Antibiotics (Cambridge O Level Biology)

Microbiologists test strains of bacteria for antibiotic resistance.

They do this by soaking paper discs in antibiotics and placing them on bacteria growing in Petri dishes.

The paper discs in the centre of Petri dishes E and F in Fig. 1 have been soaked in penicillin. 

Fig. 1

State the type of microorganism that produces penicillin.

State and explain the evidence from Fig. 1 that suggests that the bacteria in dish F are resistant to penicillin.

[6]

Some scientists investigated the effect of two antibiotics (methicillin and penicillin) on the growth of two different bacteria.

They used filter paper discs soaked in antibiotic solution and placed the discs on two agar plates, one had been spread with bacteria A and the other with bacteria B.

The filter paper discs were prepared as follows:

1. Control
2. Penicillin
3. Methicillin

Fig. 1 shows the results from the investigation:

Suggest what the paper disc might have been soaked in for the control.

Describe and explain the results in Fig. 1.

Predict and explain the appearance of a petri dish that is used to grow a strain of bacteria, Bacteria C, which possesses resistance to methicillin and penicillin.

DNA analysis of the three strains of bacteria (A, B and C) suggests an evolutionary link between them.

Bacteria C is thought to have evolved from one of the other two strains.

Suggest how bacteria C may have evolved from the other strains.

[1]

Predict and explain whether Bacteria C is more likely to have evolved from Bacteria A or Bacteria B.

[2]

Table 1 shows some of the reasons.

Place a tick (✓) in the boxes that are genuine factors why antibiotics can lose their effectiveness in combatting medical and veterinary diseases. 

Table 1

MRSA strain is a strain of bacteria often referred to as a 'superbug'. The SA part of the abbreviation 'MRSA' stands for Staphylococcus aureus, a species of bacteria.

Suggest what the 'R' of MRSA stands for. 

Scientists developing new antibiotics sometimes find a molecule that does not kill the bacteria outright, but merely stops them from reproducing.

Explain why this is generally acceptable in the development of new antibiotics.

Suggest why antibiotics tend to be effective against bacterial cells but not against animal cells.

Prior to passing the dumpsite, the stream passes through a large urban township with limited sanitation. 

The levels of antibiotics are found in Table 1.

Table 1

Calculate the percentage increase of antibiotic B detected as the stream flows from the township and past the dumpsite. 

Suggest two ways in which antibiotics may have found their way into the water as shown in Table 1.

Suggest two possible risks of antibiotics entering the environment such as in sewage and wastewater.