Cell Cultures and Characteristics Questions and Answers

Answer: A

Correct answer: pathogenic properties

A pathogen in a mixed culture can be selected by what it does to a host - inoculating a susceptible animal, from which the organism can then be recovered in pure culture while the harmless ones are eliminated. It is selection by biological property rather than by nutrient or condition.

Answer: D

Correct answer: All of these

All three are enriched media - ordinary medium with an extra nutrient added for fastidious organisms. Blood agar for streptococci and many others, Loeffler’s serum slope for Corynebacterium diphtheriae, and Bordet-Gengou medium, with potato and blood, for Bordetella pertussis.

Answer: D

Correct answer: all of the above

All three count cells. A counting chamber gives a direct microscopic total, live and dead alike; plate counting gives the viable count only, since each colony grew from a living cell; and a membrane filter concentrates organisms from a large dilute volume before they are counted or cultured, which is how water is tested.

Answer: C

Correct answer: Both (a) and (b)

Pour plate and spread plate are both quantitative: a measured volume of a known dilution is plated, the colonies are counted, and multiplying back gives the count in the original sample. The streak plate is qualitative - it separates organisms to obtain pure colonies, but nothing about it measures how many there were.

Answer: A

Correct answer: bile salts

Bile salts are natural detergents present in the intestine, so organisms that live there are adapted to tolerate them while most others are lysed. Adding bile salts to a medium therefore selects for enteric bacteria - the principle of MacConkey and DCA agars.

Answer: B

Correct answer: Microorganism cannot be isolated in pure culture

Koch’s second postulate requires the organism to be grown in pure culture, and many important pathogens cannot be: Treponema pallidum, Mycobacterium leprae, and the viruses and rickettsiae that grow only inside living cells. That is why molecular criteria have been proposed to supplement the postulates.

Answer: A

Correct answer: gram (-)ve intestinal bacteria to grow

Crystal violet inhibits gram-positive organisms and sodium deoxycholate is a bile salt, which intestinal bacteria tolerate and others do not. Between them they select for gram-negative enteric bacteria - the basis of MacConkey and similar media.

Answer: D

Correct answer: Vibrio cholerae

Vibrio cholerae is alkalophilic, growing well up to about pH 9 - which is exploited in alkaline peptone water enrichment. The other three are enterobacteria with neutral optima, and lactobacilli prefer acid.

Answer: A

Correct answer: heterotrophs

Every bacterium living in or on the human body takes its carbon from organic compounds supplied by the host, so all of them are heterotrophs. Autotrophs and chemolithotrophs fix CO₂ and oxidise inorganic compounds, which is a way of life for soil and water organisms, not for inhabitants of a body.

Answer: C

Correct answer: enrichment medium

An enrichment medium is a liquid in which conditions favour the wanted organism and suppress the rest, so that after incubation it dominates the culture and can then be plated out. The corresponding solid medium, which simply prevents unwanted colonies from appearing, is called selective.