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Indirect selection of bacterial mutants recovers a source using a related sample's tested phenotype, rather than directly challenging the retained lineage. A positive example uses a surviving replica biological colony to locate its unexposed source. A negative example uses failure to grow in one condition to locate a source that does grow elsewhere. The second is particularly useful for finding auxotrophy: dependence on an externally supplied nutrient.

Construct a four-by-four teaching layout with rows A–D and columns 1–4. The master has 13 biological colonies; A4, D1 and D4 are empty. A growth-supporting control replica grows at every occupied master site except B2 and C3. A replica lacking an assumed nutrient X grows at those same control sites except A2 and C1. All other requirements are stipulated equal. These are invented readouts, not observations from the Lederberg replica plating experiment.

Let $M$, $C$ and $S$ be the finite sets of sites with growth on the master, control and X-omission replica respectively. In this construction $S\subseteq C\subseteq M$, with cardinalitiesThe set differencecontains the two condition-dependent candidates. Pick from A2 and C1 on the master, then compare fresh samples of each recovered source with and without X. Restoration of growth when X is supplied supports an X-requirement phenotype, subject to the scientific controls; growth alone does not identify a particular gene or mutation.

By contrast,is unresolved sampling or viability in this first comparison. Neither site should be counted as an X-requirement candidate. The three sites outside $M$ have no master source to recover at all. The short logical argument is membership testing: a candidate must belong to $C$, establishing control growth, and not belong to $S$, establishing the contrasting readout. Being outside $S$ alone supplies only the second condition. Among the seven empty X-omission sites, two are candidates, two are unresolved and three have no master source. The beauty of the method is that a shared location turns this logical distinction into a practical retrieval route.

Joshua Lederberg and Esther Lederberg reported replica plating in 1952. They autoclave-sterilized velveteen and secured it pile-up on a wood or cork cylinder with a metal hoop. Inverted agar plates touched the fabric lightly. Pencil marks or pin indentations fixed orientation. Complete-medium and minimal-medium replicas compared growth of biological colonies.^[ref]

Escherichia coli W-1 grew on plain agar; serial T-1 bacteriophage replicas showed resistant biological colonies at matching sites. Corresponding unexposed master regions seeded repeated broth-and-plain-agar enrichment, yielding resistant isolates. Stray-bacteriophage checks were negative; resistance persisted without challenge. This supported preexisting heritable resistance. The authors noted variable transfer and moisture-dependent resolution.^[ref]

The Luria-Delbrück experiment uses variation between independently grown cell cultures to investigate the timing of resistant mutations. The Lederberg replica plating experiment adds a spatial route to a recoverable source: the tested samples identify an unexposed lineage. These are complementary arguments about mutation and natural selection. Growth readouts establish a tested phenotype; identifying the responsible nucleotide sequence requires other evidence, such as DNA sequencing.