EXAM: Bacteriophages, Laboratory Diagnosis, OmpathStudy

Bacteriophages, Laboratory Diagnosis, HPV & Parvovirus B19 High-Yield Revision Notes Institution: Mount Kenya University — School of Medicine & Surgery Unit: MBMM 3300 — Medical Virology and Mycology Level: MBChB Year 3 --- Papers & Examinations Covered Exam Date Paper --- --- --- Main Exam January 2022 MBMM 3300 Paper 1 Main Exam February 2021 MBMM 3300 Paper 2 Main Exam July 2019 MBMM 3300 Paper 2 Main Exam January 2023 MBMM 3300 Paper 2 End of Term CAT December 2020 MBMM 3333 Medical Virology CAT II 2018/2019 MBMM 3233 Medical Virology --- Topics Covered Bacteriophages — structure, lytic vs lysogenic cycle, lysogenic conversion, phage therapy Laboratory Diagnosis of Viral Infections — cell culture, serology, PCR, microscopy, antigen detection HPV and Parvovirus B19 — structure, diseases, oncogenesis, diagnosis, treatment, prevention --- Exam Yield Guide Used in These Notes [ESSAY — REPEATED] — appeared as Section B or C essay question more than once [ESSAY — ONCE] — appeared as essay question once [MCQ — HIGH YIELD] — appears frequently in Section A MCQs [MCQ — LOW YIELD] — appears rarely or not at all in past papers --- --- Part 1: Bacteriophages 1. Introduction [MCQ — HIGH YIELD] Bacteriophages — commonly called phages — are viruses that infect bacteria. They are the most abundant biological entities on Earth, outnumbering bacteria by a factor of 10 to 1. In medicine, they are important for several reasons — they carry toxin genes that make bacteria pathogenic through lysogenic conversion, they are used as tools in molecular biology and research, and they are increasingly being explored as therapeutic agents against antibiotic-resistant bacterial infections in what is called phage therapy. At MKU, bacteriophages appear in MCQs and the course outline specifically highlights their use in combating human and animal diseases. --- 2. Structure of Bacteriophages [MCQ — HIGH YIELD] Bacteriophages vary in morphology but the classic bacteriophage — the T4 phage — has a complex structure: Head (Capsid): Icosahedral protein capsid Contains the viral genome — double-stranded DNA in most phages Protects the genome Tail: Hollow protein tube attached to head Used to inject DNA into the bacterial cell Components: Tail sheath — contractile; contracts to drive tail core through bacterial wall Tail core — hollow tube through which DNA is injected Base plate — platform at base of tail Tail fibres — long thin protein fibres extending from base plate; mediate attachment to bacterial surface receptors Tail pins — short spikes on base plate; help anchor to bacterial surface Other phage morphologies: Filamentous phages — long thin rods; no tail; e.g., M13 Icosahedral phages without tails — e.g., PhiX174 Enveloped phages — rare --- 3. Lytic Cycle of Bacteriophages [MCQ — HIGH YIELD] The lytic cycle results in destruction of the bacterial host cell and release of new phage particles. Steps: Step 1: Adsorption Tail fibres of phage bind specifically to receptors on bacterial cell surface Receptors may be: lipopolysaccharide (LPS), teichoic acids, pili, outer membrane proteins Highly specific — phage can only infect certain bacterial strains Step 2: Injection Tail sheath contracts Tail core penetrates bacterial cell wall and membrane Phage DNA injected into bacterial cytoplasm Capsid and tail remain outside — the empty ghost Step 3: Early Gene Expression Phage DNA transcribed by bacterial RNA polymerase → early mRNA → early proteins Early proteins: Shut down bacterial gene expression — degrade bacterial DNA Encode new enzymes for phage DNA replication Step 4: Phage DNA Replication Phage DNA polymerase replicates phage genome many times Many copies of phage genome accumulate Step 5: Late Gene Expression Late mRNA transcribed → late structural proteins Capsid proteins, tail proteins, tail fibres Step 6: Assembly Heads assembled and filled with phage DNA Tails assembled separately Heads and tails joined Step 7: Lysis Phage encodes lysin (endolysin) — enzyme that degrades bacterial peptidoglycan cell wall Also encodes holin — creates pores in bacterial inner membrane allowing lysin access to cell wall Bacterial cell wall weakened → osmotic lysis → cell bursts Releases 100–200 new phage particles per infected bacterium --- 4. Lysogenic Cycle [MCQ — HIGH YIELD] In the lysogenic cycle, phage DNA integrates into the bacterial chromosome and replicates with it without killing the cell. Steps: Step 1: Adsorption and Injection Same as lytic cycle — phage DNA injected into bacterium Step 2: Integration Phage DNA circularizes Phage integrase enzyme catalyses site-specific recombination Phage DNA integrates into bacterial chromosome at a specific attachment site (attB) Integrated phage DNA called prophage Step 3: Replication with Host Prophage replicates passively every time bacterium divides All daughter cells carry prophage — vertical transmission No phage particles produced Bacterium appears normal Step 4: Induction Certain stimuli trigger ex