6.2 Applications and skills

6.2.3 Thrombosis, penicillin and HIV

We have learned in Page 6.1.3 how the body defends itself against pathogens. Now let’s examine some applications involving disease.

Causes and consequences of coronary thrombosis

  • When a blood clot forms inside an artery or vein, it is called thrombosis. A blood clot may grow in one area or detach and move through the blood vessels as an embolus.
  • Coronary blood clots are more likely to form where the arterial wall has been damaged, or where there is a restriction in blood flow due to atherosclerosis (Page 6.2.2) because when blood flow is slowed it is more likely to coagulate.
  • Coronary thrombosis can lead to myocardial infarction (heart attack) if the oxygen supply to the cardiac muscle is blocked by the occlusion.
  • Risk factors for coronary thrombosis are: family history of atherosclerosis, diabetes, high blood pressure, high blood cholesterol, smoking.

Figure 6.2.3a - ThrombusFigure 6.2.3a – Thrombus
Coronary thrombosis develops where the artery is damaged or blocked by atherosclerosis.

Florey and Chain’s experiments with penicillin

  • 1928: Alexander Fleming accidentally discovers penicillin after his bacterial cultures become contaminated by mould of the genus Penicillum.
  • 1930s: Working at Oxford, Howard Florey and Ernst Chain develop techniques that isolate penicillin in sufficient quantities to permit an investigation into its antibiotic properties.
  • 1940: Florey and Chain inject eight mice with a lethal dose of Streptococcus bacteria. Four mice are treated with penicillin, four are not. By the following day, the untreated mice are dead. The treated mice survive.
  • 1941: Human trials begin. The first patient dies because there is not enough antibiotic to finish his treatment. Penicillin proves to be very difficult to isolate and stabilise.
  • By 1944, enough penicillin is available to treat wounded troops returning from the war. Penicillin becomes the first widely used antibiotic.

The effects of HIV on the immune system

  • A virus is composed of genetic material surrounded by diverse proteins, and a simple membrane envelope.
  • Antibiotics prevent bacterial metabolism by interfering with different enzymatic processes.
  • Viruses lack metabolism; they need a living host to perform any enzymatic processes, so they cannot be treated with antibiotics.
  • HIV is a retrovirus, meaning it has RNA as its genetic material and it uses reverse transcriptase to make DNA in the host cell.

Figure 6.2.3b – Structure of the human immunodeficiency virus (HIV)Figure 6.2.3b – Structure of the human immunodeficiency virus (HIV)

  • Transmission of HIV occurs when there is blood-to-blood contact, particularly in these situations:
    • sharing hypodermic needles (for intravenous drug use)
    • tainted blood transfusions
    • sexual activity in which the mucous membranes are torn
    • mother-to-child transmission (during pregnancy, childbirth or through breast milk).
  • HIV infects lymphocytes, specifically T-cells, of the immune system, lowering the body’s ability to produce antibodies and fight disease.
  • With immunity and defence lowered, common infections become very difficult to treat. In addition, a person whose immune system is lowered can be infected with very rare diseases that would not penetrate an uncompromised immune system.
  • When an HIV positive patient suffers from multiple infections at the same time, they are diagnosed with acquired immune deficiency syndrome (AIDS).

International mindedness

Without intervention, the rate of HIV transmission from mother to child can be as high as 45%. With intervention, it can be reduced to 5%. The WHO has a programme in place that aims to eliminate mother-to-child transmission completely by 2015.

Figure 6.2.3c - Prevention mother-to-child transmission Figure 6.2.3c – Prevention of mother-to-child transmission
Stages in the prevention of mother-to-child transmission (PMTCT) as outlined by the WHO

Review concepts

  • Clotting factors cause blood to coagulate into blood clots Page 6.1.3
  • Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells > Page 5.1.2
  • Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains of bacteria have multiple resistance > Page 5.1.2
  • Viruses cannot be treated with antibiotics.

Language note

  • Do not confuse coronary thrombosis (the formation of a blood clot in an artery) and atherosclerosis (the narrowing of an artery due to the thickening of the arterial wall). It is more likely that a blood clot will form where the artery is narrowed by atherosclerosis, but these are two different phenomena.

Figure 6.2.3c - Deep vein thrombosisFigure 6.2.3d – Deep vein thrombosis
Thrombosis can develop in any vein or artery. Blood clots can grow very large in a deep vein, then detach to block vessels in another part of the body. This is called an embolism.

Consider this

  • Some people are more susceptible to thrombosis because their blood has a high proportion of clotting factors and clots more easily. The condition is called hypercoagulability.
  • There is a much higher risk of developing deep vein thrombosis for women who smoke while taking birth control pills. Deep vein thrombosis can dislodge to cause an embolism in the brain (stroke) or a heart attack.
  • The effect of penicillin on world history cannot be overstated. Florey and Chain’s first human patient died from an infection he developed after being cut by a thorn of a rose .

Figure 6.2.3d - Testing antibioticsFigure 6.2.3e – Testing antibiotics
The efficacy of antibiotics is assessed by measuring the size of the ‘halo’ in which bacteria do not grow around the drug.

Nature of Science

Risks associated with scientific research: current protocols regarding safety and ethics would have precluded Florey and Chain’s experiments with mice.

Figure 6.2.3f - Fleming, Florey and ChainFigure 6.2.3e – Fleming, Florey and Chain
Alexander Fleming discovered penicillin in 1928. Howard Florey and Ernst Chain discovered ways to isolate and produce penicillin in large quantities. The three scientists shared the Nobel Prize in 1945.

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