13. A bacterial strain has the following features with regards to growth: (i) Will grow on
agar supplemented with Arginine (arg), Tryptophan (trp) and Leucine (leu). It is not
capable of growing on media supplemented with (ii) arg and trp (iii) leu and trp,
however it grows on media supplemented with (iv) arg and leu. What is the genotype
of the bacterial strain?
a. arg-, leu+, trp-
b. leu+, arg- ,trp+
c. leu-, arg-, trp+
d. arg+, leu+, trp-
Bacterial Strain Genotype: Arginine, Tryptophan, Leucine Auxotroph Analysis for CSIR NET
Bacterial auxotrophs cannot synthesize specific amino acids and require supplementation for growth on minimal media. This CSIR NET-style question tests understanding of auxotrophy by analyzing growth patterns on media with arginine (arg), tryptophan (trp), and leucine (leu). The strain grows on arg + trp + leu, arg + leu, but not arg + trp or leu + trp, revealing its genotype as leu-, arg-, trp+.
Growth Pattern Logic
Auxotrophs (marked “-“) need the amino acid supplied; prototrophs (“+”) synthesize it. Growth on all three (arg + trp + leu) confirms no other requirements.
-
No growth on arg + trp: lacks leu biosynthesis (leu-).
-
No growth on leu + trp: lacks arg biosynthesis (arg-).
-
Growth on arg + leu: synthesizes trp (trp+), as trp absent but viable.
Prototrophs for unused supplements do not affect outcomes. Thus, the strain requires arg and leu externally but makes trp.
Option Analysis
| Option | Genotype | Arg+Trp? | Leu+Trp? | Arg+Leu? | Matches? | Reason |
|---|---|---|---|---|---|---|
| a | arg-, leu+, trp- | No (arg-) | No (trp-) | No (trp-) | ❌ | Fails arg+leu growth . |
| b | leu+, arg-, trp+ | No (arg-) | No (arg-) | Yes (trp+) | ✅ | Matches all data . |
| c | leu-, arg-, trp+ | No (leu-) | No (leu-) | Yes (trp+) | ✅ | Equivalent to b (order irrelevant) . |
| d | arg+, leu+, trp- | No (trp-) | No (trp-) | No (trp-) | ❌ | Fails arg+leu growth . |
Option b (or c) fits perfectly; others predict incorrect growth.
