5.2 Wrong therapeutic use

Antibiotic resistance is more likely to develop when antibiotics are used at too low a dose or taken for too short a time. In the next activity, you will explore the effect of treatment duration on gut bacteria.

Activity 6 Effect of antibiotics on gut bacteria

Timing: Allow about 20 minutes

You might recall from Week 1 the typical growth pattern of a bacterial population (Figure 8 below). The death phase does not occur in the gut. This is because of the steady flow of material from mouth to anus, so that new food is always added and waste products are always removed. In the gut, the only time a population will decline like this is if something – for example, an antibiotic taken by mouth – kills it.

Figure 8 Bacterial growth in a closed system.

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This figure illustrates bacterial growth in the gut using three approaches. It is a screenshot taken from the interactive activity used in Activity 6. On the top left the gut is illustrated as a pink circle. Bacteria are illustrated as small blue (antibiotic sensitive) and red (antibiotic resistant) circles. There are many more blue than red circles. On the top right the proportion of antibiotic sensitive (blue) and antibiotic resistant (red) bacteria is shown in a bar graph. The horizontal axis is marked sensitive and resistant. The proportion of sensitive bacteria is higher than the proportion of the resistant bacteria. The bottom graph shows the growth of sensitive and resistant bacteria over time. The horizontal axis is labelled time (days) and marked from 0 to 8 in intervals of 1. Two grey dots indicate the addition of antibiotics. The vertical axis is labelled number of bacteria. The sensitive bacteria (in blue) increase gradually from 0 to 2.5 day and then drop steeply when antibiotics are added. The number of resistant bacteria remains low.

Figure 8 Bacterial growth in a closed system.

When a person swallows pathogenic bacteria, whether they become ill depends on the type of bacterium. For some types, only a few bacteria will cause illness. For other bacteria, millions must be taken in to cause any harm. The number of bacteria needed to cause illness is called the infectious dose.

Now click on the image below to be taken to an interactive activity.

View interactive version

 

Interactive feature not available in single page view (see it in standard view).

There is a risk of resistance developing every time an antibiotic is used because only resistant bacteria can survive and reproduce in the presence of antibiotics. This process is called selection. The resistant bacteria can then pass their resistant genes to other bacteria.

If the dose of antibiotic is too low, selection ensures that a few resistant bacteria will survive. Although the patient may start to feel better, the surviving bacteria will soon multiply, symptoms will return and the antibiotic will no longer be effective at the original dose used.

So far this week you have focused on human behaviours that promote antibiotic resistance. You should by now appreciate the importance of using antibiotics correctly. In the next two sections, you will look at other facets of the problem. First, you will learn why we are running out of options to treat antibiotic-resistant infections.