Antimicrobial Drug Action

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Problem 3

Antimicrobial Drug Action

The following experiment was performed to study the effect of antibiotics on the growth and viability of Staphylococcus aureus.

Samples (10ml) from a culture of S. aureus growing exponentially in Mueller-Hinton broth were pipette into flasks containing synercid, or synercid and tetracycline, at the minimum concentration required to inhibit growth and incubation was continued at 37°C. A further sample served as an antibiotic-free control. Aliquots were taken at intervals for determination of culture density by an optical method and for determination of viable cell numbers by diluting the culture with sterile saline and plating duplicate 0.2ml samples onto the surface of Mueller-Hinton agar. After 24 h incubation at 37°C the number of colonies on each plate was counted and the results obtained are recorded in the Table below.

During the 50 min incubation the optical density of the antibiotic-free control suspension of bacteria increased by 75% while the optical densities of the other cultures remained constant. Synercid is a mixture of two components, quinupristin and dalfopristin in the ratio of 30:70 (w/w). The minimum growth inhibitory concentrations of quinupristin and dalfopristin are 30 and 20 µg/ml respectively. When quinupristin, dalfopristin and tetracycline were added individually to cultures of S. aureus at their minimum growth inhibitory concentrations, there was no change in viable cell numbers during 50 min incubation.

Time(min)

0

10

10

10

25

25

25

25

50

50

50

50

50

Dilution factor

105

104

105

106

103

104

105

106

102

103

104

105

106

ANTIBIOTIC

Conc. µg/ml

Colonies counteda

None

210

192

215

226

25

17

272

250

33

30

334

342

40

30

Synercid

1

136

147

10

15

415

426

40

45

90

100

10

9

Synercid + tetracycline

1

2

272

290

22

35

635

650

60

70

4

10

a Where no numbers are given the assay was not carried out.

Questions

1. What conclusions can be drawn from these data on the effects of the antibiotics on S.aureus?

Synercid is known to be bactericidal to most Gram positive bacteria including S. aureus, but the individual compounds quinupristin and dalfopristin are bacteriostatic. This shows that the combination of the two antibiotics has a synergistic effect and is much more effective used in combination. Tetracycline is also bacteriostatic; this is the reason why the number of viable cells during the 50 minute incubation did not change when the three antibiotics were added separately. The data shows that using antibiotic combination of synercid and tetracycline in this example is of greater benefit than using synercid alone. At 105 dilution factor, the number of colonies observed with the antibiotic synercid decreases by around 15 fold by the 50 minute mark. The use of synercid and tetracycline in combination obviously has a more potent effect on the bacteria than that of synercid alone, showing a round a 40 fold decrease in colony numbers, this shows that synercid and tetracycline have a synergistic effect.

2. What is the mean generation time in minutes of the organisms growing in the antibiotic free control culture?

Work out the mean of the 2 vaules of the culture containing no antibiotic.

Time(min)

0

10

10

10

25

25

25

25

50

50

50

50

50

Dilution factor

105

104

105

106

103

104

105

106

102

103

104

105

106

ANTIBIOTIC

Conc. µg/ml

Colonies counteda

None

201

220.5

21

261

31.5

338

35

Synercid

1

141.5

12.5

420.5

42.5

95

9.5

Synercid + tetracycline

1

2

281

28.5

642.5

65

7

Then work out the colony forming units using:

Number of colonies x dilution of plate / volume of incoculum

201 x 105/0.2 = 10.05 x 107 cfu

220 x 105/0.2 = 11 x 107 cfu

21 x 106/0.2 = 10.05 x 107 cfu

261 x 105/0.2 = 13.05 x 107 cfu

31.5 x 106/0.2 = 15.75 x 107 cfu

338 x 105/0.2 = 16.9 x 107 cfu

35 x 106/0.2 = 17.5 x 107 cfu

Then log these results.

Log 10.05 x 107 = 8.002

Log 11 x 107 = 8.04

Log 10.05 x 107 = 8.002

Log 13.05 x 107 = 8.116

Log 15.75 x 107 = 8.2

Log 16.9 x 107 = 8.228

Log 17.5 x 107 = 8.24

These results can now be plotted on a graph against time.

To be able to work out the mean generation time, the specific growth rate must be determined. This is achieved using the following equation:

µ=(loge Nt - loge No)/t

Where Nt = number at time t=t, No= number at time t=0 and µ= specific growth rate.

So,

µ= 8.228-8.002/50= 0.00452 h-1

Mean generation time is calculated by the following equation:

T= 0.693/µ

Where T = mean generation time and µ = specific growth rate

So,

T= 0.693/0.00452 = 153.3 mins

References

Strohl, WR (1997). Biotechnology of Antibiotics. 2nd edition. Marcel Dekker. New York.

Scotton, PG, Rigoli, R & Vaglia, A (2002). Combination of Quinupristin/Dalfopristin and Glycopeptidein Severe Methicillin-Resistant Staphylococcal Infections Failing Previous Glycopeptide Regimens. Infection. 30(3): 161-163.

Hugo, WB & Russell, AD, (1998). Pharmaceutical microbiology. Blackwell science, UK.

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.