Scientists who study amino acid biosynthesis pathwayswant to isolate auxotrophic bacteria. A technique calledpenicillin enrichment makes this task easier. This procedure starts by exposing a liquid culture of wild-type (prototrophic) bacteria growing in rich (complete)medium to a chemical mutagen. After this treatment,the cells are centrifuged to remove the liquid and themutagen. The pellet of cells at the bottom of the centrifuge tube is now resuspended in medium that lacksone amino acid (in this example, cysteine) but contains penicillin. Subsequently, the bacteria are pouredonto a filter that concentrates them and allows themto be washed free of the penicillin. The living bacteriaretained on the filter are highly enriched for cysteineauxotrophs.a. Given what you know about the action of pencillin,explain why this enrichment occurs.b. Penicillin enrichment is not a selection, becausethe drug does not kill 100% of the prototrophs.The cells on the filter thus need to be screenedfor cysteine auxotrophy. How would the scientistsperform this screen?c. If the starting strain contained a penrgene on aplasmid, would this scheme still enrich for auxotrophs? Explain.
Scientists who study amino acid biosynthesis pathways
want to isolate auxotrophic bacteria. A technique called
penicillin enrichment makes this task easier. This procedure starts by exposing a liquid culture of wild-type (prototrophic) bacteria growing in rich (complete)
medium to a chemical mutagen. After this treatment,
the cells are centrifuged to remove the liquid and the
mutagen. The pellet of cells at the bottom of the centrifuge tube is now resuspended in medium that lacks
one amino acid (in this example, cysteine) but contains penicillin. Subsequently, the bacteria are poured
onto a filter that concentrates them and allows them
to be washed free of the penicillin. The living bacteria
retained on the filter are highly enriched for cysteine
auxotrophs.
a. Given what you know about the action of pencillin,
explain why this enrichment occurs.
b. Penicillin enrichment is not a selection, because
the drug does not kill 100% of the prototrophs.
The cells on the filter thus need to be screened
for cysteine auxotrophy. How would the scientists
perform this screen?
c. If the starting strain contained a penr
gene on a
plasmid, would this scheme still enrich for auxotrophs? Explain.
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