Question
The treatment for people with active tuberculosis (TB) lasts six months and involves a combination of antibiotics. This is usually very effective if the person has a susceptible (non-resistant) strain of
Mycobacterium tuberculosis, the causative organism of TB.
Table 2.1 summarises one recommended treatment strategy that involves a combination of antibiotics.
Table 2.1
length of antibiotic mode of action of antibiotic treatment enters bacterial cells and inhibits protein rifampicin (R) 6 months synthesis prevents the synthesis of cell wall isoniazid (H) 6 months components known as mycolic acids prevents mycolic acids from being added ethambutol (E) first two months to the cell wall
pyrazinamide (Z) first two months prevents the synthesis of fatty acids
(a) Susceptible strains of M. tuberculosis will be killed using any one of the antibiotics listed in
Table 2.1. However, combination treatment is preferred as it is one method that can be used to reduce the impact to society of antibiotic resistance.
With reference to Table 2.1, explain how combination treatment for TB can help to reduce the impact of antibiotic resistance compared to single antibiotic treatment.
…… [4]
Rifampicin binds tightly to an RNA polymerase molecule close to its active site. This affects the activity of the enzyme.
(b) (i) RNA polymerase catalyses the formation of messenger RNA (mRNA) from DNA.
State the term for this process.
…… [1]
(ii) During the formation of RNA, a number of events occur that involve the action of RNA polymerase.
Suggest ways in which rifampicin can affect the activity of RNA polymerase.
…… [3]
(c) RNA polymerase is composed of five different polypeptides. Gene rpoB codes for one of these polypeptides known as the β-subunit.
One or more mutations in a specific region of rpoB result in strains of M. tuberculosis that are resistant to rifampicin. In these strains, mutations often occur in two DNA triplets in this region, in positions 526 and 531.
Table 2.2 summarises the results of an investigation into seven rifampicin-resistant strains,
A to G, that have amino acid changes for positions 526 and 531.
Table 2.2 includes:
• the change in the mRNA codon for position 526 or position 531
• the amino acid change that has occurred as a result of the mutation
• the minimum concentration of rifampicin required to inhibit growth of the bacterial strain (MIC)
• the number of other mutations occurring within the specific region of rpoB.
Table 2.2
Key
. approximately H greater than or equal to G less than or equal to
number of mRNA codon amino acid MIC / other mutations strain codon involved change µg cm–3 in the specific change region
A 526 CAC UAC His Tyr G 50 0
B 526 CAC AAC His Asn H 100 1
C 526 CAC CGC His Arg . 50–75 2
D 526 CAC CGC His Arg H 100 3
E 526 CAC CGC His Arg . 50 3
526 CAC UUC
His ……
F H 100 3
531 UCG UUG Ser Leu
526 CAC UAC
His ……
G H 100 3
531 UCG UUC Ser Phe
(i) Complete Table 2.2 to show the amino acid changes that have occurred in strains
F and G.
[1]
(ii) With reference to Table 2.2, list the strains of M. tuberculosis that show the greatest resistance to rifampicin.
…… [1]
(iii) Suggest reasons to explain why strains C, D and E show:
• resistance to rifampicin
• different levels of resistances to rifampicin.
…… [3]
[Total: 13]
Mark scheme
2(a) any four from: 4
1 (in combination treatment) antibiotics (in Table 2.1), act at different
targets / have different modes of action / AW ;
A comparison of any two antibiotics from Table 2.1
A suggestion of how two antibiotics have different ways of killing
2 idea that if resistance / mutation, occurs / exists, unlikely to be against all
antibiotics / other antibiotics should still be effective ;
3 (in combination treatment) if resistance / mutation, occurs / exists, all
bacteria will (still) be, killed / destroyed / AW ;
A no bacteria remain to develop resistance / no reservoir of resistant
bacteria
4 antibiotic resistance, not / less likely to be, spread to affect people
because no bacteria surviving (with combination treatment) ; AW
5 long treatment time / 6 months, with, combination treatment / AW,
increases chance of killing all bacteria
or
long treatment time with a single antibiotic not effective in killing all
bacteria if, resistance develops / a mutation occurs ;
6 AVP ; e.g.
combination treatment (is likely to) eliminate bacteria more quickly (so
less chance of resistance occurring)
resistance to different antibiotics involves more genes so less chance of
resistance occurring
gene for antibiotic resistance has more chance of being passed on if
using single antibiotic (and not all killed) ora
if using single antibiotic (and not all killed) more chance of being passed
on (to other bacteria) by horizontal / vertical / AW, transmission ora
2(b)(i) transcription ; 1
2(b)(ii) any three from: 3
I stops transcription
1 alters shape of / blocks, active site ;
R enters / fits into, active site as it is not competitive
R ref. to choice of competitive or non-competitive
A alters shape of enzyme only if mp2 gained
2 substrate / nucleotides, cannot, bind to / enter, active site ;
A fewer / no, enzyme-substrate complexes / ESCs, form
allow ecf from mp1 if rifampicin described as, competitive / enters active
site
3 complementary (base) pairs / (complementary) base pairs, cannot
form / form less easily (between DNA and RNA nucleotides) ;
4, 5, 6 (in context of rifampicin binding to RNA polymerase)
prevents / AW, unwinding of DNA (double helix) ;
A uncoiling
I unzipping prevents / AW, attachment to DNA (strand)
A prevents / AW, attachment to promoter
prevents / AW, movement along DNA (strand) ;
prevents / AW, elongation (of polynucleotide) / formation of polynucleotide
/nucleotides being joined ;
prevents / AW, phosphodiester bond formation ;
prevents / AW, proof reading ;
7 AVP ; e.g. prevents / AW, tRNA / rRNA, formation
2(c)(i) Phe 1
Tyr ; one mark for both correct
2(c)(ii) B D F G ; in any order 1
2(c)(iii) resistance 3
1 shape / tertiary structure, of, β-subunit / enzyme, altered / AW ;
I active site changes shape
A quaternary structure of enzyme altered
2 rifampicin / antibiotic, cannot / does not, bind (as well) ; AW
R if context is binding to active site
different levels of resistances
any two from:
3 (because) other mutations are involved ; AW
A described from Table 2.1
e.g. C has 2 (other) mutations and D and E have 3
4 mutations may result in different changes to, structure / shape of,
β-subunit / enzyme, and result in different, effects / levels of resistance ;
5 data from Table 2.2 to support mp 4 ; must be linked to concept of mp4
6 AVP ; e.g. ref. to different binding abilities (of rifampicin to enzyme)
some (of the other) mutations may cause more of a change to binding
site for rifampicin
mutation(s) may make it harder to bind
lower resistance = binds, more strongly / for longer time
lower resistance = higher proportion of transcription events hindered
e.g. idea that mutations still, produce functioning enzyme / allow catalysis
to occur