Medical microbiology

• Viruses are nonliving particles composed of nucleic acid (DNA or RNA) surrounded by protein coat called capsid, which can be enclosed within an envelope derived from the host cell's plasma membrane.

  
• Bacteriophage: a virus which parasitizes a bacterium by infecting it and reproducing inside it.

  
• Lytic cycle: A type of virus life cycle in which the virus replicates inside the host cell and then bursts out of the cell
• Lysogenic cycle: Virus DNA incorporates itself into bacteria's DNA, making a provirus which is latent. When bacteria replicates, the pro virus is also replicated.
• Retrovirus: any of a family of RNA viruses that have an enzyme (reverse transcriptase) capable of making a complementary DNA copy of the viral RNA, which then is integrated into a host cell’s DNA. The family includes a number of significant pathogens, typically causing tumours or affecting the function of the immune system, e.g. HIV.

  
• • HIV can enters the blood stream via the genitals or open wound.
  • Clinical latency: virus replicates at low levels so patients don't show symptoms.
  • Lymph node: once infected macrophages and T helper cells enter the lymph node, virus replicates rapidly.
  • CD4 cell cytosis: virus destroys T helper cells leading to low level of CD4 cells which leads to AIDS.
  • Infected macrophages transport the virus to the brain, lungs and skin.
• HIV inhibitors:

  1. Entry inhibitors: T20 blocks the virus from entering host cell.
  2. CCR5 & CXCR4 inhibitors: prevent virus from attaching to cell by inhibiting the co-receptors CCR5 and CXCR4.
  3. Reverse transcriptase inhibitor: prevents the enzyme reverse transcriptase from turning RNA to DNA.
  4. Protease inhibitor: prevent long viral peptide chain from being cut into viral proteins.

  
• Benign disease is a disease that does not spread to other parts of the body.
• Chlamydia trachomatis: Bacterial STD that cannot survive outside of a host cell. Can cause blindness and pneumonia. Asymptomatic and can lead to infertility in women and Reiter's syndrome in men. Can be transferred from mother to fetus during pregnancy.
• Measles virus: Inhaled via respiratory tract and spreads to lymph nodes and widely infects tissue cells, leading to the virus being in the eyes and CNS. Causes rash then lifelong immunity is gained. If virus spreads to CNS then can cause encephalitis which is fatal.
• Pseudomonas (opportunistic bacteria): gram negative bacteria so antibiotic resistant, rod shaped.

  • E.g., pseudomonas aeruginosa, causes hospital acquired infections.
• Borrelia burgdorferi: Spread by ticks and has 3 stages.

  1. Expanding of rash at site of bite
  2. Spreading of disease causing headache and fever
  3. Persistent infection causing arthritis and affecting nervous system which can be treated with antibiotics.

  
• Pneumonia: lung inflammation caused by bacterial or viral infection, in which the air sacs fill with pus and may become solid. Usually caused by streptococcus pneumonia.

  • In children: mainly viral e.g., parainfluenza. Or interstitial pneumonitis caused by chlamydia from mother to child.
  • In adults: mainly bacterial e.g., streptococcus pneumonia.

  
• Endospore forming bacteria: Bacillus and Clostridium. Usually gram positive and rod shaped.

  • Bacillus is aerobe e.g., bacillus anthracis which causes anthrax and bacillus cereus which causes food poisoning.
  • Clostridium is anaerobe e.g., clostridium tetani which causes tetanus, clostridium perfringens which causes food poisoning and clostridium difficile which causes severe form of colitis.
• Bacillus anthrax: can enter the body through the intestines, lungs or skin. It's non-contagious.

  • Pulmonary anthrax initially starts with flu-like symptoms followed by severe respiratory problems and if not treated can be fatal.
  • GI tract anthrax presents vomiting of blood and severe diarrhoea.
  • Skin anthrax presents with a large, painless ulcer (black dot) at site of infection.
• Clostridium tetani: first signs of tetanus is lockjaw and stiffness of back. Stiffness and spasms of muscles can expand throughout the body.

  • Spasm occurs due to tetanospasmin blocking the inhibiting transmitter which causes a continuous stimulation of excitatory transmitter.
• Vibrio cholerae: Ingested via contaminated water causing cholera which leads to death via dehydration due to diarrhoea and vomiting.

  • Vibrio cholerae releases cholera toxins in the GI tract that bind to GM1 receptors. A&B subunit of cholera toxin enters the cell and binds to G alpha S which activates cyclic AMP. Cyclic AMP activates CFTR (cystic fibroses transmembrane regulator) that leads to an efflux of chloride out the cell into the lumen of GI tract causing watery diarrhoea.
• Pyogenic cocci: spherical bacteria that causes pus-producing infections. E.g., staphylococcus aureus and MRSA.

  • Staphylococcus aureus lives on skin and nasal membranes. Different strains differ in severity of disease they can cause. e.g., the exfoliate toxin causes staphylococcus scaled skin syndrome (SSSS).
  • Methicillin resistant staphylococcus aureus: acquired in hospitals and treated by IV vancomycin.

  
• Enteric bacteria: Gram negative bacteria, rod shaped, anaerobic and live in intestinal tracts of animals. E.g., Escherichia coli and Shigella dysenteriae.

  • Escherichia coli is an important bacteria in the gut and each person contains around 1kg.
  • Shigella dysenteriae is the cause of bacillary dysentery.
• Salmonella: A gram negative bacterial disease that causes fever, diarrhoea, and abdominal cramps.

  • Salmonella enteriditis is ingested and absorbed to epithelial cells in terminal portion of small intestine. Bacteria penetrates cells and migrate to lamina propria layer. They multiply in lymphoid follicles causing reticuloendothelial hyperplasia and hypertrophy (increase bacteria cells and size). Polymorphonuclear leukocytes confine infection to GI tract. Inflammatory response also mediates release of prostaglandins, stimulating cyclic AMP and active fluid secretion leading to diarrhoea.
• Bacterial diarrhoeal disease:

  
• Top 10 STDs.