Problem 2 - Brock Biology of Microorganisms

Question

Explain why an Escherichia coli strain that is His \(^{-}\)is an auxotroph and one that is \(\mathrm{Lac}^{-}\)is not. (Hint: Think about how \(E\). coli metabolizes histidine and lactose and what each compound is used for.) (Section 10.1)

Step-by-Step Solution

Answer

A His ^{-}strain is an auxotroph because it needs external histidine to survive, whereas a Lac ^{-}strain is not, as it can use other carbon sources.

Step 1: Understand Histidine and Lactose

Histidine is an essential amino acid required for protein synthesis in cells. Lactose is a sugar that serves as a source of energy when metabolized. Recognizing these roles is crucial for understanding auxotrophy.

Step 2: Explain Auxotrophy

An auxotroph is an organism that cannot synthesize a certain compound required for its growth and must obtain it from the environment. If a strain is His ^{-}involves histidine biosynthesis, it cannot produce histidine and needs it in the growth medium.

Step 3: Analyze His ^{-}Strain

A His ^{-}strain is deficient in the ability to produce histidine. Since histidine is required for protein synthesis, the inability to synthesize it classifies the strain as an auxotroph, requiring histidine supplementation.

Step 4: Examine Lac ^{-}Strain

A Lac ^{-}strain cannot metabolize lactose. However, as long as other sources of carbon and energy (like glucose) are available, the strain can survive. Thus, it is not dependent on lactose specifically for survival.

Step 5: Compare Dependence

His ^{-} strains are auxotrophs because their survival depends on an external supply of histidine. Lac ^{-} strains, on the other hand, can survive using other carbon sources available in the environment, so they are not considered auxotrophs.

Key Concepts

Protein Synthesis

Protein synthesis is a fundamental process in all living cells. It begins with the transcription of DNA into mRNA, followed by translation where ribosomes decode the mRNA into a specific sequence of amino acids. This sequence forms a protein, which performs various functions essential to life. Histidine, like other amino acids, is a building block in this process. Without sufficient histidine, cells cannot effectively produce proteins. Escherichia coli (E. coli) strains that cannot synthesize histidine (His^{-}) are unable to complete protein synthesis without external histidine supplementation. This biochemical incapacity defines them as auxotrophs.

Carbon Source Metabolism

Carbon source metabolism is the process by which cells extract energy from organic molecules. E. coli can metabolize various carbon sources, including glucose and lactose. Lactose metabolism involves converting lactose into glucose and galactose, which can be further processed via glycolysis and cellular respiration to produce ATP, the cell's energy currency. Strains lacking the ability to metabolize lactose (Lac^{-}) can still thrive if other carbon sources, like glucose, are available. Therefore, they are not considered auxotrophs as they do not depend solely on lactose for survival.

Histidine Biosynthesis

Histidine biosynthesis is a complex biochemical pathway that involves multiple enzymes to convert precursor molecules into histidine. E. coli strains that are His^{-} lack one or more of the enzymes required for this pathway. Since histidine is a vital amino acid for protein synthesis, His^{-} strains must obtain histidine from their environment to grow and function properly. This inability to synthesize an essential growth compound categorizes His^{-} strains as auxotrophs. In contrast, other strains that can synthesize histidine are not dependent on external sources for this amino acid.

Escherichia coli Metabolism

Escherichia coli is a versatile bacterium with a diverse metabolic capability. It can utilize a wide range of carbon sources and synthesize most of its required growth factors. The metabolic pathways of E. coli include glycolysis, the citric acid cycle, and various fermentation processes. However, the metabolic versatility has limits. Strains like His^{-} are auxotrophs due to their inability to synthesize essential compounds like histidine. Lac^{-} strains, on the other hand, do not face this limitation as they can metabolize other carbon sources to meet their energy needs. This distinction helps illustrate why certain metabolic deficiencies impact bacterial survival differently.