Antibiotics (Cambridge (CIE) A Level Biology)

Scientists investigated the sensitivity of two different species of bacteria (X and Y) in the presence of different types of antibiotics. An inoculum of each species of bacteria was spread onto agar plates before filter paper discs soaked in different antibiotics were placed onto the surface of each plate.

The substances that the filter paper discs were soaked in were as follow:

1 - Penicillin G, a natural penicillin
2 - Penicillin V, a natural penicillin
3 - Amoxicillin, a synthetic penicillin
4 - Mezlocillin, a synthetic penicillin
5 - Distilled water

The plates were incubated at 30°C for 24 hours, after which they were removed and the clear zone, where no bacterial growth occurred, was measured around each filter paper disc.

Fig. 1 shows the appearance of the incubated plates at the end of the investigation.

Fig. 1

Explain the effects that penicillin V had on bacterium Y.

Discuss why bacterium Y would be of particular concern to the scientists.

State the reason why penicillin is not effective in treating viral diseases.

Fig. 2 shows a diagram of part of the cell walls of both bacterium X and Y.

Fig. 2

Suggest why bacterium X would be more susceptible to the action of penicillin than bacterium Y.

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.

Scientists investigated the presence of bacteria resistant to the antibiotic tetracycline in poultry and in the farmers who kept them. They looked for Escherichia coli (E.coli) resistant to tetracycline. The scientists took samples of faeces from the poultry birds and the farmers. Turkey farmers often used food containing tetracycline, whereas chicken farmers did not very often.

The bacteria were grown on nutrient agar containing tetracycline. Resistant bacteria grew and were visible as colonies on the agar plates.

The results are shown in the table below.

 Suggest a hypothesis the farmers were testing in this investigation.

Describe the results of the scientists’ investigation described in part (a).

Scientists investigated treatment of a human respiratory infection caused by a species of bacterium. This species of bacterium is often resistant to the antibiotics currently used for treatment. They investigated the use of a new antibiotic to treat the respiratory infection. The new antibiotic blocks DNA replication in bacterial cells.

The scientists tested the new antibiotic on mice with the same respiratory infection.  The antibiotics were given to the mice at a dose of 25 mg kg⁻¹ per day.

Calculate how much antibiotic would be given to a 33 g mouse each day.

The antibiotic tetracycline is used to treat human bacterial infections such as pneumonia and other respiratory tract infections.  This antibiotic is safe to use in humans as it does not inhibit or block processes such as DNA replication, ribosome function, transcription or translation.

Suggest why these processes are not inhibited in humans but can be in bacteria.

Resistance to antibiotics within a population of bacteria is due to selection pressure. This can be linked to the use of antibiotics by patients.

A study was carried out into the link between antibiotic use and the presence of resistant Escherichia coli (E. coli) populations in human communities.

• Over 30 000 patients were involved in the study.

• Only patients attending large medical clinics took part in the study.

• The number of prescriptions issued by each clinic was used as an estimate of antibiotic use.

• Urine from patients attending the clinics was used as a possible source of antibiotic resistant E. coli.

• Antibiotic resistance of E. coli in the urine samples was measured using the disc diffusion method.

The disc diffusion method measures sensitivity of bacteria to an antibiotic. A bacterial population with low sensitivity to an antibiotic is resistant to that antibiotic.

In the disc diffusion method a Petri dish is filled with nutrient agar and urine samples containing E. coli are spread evenly across the agar.

Discs containing different antibiotics are placed on top of the agar. A lid is put on the Petri dish and the plate is incubated overnight.

Fig. 1 shows an example of a Petri dish from the study after incubation.

Fig. 1

(i) Suggest two variables that need to be standardised when using the disc diffusion method in this study.

[2]

(ii) Describe how you would determine the sensitivity of E. coli to each antibiotic.

[2]

Table 1 shows the results of this investigation.

Table 1

(i) Comment on the standard deviations for antibiotic use as shown in Table 1.

[2]

(ii) Suggest two reasons why the number of prescriptions issued for antibiotics may not give an accurate measure of antibiotic use by patients.

[2]

The number of prescriptions issued for antibiotics varied considerably between clinics.

The researchers wanted to find out whether there was a correlation between the number of prescriptions for each of the five antibiotics issued by a clinic and the percentage of urine samples containing resistant E. coli.

Spearman’s rank correlation test was used for this analysis.

The results of this analysis are shown in Table 2.

Table 2

Table 3 shows the critical values for r_s at five levels of significance for the data collected in this study.

Table 3

(i) Suggest why the Spearman’s rank correlation test was used in this study.

[1]

(ii) State a null hypothesis for the Spearman’s rank correlation test for this study.

[1]

(iii) Using Table 2 and Table 3, identify which antibiotics showed a statistically significant correlation between the number of prescriptions and the presence of resistant strains of E. coli in urine samples. Give a reason for your answer.

[2]

The percentage of patients with ampicillin-resistant E. coli infections varies with age and gender, as shown in Fig. 2.

Fig. 2

Describe and explain the trends shown by these data.

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

(i)  State two conclusions that can be made about the quantity of these antibiotics used and the percentage of S. pneumoniae resistant to these two types of antibiotic.

[2]

(ii) Calculate the ratio of penicillin use between Spain and the Netherlands.

[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.

(i) State two reasons why this statistical test is suitable for these data.

[2]

(ii) The results of the test are shown in Table 2.

Table 2

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.

Fig. 1

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:

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

Mutant strain 1 

[2]

Mutant strain 2 

[2]

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

[4]

Researchers have identified a new category of antibiotics capable of targeting bacteria that show widespread resistance to drugs.

The antibiotic Zosurabalpin has demonstrated efficacy against Carbapenem-resistant Acinetobacter baumannii (CRAB) in experiments involving mouse models. Human trials are now underway to assess its potential.

(i) Describe the implications of widespread antibiotic resistance on public health

[3]

(ii) Discuss two potential strategies to address this global challenge.

[4]

Explain how horizontal gene transfer, mutation, and selective pressure contribute to the evolution of antibiotic-resistant bacteria.

Zosurabalpin is currently going through clinical trials in which the antibiotics will be tested on healthy human volunteers. Following this, further tests will be carried out with a small group of patients.

Suggest why these trials are necessary before a new drug can be made available to human patients.

In a series of experiments, it was found that Zosurabalpin significantly reduced levels of bacteria in mice. However, further tests are required to establish the full extent of the antibiotic action.

Explain why a reduction in bacteria does not imply a complete solution to the issues encountered with antibiotic-resistance.