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These colonies show pigmentation due to metabolism of X-gal by the B-galactosidase gene in transformed cells. Only transformed cells can grow on the ampicillin-containing medium. After an extended incubation period, so much ampicillin in the medium may become inactivated that non-transformed cells begin to reproduce and form colonies. These unrepressed cells generally form a "halo" of colonies around the original amp-resistant colony, as shown in the figure below. Does this result indicate that ampicillin is bacteriocidal or bacteriostatic toward non-transformed cells? |
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