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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.

Figure 13.2.1a – Structure of Gram+ (left) and Gram– (right) cell walls.Figure 13.2.1a – Structure of Gram+ (left) and Gram (right) cell walls.

Figure 13.2.1b – Outline of Gram staining procedureFigure 13.2.1b – Outline of Gram staining procedure


Figure 13.2.1c – Results of the Gram staining procedureFigure 13.2.1c – Results of the Gram staining procedure

  • 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 disksFigure 13.2.1d – Zones of inhibition develop around antibiotic disks

Try it!

  • 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.

Figure 13.2.1e – Small-scale biogas generatorFigure 13.2.1e – Small-scale biogas generator

  • 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 FlemingFigure 13.2.1f – Alexander Fleming
Alexander Fleming with a culture of Staphylococcus , a Gram+ bacterium.

Figure 13.2.1g – E. coliFigure 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.

Figure 13.2.1h – HomemadeFigure 13.2.1h – Homemade

  • How will you know when you are producing methane in your biogas reactor? Test the gas in a bunsen burner!