10.2 Antibiotics

Antibiotic resistance of pathogenic bacteria is a worldwide cause for concern. Investigations into the use of antibiotics and their effects use an international unit of measurement, called the defined daily dose (DDD).

The DDD represents the assumed average dose for a drug per day being used to treat a specific disease in adults.

A study investigated the antibiotic resistance of Streptococcus pneumoniae. The study was carried out in some European countries and the percentage of S. pneumoniae resistant to the antibiotics penicillin and macrolide was determined.

The results are shown in Table 1.

Table 1

[1]

To find the strength of the relationship between antibiotic use and the percentage resistance in S. pneumoniae, a linear correlation test was used. The probability for each value was found using the critical values from a probability table.

State what the values in Table 2 indicate about the relationship between antibiotic use and antibiotic resistance in S. pneumonia.

[2]

The disc diffusion method is used to test the effectiveness of different concentrations of antibiotics. Fig. 1 shows the results of a disc diffusion test.

Starting with a stock solution of an antibiotic, outline how you would make a serial dilution to produce a range of concentrations of the antibiotic.

The disc diffusion test is also used to test for antibiotic resistance. The test may take about five days to complete.

Escherichia coli is a common bacterium in the human intestines that can develop antibiotic resistance and transmit it to other bacteria. It is present in faeces and can also be present in waste water that is discharged into rivers.

Environmental agencies test samples of waste water for antibiotic resistant bacteria. This can be done with the disc diffusion method. A new test was developed to allow estimates of antibiotic-resistant E. coli to be obtained within 24 hours.

Suggest two criteria that environmental agencies could apply when assessing the new test to see whether it is suitable.

Penicillin belongs to a group of antibiotics known as β lactams, which all act in the same way on bacteria.

Describe how penicillin kills non-resistant bacteria. 

One of the ways in which a bacterium may be resistant to an antibiotic, such as a β lactam, is by having protein pumps in its cell surface membrane which expel the antibiotic from the bacterium.

The gene coding for such an efflux pump is carried on a plasmid.

Outline how the bacterium produces an efflux pump from a gene on a plasmid.

A strain of the bacterium Pseudomonas aeruginosa, strain R, has a gene coding for an efflux pump and is resistant to a β lactam antibiotic.

The minimum inhibitory concentration (MIC) of the β lactam for strain R was determined. The MIC is the lowest concentration of antibiotic that prevents a colony of bacteria growing.

The MICs were also determined for two mutant strains derived from strain R, mutant strain 1 and mutant strain 2. Each of these strains differs from strain R in the expression of the gene coding for the efflux pump. 

The MICs for the three strains of P. aeruginosa are shown in Table 1.

Table 1

With reference to Table 1, suggest:

Why the MICs for mutant strains 1 and 2 differ from that for strain R.

Mutant strain 1 

[2]

Mutant strain 2 

[2]

How a population of strain R of P. aeruginosa could be replaced by mutant strain 2.

[4]

The bacterium Staphylococcus aureus lives in the nose and on the skin of humans where it may cause minor infections that can easily be treated with antibiotics. There are however strains of S. aureus that have developed resistance to several of these antibiotics and are known as methicillin-resistant Staphylococcus aureus or MRSA.

MRSA may lead to very serious infections in the human body and are commonly found in environments of high antibiotic use, such as hospitals. Vancomycin is a powerful antibiotic that is prescribed for serious bacterial infections, such as MRSA, but in recent years there has been an increase in the occurrence of vancomycin resistant strains of MRSA.

Fig. 1 shows the results of a study that investigated the increase in vancomycin resistance in MRSA.

Fig. 1

Describe the results shown in Fig. 1

Explain how these strains of MRSA may have become resistant to vancomycin.

Antibiotic resistance has become an increasingly serious problem around the globe over the past few decades.

Suggest how the risk of bacteria developing antibiotic resistance can be reduced.

Klebsiella pneumoniae is a species of bacteria commonly found in the human digestive tract where they are harmless. They can however, cause serious infections in other parts of the body. K. pneumoniae can form a biofilm, consisting of a thin layer of bacteria that are attached to each other, on surfaces and enter the body through direct contact with them.

A mutant strain of K. pneumoniae was found to form biofilms similar to the normal strain of the bacteria. Scientists grew normal and mutant bacteria on agar plates and exposed them to colistin, an antibiotic used to treat serious infections of K. pneumoniae.

Fig. 1 shows the change in the number of live bacterial colonies over time.

Fig. 1

Describe the changes in the number of live bacterial colonies of normal and mutant bacteria, with reference to Fig.1.

At the end of the investigation, scientists concluded that the mutant strain of K. pneumoniae was resistant to colistin.

Explain the role of natural selection in the development of antibiotic resistant strains of K. pneumoniae.

Suggest two possible ways in which K. pneumoniae can resist the effects of an antibiotic.

Explain the importance of completing a course of antibiotics, with reference to Fig.1.

Scientists investigated the effectiveness of a penicillin antibiotic against four different strains (A, B, C and D) of a species of bacterium. They were grown on a divided petri dish containing nutrient agar until they covered the entire surface.

Four filter paper discs were soaked in a penicillin antibiotic and placed in contact with each bacterial strain for 20 minutes. The petri dish was left for 24 hours and thereafter the diameter of the zone of clearance, where bacterial cells were killed, for each strain was measured. The area of the zone of clearance was calculated for strain A, C and D, using the formula πr² .

Fig.1 shows the appearance of the petri dish after 24 hours, while Table 1 shows the results of their investigation.

Fig. 1

Table 1

Calculate the area of the zone of clearance for strain A.

Show your working and round your answer to the nearest whole number.

Explain the effects of penicillin on the different bacterial strains, with reference to Fig.1.

Further investigation found that strain B was very common amongst individuals from the local hospital and prison.

Suggest possible reasons for this.

State one measure that can be implemented at the local hospital and prison to limit the spread of strain B.