viral genome also directs the synthesis of new viral nucleic acids and proteins inside the host cell. DNA viruses often use the host’s replication machinery, while RNA viruses typically bring their own enzymes such as RNA-dependent RNA polymerase to replicate their genome Once inside a host cell, the viral nucleic acid is used to produce viral mRNA, which is then translated by the host’s ribosomes into viral proteins. The strategy for this varies depending on whether the virus is DNA or RNA based Integration of the viral genome into the host’s genome, for example HIV virus  TYPES OF VIRAL NUCLEIC ACID Viruses can be classified based on the type of nucleic acid they contain, which determines their replication process and host interactions. There are two main types of nucleic acids in viruses: Deoxyribonucleic acid (DNA), which is further classified as follows: Single-stranded DNA (ssDNA).Viruses that contain this DNA have to be converted to dsDNA before replication Double-stranded DNA (dsDNA) is the genome found in most DNA viruses The replication of most viruses with DNA replicate their genome within the host cell nucleus using the host’s DNA-dependent DNA polymerase for DNA synthesis and RNA polymerase for transcription of viral genes into mRNA Some larger DNA viruses, like poxviruses, replicate in the cytoplasm using their viral enzymes  RNA viruses replicate primarily in the cytoplasm of the host, often using their own viral RNA-dependent RNA polymerase because host cells typically do not possess enzymes that can replicate RNA from an RNA template  GENOME TYPE EXAMPLES KEY FEATURES REPLICATION SITE dsDNA Herpesviruses, Stable genomes, rely on host Nucleus (most) COMPARI Adenoviruses DNA polymerase, can establish latent infections SON OF ssDNA Parvoviruses Requires conversion to dsDNA Nucleus THE before replication SEVEN dsRNA Rotavirus Requires conversion to dsDNA Cytoplasm VIRAL before replication GENOME +ssRNA Poliovirus, RNA can act directly as mRNA Cytoplasm SARS-CoV 2 TYPES -ssRNA Influenza, rabies Must package RNA polymerase Cytoplasm and nucleus virus for transcription of RNA ssRNA with DNA HIV RNA reverse transcribed into Cytoplasm and nucleus intermediate DNA, integrates into host genome Reverse transcribing Hepatitis B virus DNA replicates via RNA Cytoplasm and nucleus DNA (dsDNA and RNA intermediate  The capsid is a crucial structural component of the virus, made up of protein subunits called capsomeres. It performs several vital functions, including protection of the viral genome, assisting in CAPSID viral attachment to host cells, and (COAT determining the virus’s overall shape and PROTEIN) classification  CAPSID STRUCTURE AND COMPOSITION The capsid is constructed from numerous identical protein subunits called capsomeres. Capsomeres may be arranged symmetrically to form distinct shapes like helical, icosahedral or more complex  With the helical-shaped capsid, the capsomeres are arranged in a spiral or helical pattern around the viral genome, which is often RNA. The capsid forms a rod-like structure that can be flexible or rigid. Examples of viruses with this shape include tobacco mosaic virus (TMV) and Influenza virus  Icosahedral-shaped capsids have capsomeres that form a polyhedral shape of about 20 triangular faces, creating a highly symmetrical structure The shape allows for efficient packaging of the viral genome in a compact form It includes adenoviruses and polioviruses  Some viruses, especially bacteriophages have a more intricate structure combining both helical and icosahedral elements, often with additional components like tails and fibers Examples: T4 bacteriophages  The capsid shields the viral nucleic acid from environmental damages such as enzymes, UV light and chemical agents. This helps the virus survive outside a host Capsids often have specific binding sites that interact with receptors on the surface of host cells, enabling the virus to attach and VIRAL CAPSID penetrate host cells FUNCTIONS The capsid encloses and packages the viral genome tightly, making sure that there is efficiency in transporting and delivering the virus into the host cells during infection  The viral envelope is a lipid bilayer derived from the host cell’s membrane during the viral budding process It surrounds the virus’s protein capsid and incorporates viral glycoproteins VIRAL The envelope is present in many viruses that ENVELOPE infect animals, for example influenza virus  COMPOSITION OF THE ENVELOPE The envelope is made up of lipids, proteins, and carbohydrates, similar to the host cell membrane from which it originates Embedded within the lipid bilayer are viral glycoproteins (spikes), which are essential for binding to host cell receptors Glycoproteins have both carbohydrate and protein components, and they play a critical role in host recognition, immune evasion and membrane fusion during viral entry. Examples include GP120 in HIV  ROLE OF ENVELOPES IN INFECTION The envelope plays a role in the following: Attachment: the viral glycoproteins embedded in the envelope bind to specific receptors on the surface of the host, initiating infection. This specificity determines which cells and tissues the virus can infect Membrane fusion: after binding to the host the host cell, the envelope fuses with the host’s cell membrane, allowing the viral capsid and genome to enter the host cell. Glycoproteins mediate the fusion process; for example, HIV GP41 glycoprotein fuses with the host’s T-cell membrane The viral envelope helps viruses evade host immune system detection. Since the envelope is derived from the host cell membrane, it can partially “disguise” the virus, making it less recognizable to immune cells CLASSIFICATION OF VIRUSES  Initially, when viruses were discovered, they were not classified. Because of that, viruses were named haphazardly. Based on: The disease they cause e.g poliovirus, rabies virus The type of disease e.g murine leukemia virus Geographic locations e.g Sendai virus, Coxsackie virus Their discovers e.g Epstein-Barr virus How they were originally thought to be contracted e.g dengue virus (“evil spirit”), influenza virus (the “influence” of bad air) Combinations of the above e.g Rous Sarcoma virus  CLASSIFICATION OF VIRUSES Classification of viruses can be based on Presence or absence of a viral envelope Nucleic acid present Example, adenoviruses and herpiviruses have double-stranded DNA Morphology Helical viruses Icosahedral viruses Complex viruses  The classification of viruses based on their envelope is an important aspect of virology that impacts how viruses interact with host cells, how they are transmitted and how they can be targeted by vaccines CLASSIFICATIO and antiviral treatments N OF VIRUSES Viruses can be categorised into two main BASED ON THEIR groups based on the presence or absence ENVELOPE of an envelope:  Enveloped viruses have a lipid bilayer membrane (the viral envelope) surrounding their capsid The envelope is derived from the host cell membrane during the viral budding process Enveloped viruses: Are sensitive to environmental conditions such as heat, detergents and desiccation due ENVELOPED to the fragility of the lipid bilayer VIRUSES Often require close contact for transmission e.g. via respiratory droplets or bodily fluids On the envelope, there are viral proteins (glycoproteins) that facilitate binding and entry into host cells through mechanisms such as fusion with the host membrane Example: HIV, SARS-CoV-2, Herpes simplex Virus  Non-enveloped viruses lack a lipid envelope and consist solely of a protein coat (capsid) that encases their nucleic acid These viruses are: Generally, more resistant to environmental conditions such as heat and detergents and can survive longer outside a host NON-ENVEL OPED VIRUS Able to be transmitted via fomites (contaminated surfaces), water or food, often spreading through the faecal-oral route Usually entering the host cells through endocytosis and rely on capsid proteins for attachment and penetration Examples include adenovirus and poliovirus  The Baltimore classification system categorises viruses on their type of nucleic acid and their method of replication. CLASSIFICATIO This system has seven classes of viruses, N BASED ON each defined by specific characteristics of THE their genetic material and replication BALTIMORE strategies CLASSIFICATIO N SYSTEM  GENOME TYPE EXAMPLES KEY FEATURES REPLICATION SITE dsDNA Herpesviruses, Stable genomes, rely on host Nucleus (most) COMPARI Adenoviruses DNA polymerase, can establish latent infections SON OF ssDNA Parvoviruses Requires conversion to dsDNA Nucleus THE before replication SEVEN dsRNA Rotavirus Requires conversion to dsDNA Cytoplasm VIRAL before replication GENOME +ssRNA Poliovirus, RNA can act directly as mRNA Cytoplasm SARS-CoV 2 TYPES -ssRNA Influenza, rabies Must package RNA polymerase Cytoplasm and nucleus virus for transcription of RNA ssRNA with DNA HIV RNA reverse transcribed into Cytoplasm and nucleus intermediate DNA, integrates into host genome Reverse transcribing Hepatitis B virus DNA replicates via RNA Cytoplasm and nucleus DNA (dsDNA and RNA intermediate  Based on genetic contents and replication strategies of viruses in the synthesis of mRNA. Viruses are divided into seven classes: 1. dsDNA viruses 2. ssDNA viruses 3. dsRNA viruses CLASSIFICATION 4. (+) sense ssRNA viruses (codes directly for BASED ON THE protein) BALTIMORE CLASSIFICATION 5. (-) sense ssRNA viruses SYSTEM 6. RNA reverse transcribing viruses 7. DNA reverse transcribing viruses where "ds" represents "double strand“ "ss" denotes "single strand". EVENTS INVOLVED IN VIRAL REPLICATION  The viral replication process is a complex series of steps that enables viruses to reproduce within host cells Unlike other microorganisms like bacteria, viruses cannot replicate on their own They require a host cell’s machinery to produce new viral particles (virions) The steps involve: Recognition Attachment (adsorption) Entry (penetration) Uncoating Replication and transcription Assembly Release (budding or lysis)  Adsorption ∙ The first step in infection of a cell is attachment to the cell surface. ∙ The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of ATTACHMENT / ADSORPTION the cell. ∙ Cells without the appropriate receptors are not susceptible to the virus.  ∙ Penetration occurs almost rapidly after attachment and is a next step for gaining entry into the cytoplasm by crossing the plasma membrane. ∙ Thus, penetration allows the viruses to deliver their genome into the host cells to initiate replication. PENETRATION  Uncoating: Release of the viral genome from its protective capsid to enable the viral nucleic acid to replicate. Transcription: Synthesis of m-RNA Translation: The viral m-RNA is translated on cell ribosomes into structural and non-structural proteins.  6-Replication of the viral genome 7-Assembly: New virus genomes and proteins are assembled to form new virus particles. 8-Release: Enveloped viruses are released by budding Unenveloped viruses are released by rupture and lysis of the infected cells. BACTERIOPHAGES  Bacteriophages are viruses that can infect and destroy bacteria They have been referred to as bacterial parasites, with each phage type depending on a single strain of bacteria to act as host They infect bacteria by injecting genetic BACTERIOPHA material, which they carry enclosed in an GES outer protein capsid The genetic material can be RNA or DNA Infection may or may not lead to the death of the bacterium  Typically, they carry only the genetic information needed for replication of their nucleic acid and synthesis of their protein coats When phages infect their host cell, they order of business is to replicate their nucleic acid and to produce their protective protein coat They do this with precursors, energy generation and ribosomes supplied by their bacterial host cell  Bacteriophages can be classified based on two major criteria Phage morphology Nucleic acid properties Now, over 5000 bacteriophages have been studied by electron microscopy and can be divided into 13 virus families  COMPOSITION AND STRUCTURE Typically, bacteriophages have the following structure Head (capsid), which is often icosahedral in shape and serves as a protective shell for the viral nucleic acid. It is made up of protein subunits called capsomeres and functions to protect the genome from degradation and assists in the attachment to host bacteria Bacteriophages contain either dsDNA or ssDNA as their genetic material, with most of them being dsDNA viruses, which is necessary for replication and assembly of new virions within the host The tail structure that they contain is often long and tubular, connecting the head to the base plate Tail sheathe is an outer sheath that contracts during the injection process The tail fibers protrude at the end of the tail. They help the phage attach to specific receptors on the bacterial surface Base plate structure is at the end of the tail and it helps in the attachment and injection of the  COMPOSITION AND STRUCTURE Composition Nucleic acid Proteins  LIFE CYCLE OF BACTERIOPHAGE Two cycles Lytic cycle Lysogenic cycle  Attachment: proteins in the “tail” of the phage bind to a specific receptor on the surface of the bacterial cell Entry: the phage injects its double-stranded DNA genome into the cytoplasm of the bacterium THE STAGES DNA copying and protein synthesis: phage DNA is copied, and phage genes are OF THE expressed to make proteins e.g. capsid LYTIC CYCLE proteins Assembly of new phage: capsids assemble from the capsid proteins and are stuffed with DNA to make lots of new phage particles  Lysis: late in the lytic cycle, the phage expresses genes for proteins that poke holes in the plasma membrane and cell wall Holes let water flow in, making the cell expand and burst (lysis) Cell lysis releases hundreds of new phages, which can find and infect other host cells  It allows a phage to reproduce without killing its host. Some phages can only use lytic cycle In lysogenic cycle, attachement and DNA injection are occur the same way as the lytic cycle LYSOGENIC Once the phage DNA is inside the cell, it CYCLE is not immediately copied ore expressed to make proteins Instead it recombines with the bacterial chromosome and its DNA integrates with the chromosome  This integrated phage DNA (prophage) is not active. Its genes are not expressed, and it does not drive production of new phages However, each time a host cell divides, the prophage is copies along with the host DNA Prophage can become active, coming out of the bacterial chromosome, thereby triggering the remaining steps of the lytic cycle (DNA copying and protein synthesis, phage assembly and lysis)  TYPES OF BACTERIOPHAGES T-even phages such as T2, T4 and T6 that infect E. coli Temperate phages e.g. lambda and mu Spherical phages e.g. PhiX174 Filamentous phages e.g. M13 RNA phages  T-EVEN PHAGES T-phages are specific class of bacteriophages, include T4 and T7 that infect E.coli They have an icosahedral head, double stranded DNA, tail of the bacteriophage includes the tail sheath, base plate and tail fibers Replication Attachement of bacteriophage to bacterium Insertion of genetic material Replication Bacterium is lysed New viruses released, and cycle repeated  TEMPERATE PHAGES Lambda phages have icosahedral head about 50 – 60 nm in diameter A flexible, long noncontractile tail about 150nm in length Capsid head Single, double stranded DNA molecule about 48kb in length  FILAMENTOUS PHAGES M13 is filamentous bacteriophage composed of circular single stranded DNA PIII(P3) attaches to the receptor at the tip of the F pilus of the host Does not kill the host but is released by budding Able to infect E.coli  SPHERICAL PHAGES Spherical phages Bacteriophage phiX174 is an icosahedral phage Attaches to the host cells without the aid of a complex tail assembly Virions consists of a protein coat which envelopes a core that contains DNA and proteins RNA phages Phages with RNA genomic material E.g. Q beta phage – one of the smallest known viruses (24 nm in diameter) with an icosahedral ccapsid It infects E.coli  Temperate phage can go through one of two life cycles upon entering a host cell Lytic – growth results in lysis of the host and release of progeny phage Lysogenic – growth results in integration of the phage genome into the host chromosome or VIRULENCE stable replication as a plasmid FACTORS Most of the gene products of the lysogenic CARRIED phage remains dormant until it is induced to ON PHAGE enter the lytic cycle  They only infect bacteria, and not humans However, they are able to alter the genome of a non-virulent bacteria strain, making virulent strains For example, Cholera – most strains are harmless DISEASES CAUSED BY BACTERIOPHAGES Scarlet fever – commonly affects children  Signs and symptoms of bacteriophages Sore throat, fever, and a red rash Usually spread by inhalation Most of the clinical features are caused by erythrogenic toxin – substance produced by the bacterium Streptococcus when it is infected by bacteriophage T 12 MEDICAL IMPORTANT VIRUSES classification DNA VIRUSES  The classification of viruses is based on chemical and morphological criteria The two major components of the virus used in classification are The nucleic acid (its molecular weight and structure) The capsid (its size and symmetry and whether it is enveloped)  DNA viruses Three naked (nonenveloped) icosahedral virus families The parvoviruses Papovaviruses Adenoviruses The three enveloped families The hepadnavirus family (icosahedral) The herpesviruses (icosahedral) Poxviruses Largest and have a complex internal symmetry  These are very small (2nm in diameter), naked icosahedral viruses Single stranded linear DNA There are two types of parvoviruses: defective and nondefective 1- The defective parvoviruses, e.g., adeno-associated virus, require a helper virus for replication PARVOVIRIDAE The DNA of defective parvoviruses is unusual because plus-strand DNA, and minus-strand DNA are carried in separate particles 2-The nondefective parvoviruses are best illustrated by B19 virus is associated with aplastic crises in sickle cell anaemia patients and with erythema infectiosum, an innocuous childhood disease characterized by a “slapped-cheeks” rash  Poxviruses are the largest and most complex of viruses that infect vertebrates They are large enough to be seen under the light microscope Smallpox, the great success story in the fight against infectious disease Poxviridate contains two subfamilies POXVIRIDAE The chordopoxvirinae – the poxviruses of vertebrates The entomopoxvirinae – the poxviruses of insects Chordopoxvirinae are placed in eight genera Viruses are distinguished on the basis of morphology, genome st  Large, enveloped viruses (100 nm in diameter) with an icosahedral nucleocapsid They have a double-stranded linear DNA The five important human pathogens are herpes simplex virus type 1 and 2, HERPESVIRIDAE varicella-zoster virus, cytomegalovirus, and Epstein-barr virus  Small, nonenveloped, circular, double stranded DNA viruses Cause infection in humans, dogs, cattle, monkeys and may other species Included in the family are Human papillomaviruses (HPV) – cause warts PAPILLOMAVIRIDAE There are over 200 genotypes of this virus, classified based on their DNA sequence, much attention given to the 30 that are transmitted sexually  Each type has different clinical presentation: HPV-1 associated with plantar warts HPV -2 and HPV-4 associated with common warts of the hands HPV-6 and HPV-11 associated with genital warts HPV-16 and HPV -18 associated with cervical cancer  Adenoviruses are DNA viruses first isolated from adenoidal tissue in 1953 This family consists of double-stranded DNA viruses with an icosahedral nucleocapsid. They have been recovered from many mammalian and avian species. Many are found in the respiratory tract and ADENOVIRIDAE infections are often persistent. Only a small number cause significant veterinary diseases. Consists of two genera (Mastadenovirus – causes disease in mammals aviadenovirus – causes disease in birds  ADENOVIRUS STRUCTURE Non-enveloped DNA virus 80-100 nm in size Single Linear ds DNA genome with core proteins  HEPADNAVIRIDAE These are double-shelled viruses (42 nm in diameter) with an icosahedral capsid covered by an envelope The DNA is a double-stranded circle that is unusual because the complete strand is not covalently closed circle and the other strand is missing approximately 25% of its length Hepatitis B is the human pathogen in this family  They are small, nonenveloped (naked), circular, DNA viruses Have been isolated from many species, including humans Included in this family are JC and BK viruses POLYOMAVIRIDAE Infection occurs during childhood Usually latent but can be symptomatic during periods of immune suppression  INFECTION OF IMMUNOCOMPROMISED INDIVIDUALS – JC AND BK VIRUSES When JC virus reactivates it causes disease in the central nervous system BK virus causes a hemorrhagic cystitis RNA VIRUSES  There are 14 families of RNA viruses: Three naked icosahedral virus families Three enveloped icosahedral The remaining eight families are enveloped helical viruses Some of them have single-stranded linear RNA as a genome, while others have single-stranded circular RNA  The picornaviruses are small (22 to 30nm) nonenveloped, single-stranded RNA viruses with cubic symmetry The virus capsid is composed of 60 protein subunits, each consisting of four polypeptides Because they contain no essential lipids, they PICORNAVIRIDAE are ether resistant They replicate in the cytoplasm Consists of five genera Enterovirus, hepatovirus, rhinovirus, apthovirus, cardiovirus (last two infect hoofed animals and rodents respectfully)  1- They are transmitted by the fecal oral route. 2- They are acid stable. 3- They replicate in the pharynx and small intestine. GENERAL 4- They cause neurological and CHARACTERISTICS OF ENTEROVIRUSES non-neurological diseases. 5- They shed in stool. 6- Do not cause diarrhea.  REOVIRUSES The name reoviridae is derived from respiratory enteric orphan viruses The term orphan virus means that it is not associated with any known disease Even though the viruses in this family have more recently been identified with various diseases, the original name is still used A family of viruses that can affect the gastrointestinal system and respiratory tract  They are naked viruses (75 nm) with two icosahedral capsid coats They have 10 segments of double stranded linear RNA The main human pathogen is rotavirus, which causes diarrhea mainly in infants  Enveloped viruses with an icosahedral capsid Single-stranded, linear, nonsegmented, positive polarity RNA Includes hepatitis C virus, yellow fever virus, dengue fever, west nile virus FLAVIVIRIDAE  The togaviridae (togaviruses) can be classified into the following major genera: rubivirus, and arterivus No known arteviruses can disease in humans Members of the togaviridae are responsible for two very different kinds of human disease All alphaviruses are transmitted by TOGAVIRIDAE arthropods and cause encephalitis, arthritis and rashes Enveloped viruses with an icosahedral capsid with single-stranded, linear, nonsegmented, positive polarity RNA  RETROVIRIDAE The retroviridae are a family of enveloped (+) sense ssRNA viruses that have been intensely studied because of their association with cancers, leukemias and AIDS syndrome They replicate through a DNA intermediate using reverse transcriptase Two medically important groups Oncovirus group – contains the sarcoma and leukemia viruses e.g. human T-cell leukemia virus (HTLV) Lentivirus group – includes human immunodeficiency virus (HIV)  ORTHOMYXOVIRIDAE Enveloped, with a helical nucleocapsid Has eight segments of linear, single-stranded, negative polarity RNA They cause highly contagious airbone respiratory illness Influenza virus is the main human pathogen  PARAMYXOVIRIDAE These are enveloped viruses with a helical nucleocapsid. With a diameter of about 100 – 800 nm *Single-stranded, linear, nonsegmented, negative-polarity RNA. The important human pathogens are measles, mumps, parainfluenza, and respiratory syncytial viruses.  These are bullet-shaped enveloped viruses with a helical nucleocapsid. Single-stranded, linear, nonsegmented, negative-polarity RNA. RHABDOVIRIDAE The term "rhabdo" refers to the bullet shape. Included under family rhabdoviridae are viruses that infect mammals, reptiles, fish, insects and plants  Rhabdoviruses infecting mammals are grouped into two genera Vesiculovirus – containing vesicular stomatitis virus and related viruses like chadipura virus (arbovirus) Lyssavirus – containing rabies virus and related viruses Rabies virus is the only important human pathogen.  FILOVIRIDAE These are long threadlike viruses, hence the name (filum means thread) These are enveloped viruses with a helical nucleocapsid. Single-stranded, linear, nonsegmented, negative-polarity RNA. They are highly pleomorphic, long filaments that are 80 nm in diameter but can be thousands of nanometers long. The two human pathogens are Ebola virus and Marburg virus They cause severe hemorrhagic fevers  These are enveloped viruses with a helical nucleocapsid -Single-stranded, linear, nonsegmented, positive-polarity RNA. - The term "corona" refers to the CORONAVIRIDAE prominent halo of spikes protruding from the envelope. -Coronaviruses cause respiratory tract infections, such as the common cold and SARS (severe acute respiratory syndrome), in humans. Two genera found under this family (coronavirus and torovirus)  -These are enveloped viruses with a helical nucleocapsid. - Single-stranded, circular, negative-polarity RNA in three segments. -Some bunyaviruses contain ambisense RNA in their genome. -The term "bunya" refers to the prototype, Bunyamwera BUNYAVIRIDAE virus, which is named for the place in Africa where it was isolated. -These viruses cause encephalitis and various fevers such as Korean hemorrhagic fever. - Hantaviruses, such as Sin Nombre virus, are members of this family.  Common name is arenavirus They are viruses that are Spherical in shape that are enveloped Contain T shaped glycoprotein spikes that surround the membrane They normally infect a variety of mammalian species, especially bats and ARENAVIRIDAE rodents Humans get infected by Coming in contact with infected rodents Aerosol inhalation of urine, saliva, faeces or nasal secretions from a rodent  In humans, signs and symptoms would typically range from Asymptomatic Fever, prostration, headache and vomiting More severe case of meningitis and haemorrhagic fever Arenaviruses that cause infections in humans include Lymphocytic choriomeningitis virus (LCMV) Lassa fever virus  ASTROVIRIDAE Includes viruses that can infect humans and animals Viral structure is Small, round virus with distinctive 5/6 pointed star-like appearance (28 to 30 nm in diameter) Icosahedral, nonenveloped RNA virus They have been isolated from a number of animals, and are associated with gastroenteritis They are transmitted via the fecal-oral route, and through contaminated food or water  They are small (30 -38 nm) , rounded, nonenveloped, RNA viruses that cause gastroenteritis in humans Have a broad host range and disease manifestation, including cats, rabbits Included in this family are CALCIVIRIDAE Noroviruses (Norwalk-like viruses) and sapoviruses cause viral gastroenteritis in humans  Clinical symptoms of noroviruses (occur after 1 – 2 days incubation period) Nausea, abdominal cramps, vomiting (mostly in children), watery diarrhoea Sapovirus infection causes the same symptoms Infants and toddlers are more prone to infection than older children  REVIEW QUESTIONS With examples, describe differences between enveloped and naked viruses Describe the basic structure of a virus How do bacteriophages replicate? Using tables, describe the DNA and RNA viruses families. Include the following details: Basic structure Pathogenesis Transmission