Cell Culture Techniques Questions and Answers

Answer: C

Both are true and the reason gives the difficulty: an undefined and variable ingredient makes it hard to attribute a result to the treatment rather than to the medium, so a defined formulation removes one source of doubt.

Answer: D

Correct answer: a bicarbonate system needing carbon dioxide

Most media are buffered by bicarbonate, and that buffer only holds its pH in equilibrium with a defined partial pressure of carbon dioxide - typically five per cent. In ordinary air the dissolved carbon dioxide escapes and the medium turns alkaline, which the phenol red indicator shows as a shift towards purple. Media buffered with HEPES instead can be handled outside an incubator for the same reason.

Answer: A

A filtered stream of air moving across the work surface keeps microorganisms away from the open vessels. Temperature and gas are controlled by the incubator instead. The hood protects the culture, and in some designs the worker as well.

Answer: B

These very small bacteria cause no turbidity and are not visible in an ordinary microscope, yet they alter cell metabolism and behaviour. Regular specific testing is the only reliable way to find them. Many published results have been traced to undetected mycoplasma.

Answer: C

The organisms are far too small to make the medium turbid, so a heavily contaminated culture can look entirely normal. This is exactly what makes them a serious problem for reproducible work. Absence of visible signs is not evidence of a clean culture.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the necessity. Mycoplasma are too small to see by light microscopy, do not cloud the medium and do not kill the cells, so an infected culture looks entirely normal - while its metabolism, gene expression and drug responses are all altered. Nothing about routine observation would reveal it, so periodic testing by PCR or staining is the only defence.

Answer: A

Yellow indicates acidity, usually from cells respiring heavily or from bacterial contamination, and purple indicates alkalinity from loss of carbon dioxide. It gives an immediate visual check without opening the flask. A quick glance at the colour tells an experienced worker a great deal.

Answer: B

Phenol red is yellow below about pH 6.8, and the usual causes are cells that need feeding or a bacterial contamination. Either way the flask requires attention promptly. Purple at the other extreme suggests carbon dioxide has been lost.

Answer: C

The flask holds 10 million cells in total, and a tenth of those is 1 million. Split ratios are quoted in terms of the fraction transferred rather than the final volume. A one in ten split therefore discards nine tenths of the cells.

Answer: D

Reaching sixteen times the starting number needs four doublings, since two to the fourth is 16, and four doublings at one a day is four days. Cultures are usually split before they reach that density. Exponential growth means the last doubling adds the most cells.