Question

You’d like to test whether the same culture of bacterial cells produces lipopolysaccharides. Luckily, there’s a basic chemical test that you can perform to find out. The sample for your test will be an aliquot of the bacterial culture, which is growing in standard LB media. How would you prepare the (a) positive and (b) negative controls for this chemical test?

Verbally OR mathematically communicate the relationships between the intensity of light incident on a sample, the intensity of light transmitted through a sample, absorbance, and transmittance.

Answer 1

Experiment: To test whether the same culture of bacterial cells produce lipopolysaccharides.

Positive control: Inoculate the starting culture into LB broth. Incubate it for sufficient time for the growth of the culture. Take two aliquots from the same culture and perform the chemical test. Since two aliquots are from the same tube, they should show the same result.

Negative control: Inoculate two different bacterial cultures (that are known to produce different lipopolysaccharides or one produces and the other do not) into two different tubes. Incubate them and perform the test. One bacterial culture must show positive result while the other should show the negative result.

Absorbance and transmittance are negatively related to each other.

If absorbance of a sample is high, the intensity of the transmitted light will be low.

If absorbance of a sample is low, the intensity of the transmitted light will be high.

If all the light passes through a sample without any absorption, absorbance = 0 and transmittance = 100%.

If all the light is absorbed by a sample, absorbance = 100% and transmittance = 0%.