Plants and Bacterial Growth

The antimicrobial properties of a plant are its ability to produce compounds that kill microorganisms such as bacteria, or prevent their growth, as a chemical defence against infection.

Such plants are of interest because their antimicrobial compounds are a potential source of new antibiotics, which are needed as more bacteria become resistant to existing antibiotics.

A zone of inhibition is the clear area on the agar around a disc of plant extract, where no bacteria have grown because the extract has prevented their growth.

The diameter (or area) of each clear zone is measured. The larger the clear zone, the more effective the extract is as an antimicrobial, because it has inhibited bacterial growth over a greater distance from the disc.

Comparing the zones produced by different extracts therefore allows their relative effectiveness to be ranked.

Preclinical testing is testing carried out before the drug is given to human patients. The drug is first tested on cells and tissues to check it has the intended effect, then on animals to check for toxicity and side-effects and to find a safe dose range.

In Phase 1 of the clinical trial, the drug is given to a small number of healthy volunteers, mainly to check that it is safe in humans and to study how the body handles it.

In Phase 2, a larger group of patients who actually have the disease is given the drug to test whether it is effective and to help refine the dose.

Before starting, the work surface should be wiped with disinfectant and all glassware and the agar should be sterilised (for example in an autoclave). This kills microbes already present so they cannot contaminate the culture and affect the results.

The inoculating loop used to spread the bacteria should be passed through a Bunsen flame until it glows red-hot and then allowed to cool. Flaming sterilises the loop, killing any microbes on it, so it does not introduce contamination; cooling prevents it killing the bacteria being transferred.

All transfers should be carried out close to a lit Bunsen burner. The upward convection air current above the flame carries airborne microbes upwards and away from the open plate, reducing the chance of contaminants falling onto the agar.

When adding the discs, the lid of the Petri dish should be lifted only slightly and at an angle rather than removed completely. This keeps the gap small, so fewer airborne microbes can fall onto the agar surface.

After inoculation, the lid should be secured with two pieces of tape rather than sealed all the way round, and the plate should be incubated at no more than 25 °C. Taping but not sealing prevents the lid coming off and stops the growth of harmful anaerobic pathogens, while incubating below body temperature means human pathogens are unlikely to grow, protecting the investigator.

Finally, the used plates should be sterilised by autoclaving before disposal, to kill all the microbes so they cannot infect anyone. Throughout, the purpose of aseptic technique is to prevent contamination by unwanted microbes and to protect the investigator.

The conclusion is partly supported, because a larger zone of inhibition does generally indicate a more effective antimicrobial: the extract has prevented bacterial growth over a greater distance from the disc.

However, the conclusion may not be fully valid. Zone size also depends on how easily the compounds in the extract diffuse through the agar; a very potent compound that is large or diffuses poorly could produce a small zone, so a small zone does not always mean a weak antimicrobial. The conclusion also assumes other variables were controlled — the bacterial strain, disc size, volume and concentration of extract, agar, temperature and incubation time must all have been the same for the comparison to be fair.

To improve the investigation, the student should repeat each extract several times and calculate a mean zone size, which makes the results more reliable and allows anomalies to be identified. They should also include a control disc soaked in the solvent only, to check that any zone is due to the extract and not the solvent.

In conclusion, the data are consistent with extract X being the most effective under these conditions, but the claim that it is definitively the 'strongest' is overstated without controlling diffusion and repeating the test to confirm reliability.

A placebo is a dummy treatment that looks identical to the real drug but contains no active ingredient. Some patients improve simply because they believe they are receiving treatment — the placebo effect. By giving a comparison group the placebo, this effect occurs in both groups, so any additional improvement in the group receiving the real drug can be attributed to the drug itself rather than to belief.

A double-blind design means that neither the patient nor the doctor assessing them knows whether they have been given the real drug or the placebo. This removes bias from both directions: patients cannot be influenced in how they report their symptoms, and the doctor cannot, consciously or unconsciously, record or interpret the results more favourably for the drug group. The results are therefore a fairer reflection of the drug's true effect.

Using a large number of patients reduces the effect of chance and of individual variation between people, since patients differ in age, health and how they respond. A large sample makes anomalies less significant and allows the results to be analysed statistically.

Together, the placebo controls for the placebo effect, the double-blind design removes bias, and the large sample reduces the influence of chance, so the trial produces results that are both reliable (repeatable and consistent) and valid (genuinely measuring the effect of the drug), and that can be generalised to the wider population.

Plants produce a wide range of antimicrobial compounds as a chemical defence against the bacteria, fungi and other microbes that attack them. Because these compounds already kill bacteria or stop their growth, they are promising candidates for new antibiotics.

New antibiotics are urgently needed because many bacteria have evolved resistance to existing antibiotics, so current treatments are becoming less effective. Plants offer a huge variety of compounds, most of which have not yet been tested, and these may act in new ways to which bacteria are not yet resistant.

The first laboratory step is to test extracts of the plant against bacteria using aseptic technique. The scientist spreads bacteria evenly over an agar plate to make a lawn, then places paper discs soaked in different plant extracts onto the agar. The plate is taped and incubated at no more than 25 °C.

After incubation, the scientist looks for clear zones of inhibition around the discs, where the extract has prevented bacterial growth. A larger zone indicates a more effective antimicrobial, identifying which extracts are worth investigating further.

The area of a circular zone is given by A = πr², where r is the radius (half the diameter).

For extract P: r = 12 ÷ 2 = 6 mm, so A = 3.14 × 6² = 3.14 × 36 = 113 mm² (3 s.f.).

For extract Q: r = 20 ÷ 2 = 10 mm, so A = 3.14 × 10² = 3.14 × 100 = 314 mm².

Extract Q is the more effective antimicrobial, because it produced the larger zone of inhibition — it has prevented bacterial growth over a greater area of the plate.

Area is a better measure than diameter because the area increases with the square of the radius, not in proportion to it. Q's diameter is less than twice P's diameter (20 mm versus 12 mm), yet its area is almost three times larger (314 mm² versus 113 mm²). Area therefore reflects the actual region of agar over which growth has been inhibited and spreads out the differences between extracts, making the comparison fairer.

It should be remembered, however, that zone size also depends on how readily each compound diffuses through the agar, so this comparison reflects relative effectiveness under these specific conditions.

There are strong reasons to develop the compound. It gives large zones of inhibition in the laboratory, showing it is effective at preventing bacterial growth, and as a naturally produced plant defence compound it may act in a way that differs from existing antibiotics. This is valuable because many bacteria have become resistant to current antibiotics, so a new antibiotic with a different mode of action could treat infections that no longer respond to existing drugs, potentially saving many lives.

However, a large zone of inhibition on an agar plate is only an early laboratory result and does not prove the compound is suitable as a medicine. Zone size partly depends on how well the compound diffuses through agar, and conditions on a plate are very different from those inside the human body, where the compound must reach the infection, work at a safe concentration and not be broken down too quickly. The compound could also be toxic to human cells or cause serious side-effects, which the plate test cannot reveal.

For this reason the compound must go through the full development process before it could be marketed. It would first undergo preclinical testing on cells, tissues and then animals to check for effectiveness and toxicity, followed by clinical trials in phases — first a small number of healthy volunteers to test safety, then larger groups of patients to test effectiveness, using placebos, a double-blind design and large samples to give reliable and valid results. This process is expensive and slow and the compound may fail at any stage, for example if it proves toxic or no more effective than existing drugs.

In conclusion, the laboratory result makes the compound a promising candidate that is well worth developing further, especially given the urgent need for new antibiotics to combat resistance. However, it should not be marketed as a new antibiotic until it has passed the full sequence of preclinical and clinical testing to prove it is both safe and effective in humans; the large zone of inhibition alone is not sufficient evidence.

The plates are taped rather than left loose so that the lid does not come off accidentally. This protects the investigator and others from the microbes growing inside, and keeps unwanted airborne microbes from entering and contaminating the culture.

However, the lid is taped but not sealed all the way round. This still allows some air, and therefore oxygen, to reach the culture. Sealing it completely would create anaerobic conditions, which could favour the growth of harmful anaerobic pathogens, so a partial seal makes the cultures safer.

The plates are incubated at no more than 25 °C rather than at human body temperature (around 37 °C). Below body temperature, microbes that are pathogenic to humans are far less likely to grow, because they are adapted to grow best at body temperature. This greatly reduces the risk of culturing a dangerous human pathogen, again protecting students.

Finally, the used plates are sterilised, for example by autoclaving, before disposal. This kills all the microbes that have grown so that they cannot infect anyone after the experiment or escape into the environment. Together, these precautions follow the central aim of aseptic technique: to prevent contamination and to protect the investigator.