Wild type Escherichia coli forms dark purple colonies (often with a green metallic sheen) on EMB-lactose agar because it ferments lactose vigorously, producing acid that lowers the pH and promotes dye uptake.

Correct answer: Decrease in pH of the medium.

Principle of EMB-Lactose Agar and E. coli

Eosin Methylene Blue (EMB) agar is a selective and differential medium used mainly for Gram-negative enteric bacteria and to differentiate lactose fermenters from non-fermenters.

• The medium contains lactose plus the dyes eosin Y and methylene blue, which act as pH indicators and also inhibit most Gram-positive bacteria.

• Strong lactose fermenters like wild-type E. coli produce large amounts of acid, lower the pH, and cause the dye complex to precipitate into the colonies, making them dark purple to black with a characteristic green metallic sheen.

Explanation of the Correct Option

Decrease in pH of the medium (Correct)

• E. coli ferments lactose and produces organic acids, significantly lowering the pH around the colonies.

• At low pH, eosin and methylene blue form a precipitated dye complex that is taken up by the colonies, making them dark purple (often with a metallic green sheen in strongly acid-producing strains).

• Therefore, the purple coloration directly reflects acid production and hence a decrease in pH of the medium, not an increase.

Why the Other Options Are Wrong

Increase in pH of the medium (Incorrect)

• Non-lactose fermenters or organisms that deaminate peptones tend to increase the pH by oxidative deamination of proteins.

• When the pH rises, the methylene blue–eosin dye complex becomes more soluble and is not taken up by colonies, so they appear colorless or pale, not purple.

• Thus, an increase in pH is associated with colorless or light colonies, not the characteristic purple colonies of E. coli on EMB agar.

Secretion of purple colored pigment (Incorrect)

• The purple color is not due to secretion of a true pigment by E. coli but due to absorption/precipitation of the eosin–methylene blue dye complex into the colony at low pH.

• The dyes are already present in the medium, and their interaction with acid produced during lactose fermentation is what changes color, not bacterial synthesis of a purple metabolite.

• Therefore, calling it “secretion of purple colored pigment” is mechanistically inaccurate for EMB agar.

Secretion of β-galactosidase (Incorrect as the direct cause of purple color)

• E. coli does produce β-galactosidase, which hydrolyzes lactose into glucose and galactose, enabling lactose fermentation.​

• However, the observed purple colony color is a downstream effect of acid production and pH drop, not directly of enzyme secretion; the indicator reaction here is pH-dependent dye uptake, not detection of the enzyme itself.

• So, while β-galactosidase is involved in the pathway, the specific reason for purple colonies on EMB-lactose agar is the decrease in pH, making this option indirectly related but not the correct answer in the context of this question.

Quick Reference Table: Colony Color on EMB Agar

This makes it clear that wild type E. coli forms purple colonies on EMB-lactose agar specifically due to acid-mediated pH decrease in the surrounding medium