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These colonies show pigmentation due to metabolism of X-gal by the Beta-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 will begin to form colonies (white, because they are untransformed) around amp-resistant colonies, as indicated by arrows at higher magnification in the inset image. Does this result indicate that ampicillin is bacteriocidal or bacteriostatic toward non-transformed cells? |
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