13.2 Applications and skills
13.2.1 Innovations and techniques in microbiology
- One aspect of the nature of science is that sometimes serendipity leads to scientific discovery.
- Alexander Fleming was a physician and bacteriologist who discovered penicillin in England in 1928.
- Early attempts to produce penicillin in shallow trays were unsuccessful. The products were unstable and the cultures were easily contaminated.
- ‘Deep tank’ batch fermentation made the mass production of penicillin possible. Food chemists had been using a similar technique to produce citric acid, as well as other food additives.
- The story of penicillin is not only about serendipity, but also of co-operation, innovation and technological advancement.
Skill: Gram staining procedure
- Antibiotics like penicillin, work by inhibiting the synthesis of peptidoglycan, a major component of bacterial cell walls.
- The Gram staining procedure is a simple test that differentiates between two types of bacterial cell wall.
- Bacteria are classed as either Gram+ or Gram- based on the results of this test.
- Gram+ bacteria stain purple
- Gram– bacteria stain pink
Skill: Measuring zones of inhibition in bacterial culture
- To test antimicrobial properties, bacteria may be cultured on agar plates interspersed with small discs of filter paper that have been soaked in various test substances.
- During incubation, the antimicrobial agent will diffuse out of the disc, so that a concentration gradient is achieved.
- After one or two days of incubation, the bacterial lawn becomes interrupted by areas with no bacterial growth, called zones of inhibition, that develop around the discs.
- The dimensions of the zone of inhibition can be measured and compared.
- If a zone of inhibition is very large, it means that small concentrations of substance are effective at preventing microbial growth.
Figure 13.2.1d – Zones of inhibition develop around antibiotic disks
- Which of the antibiotics shown in Figure 13.2.1d is the most effective?
- Which is the least effective?
- How could you determine the minimum concentration of antimicrobial agent necessary to inhibit bacterial growth?
Skill: Producing biogas in a small-scale fermenter
- It is simple to set up a small-scale fermenter, using household materials and manure.
- Use opaque materials – you do not want to introduce photosynthetic organisms.
- For the first few weeks, it will produce only carbon dioxide, but if kept in a warm place (about 20°C), methanogenic bacteria will eventually reduce all the carbon dioxide into methane gas.
Figure 13.2.1f – Alexander Fleming
Alexander Fleming with a culture of Staphylococcus , a Gram+ bacterium.
Figure 13.2.1g – E. coli
Escherichia coli is a Gram– bacterium.
To what extent are ‘accidental’ discoveries a product of luck?
Nature of Science
Serendipity and technological innovation: Deep tank fermentation allowed the discovery of penicillin to become the mass-produced wonder drug that saved thousands of soldiers' lives during the Second World War.
In the lab
- Inoculating slides with bacteria takes practice and skill. Petri dishes are easily contaminated. In the video on top of this column a simple and aseptic method to perform a disc diffusion test is shown.
- You can soak filter paper discs with household cleaners or ethanol to perform your own Zone of Inhibition tests.
- How will you know when you are producing methane in your biogas reactor? Test the gas in a bunsen burner!