"SLE334 Medical Microbiology and Immunology, T2, 2017Topic 1: The Enemy – MicrobesViruses, Fungi & Parasites(Mims’ Chapters 3 - 6)Lecture 4Thursday 20 July 2017LT13 (HC2.005), 1-2 pm(date, time and venue different for Geelong students)Dr Sharon La [email protected] Medical Microbiology and Immunology, T2, 2017This lecture will cover……•? Basic biology of viruses- the basic structure of viruses- classification of viruses•? Genetics and replication of RNA viruses•? Virus infection and replication•? Classification of fungi•? Parasitic infections- protozoal diseases- helminth infections SLE334 Medical Microbiology and Immunology, T2, 2017What is a virus?•? Viruses are agents that are unable to multiply outside ahost cell. •? The basic structure of a virus is a nucleic acid genomecontained within a protein capsid that may be surroundedby a membrane derived from the host cell. SLE334 Medical Microbiology and Immunology, T2, 2017Definitions•? Virus particle or virion–? Infectious agent composed of nucleic acid (RNA or DNA), a proteinshell, and in some cases a lipid envelope•? Capsid–? Protein coat that surrounds the viral nucleic acid–? Comprised of repeating structures called capsomeresHelical virus–? Have either icosahedral, helical or complex symmetry•? Nucleocapsid–? Complete protein-nucleic acid complexIcosahedral virus•? Envelope –? Lipid membrane derived from host cell membrane and viral geneproducts SLE334 Medical Microbiology and Immunology, T2, 2017A virus differs from a cell inthree fundamental ways1.? A virus usually has only a single type of nucleicacid serving as its genetic material.This canbe single or double stranded DNA or RNA;2.? Viruses contain no enzymes of energymetabolism, thus cannot make ATP;3.? Viruses do not encode sufficient enzymaticmachinery to synthesize their componentmacromolecules, specifically, no proteinsynthesis machinery.SLE334 Medical Microbiology and Immunology, T2, 2017Viruses are classified by sharedcharacteristics•? Example of a classification scheme forDNA viruses SLE334 Medical Microbiology and Immunology, T2, 2017Major Groups of VirusesDNA viruses and well known diseases: hepatitis B, herpes simplex,chicken pox, small pox, glandular fever, encephalitisSLE334 Medical Microbiology and Immunology, T2, 2017Major Groups of Viruses…cont’dRNA viruses (13 families) and well known diseases: polio, hepatitis A &C, diarrhea,encephalitis, Dengue fever, AIDS, SARS, acute gastroenteritis, influenza, measles, mumps, RSV, Ebola SLE334 Medical Microbiology and Immunology, T2, 2017MajorGroups ofViruses… cont’dSLE334 Medical Microbiology and Immunology, T2, 2017•? A virus that has a single-stranded RNAgenome with the same orientation as itsmRNA is said to be a positive-strandRNA virus. •? A virus whose single-stranded RNAgenome is complementary to its mRNA issaid to be a negative-strand RNA virus. SLE334 Medical Microbiology and Immunology, T2, 2017Molecular Genetics of RNA virusesRNA viruses produce mRNA by several different routes: 1.? In dsRNA viruses, -ve strand is first transcribed by viral polymerase into mRNA2.? In ssRNA viruses, there are three distinct routes to the formation of mRNASLE334 Medical Microbiology and Immunology, T2, 2017Molecular Genetics of RNA viruses•? Where the single strand has thepositive (+) sense configuration (i.e.has the same base sequence as thatrequired for translation), it can beused directly as mRNA•? Viral mRNA is usually'monocistronic' (i.e. has a singlecoding region) and can displace hostmRNA from ribosomes so that viralproducts are synthesizedpreferentially. •? If viral genome is a single nucleicacid molecule, translation producesa large multifunctional protein, apolyprotein, which is then cleavedenzymatically to produce a numberof distinct proteins.SLE334 Medical Microbiology and Immunology, T2, 2017Retroviruses•? Retroviruses are RNA viruses thatreplicate through a DNAintermediate. •? The retrovirus called humanimmunodeficiency virus (HIV) causesAIDS •? The retrovirus virion contains anenzyme, reverse transcriptase, thatcopies the information from its RNAgenome into DNA, a process calledreverse transcription•? dsDNA enters the nucleus andThe three major proteins encodedwithin the retroviral genome: Gag, Pol,becomes integrated into the hostand Env.genome. •? Gag is a polyprotein and is an•? This integrated viral DNA is thenacronym for Group Antigens (ag).•? Pol is the reverse transcriptase.transcribed by host polymerase into•? Env is the envelope protein.mRNA.SLE334 Medical Microbiology and Immunology, T2, 2017Replication of RNA viruses•? -ve sense RNAtranscribedrepeatedly into more•? Viruses must alsopositive strandsreplicate their nucleicacid to providegenetic material forpackaging into thecapsids.SLE334 Medical Microbiology and Immunology, T2, 2017Viruses come in all shapes and sizesSLE334 Medical Microbiology and Immunology, T2, 2017Virus infection and replication•? Viruses show host specificityand usually infect only one or arestricted host range.•? Basis of specificity is ability ofvirus particle to attach to host cellvia host receptor. SLE334 Medical Microbiology and Immunology, T2, 2017Viral infection:Host specificity •? Most viruses can infect only a limited range of cell types. •? This selectivity exists largely because viral attachment tothe host cell membrane is dependent on the interactionof viral protein with specific host cell receptors.•? Eg. Influenza virus attaches by its hemagglutinin to aglycoprotein (sialic acid) found on cells of mucousmembranes and on red blood cellsSLE334 Medical Microbiology and Immunology, T2, 2017Viral infectionsof animal cells•? Not all infections of animal hostcells result in cell lysis or death;latent or persistent infections arecommon, and some animalviruses can cause cancer•? 'lytic' infections - many newvirus particles released by celllysis•? 'persistent' infections – cellremains alive and continues torelease virus particles at a slowrate•? latent infections - virus remainsquiescent, andviral geneticmaterial may: -exist in the host cell cytoplasm - be incorporated into the genome SLE334 Medical Microbiology and Immunology, T2, 2017Under attack!Human viruses•? Most virusesinfect by aparticularroute andinfectparticular celltypes only.SLE334 Medical Microbiology and Immunology, T2, 2017The common cold•? Mainly involves infection of the nasal passages•? Blocked or runny nose, with excessive nasal discharge andsneezing, and a sore throat•? The illness is self limiting with no effective treatment•? Caused by a variety of virusesRespiratory AdenovirusRhinoviruses CoronavirusesParainfluenzasyncitial virus(100 serotypes)virus•? Due to the large numbers of viruses responsible there islittle hope for a vaccine"