59. Limulus Amoebocyte Lysate (LAL) assay is used to identify the presence of
(A) endotoxin
(B) exotoxin
(C) anthrax toxin
(D) tetanus toxin

The Limulus Amoebocyte Lysate (LAL) assay serves as a critical tool in microbiology and pharmaceutical quality control for identifying bacterial contamination. Derived from horseshoe crab blood cells, it triggers a clotting reaction specifically with endotoxins.​

Correct Answer

(A) endotoxin

The LAL assay precisely detects endotoxins, which are lipopolysaccharides (LPS) in the outer membrane of gram-negative bacteria. This reaction forms a gel clot, enabling sensitive quantification in drugs, medical devices, and biologics to prevent pyrogenic reactions.​

Option Explanations

Endotoxin (A) – Correct

Endotoxins are heat-stable components released upon gram-negative bacterial cell lysis. LAL’s enzymatic cascade activates upon LPS binding, producing measurable turbidity or color change in variants like chromogenic or turbidimetric tests.​

Exotoxin (B) – Incorrect

Exotoxins are secreted, heat-labile proteins from both gram-positive and gram-negative bacteria, such as botulinum toxin. LAL does not react with these protein-based toxins, as its mechanism targets LPS specifically.​

Anthrax Toxin (C) – Incorrect

Anthrax toxin, produced by Bacillus anthracis (gram-positive), consists of protective antigen, edema factor, and lethal factor. As an exotoxin complex, it evades LAL detection, requiring alternative assays like ELISA.​

Tetanus Toxin (D) – Incorrect

Tetanus toxin (tetanospasmin) is a potent neurotoxin secreted by Clostridium tetani (gram-positive). LAL fails to detect it due to its protein nature and lack of LPS structure.​

LAL Assay Applications

Pharmaceutical industries rely on LAL for bacterial endotoxin testing (BET) in injectables and implants. Regulatory bodies like the FDA and USP endorse its use, with sensitivity down to 0.001 EU/mL in kinetic methods.​

This MCQ tests core microbiology knowledge, especially for exams in biotechnology or pharmacy, emphasizing endotoxin-specific detection over other toxins.​