Edexcel IAL Biology AS Core Practical 9

[Maximum number: 9]

Petri dishes with nutrient agar were used in an investigation of the antimicrobial properties of plant extracts. Bacteria were spread on the surface of the agar to form a lawn. Plant extracts were placed in wells cut in the agar. After incubation, the size of the zone of inhibition is a measure of the antimicrobial effect of the plant extract. The diagram shows a Petri dish after the incubation period.

Figure for Question Edexcel A-Level Biology AS 4.12 Core Practical 9 Antimicrobial Properties of Plants Questions question 1 — Edexcel A-Level Biology AS

(a)

Explain three aseptic techniques that should be used when transferring bacteria from one culture to another. (3)

[ 3 ]

(b)

The technique described was used to investigate the antimicrobial effects of oil A and oil B, extracted from two species of plant. Different bacterial lawns were made using E. coli and B. subtilis.

[ 6 ]

(i)

Explain how the area of the zone of inhibition could be measured accurately.

[ 2 ]

An explanation that includes the following points:

- use of squared paper (1)

- detail of what to do with paper to get area (1)

OR

- measure \{diameter / radius\} \{using callipers / in more than one direction\} (1)

- detail of calculation of area from this (1)

e.g. treatment of part squares do not accept unqualified count squares

accept micrometer and/or graticule accept different angles

e.g. $\pi \mathrm{r}^{2}$

(ii)

The table shows the results from this investigation.

Table for Question (ii) — Edexcel A-Level Biology AS

Determine the effect of these two oils on the growth of these bacteria.

[ 4 ]

An answer that includes the following point:

- a calculation to compare two relevant results between different \{oils / bacteria\} (1)

and any three of the following:

- oil A is a more effective (inhibitor of bacterial growth) than oil B (on both bacteria) (1)

- both oils are more effective (inhibitors of bacterial growth) on B. subtilis (than on E. coli) (1)

- oil B (inhibits only B. subtilis / does not inhibit E.coli) / oil A inhibits both bacteria (1)

- biggest (anti-bacterial) effect is A on B. subtilis / smallest (anti-bacterial) effect is B on E. coli (1)

penalize the idea that the effect is a stimulus to growth once only. e.g. B. subtilis $20.66 \div$ $11.33=1.82 \times$ more
inhibited by A than B
oil A has 9.33 greater
zone of inhibition than oil
B on B. subtilis
oil A affects B. s. 2.4
times more than it affects
E. c. or reverse
calculation
many other calculations
are possible for the mark
accept Z.o.l. bigger for
more effective
accept reverse argument
do not piece together,
must be clearly stated

Edexcel A-Level Biology AS 4.12 Core Practical 9 Antimicrobial Properties of Plants Questions question 1

Question (a)

Question (b)

Question (i)

Question (ii)