Question

Diphtheria is caused by
(a) Poisons released by living bacterial cells into the host tissue
(b) Poisons released from dead bacterial cells into the host tissue
(c) Poisons released by virus into the host tissue
(d) Excessive immune response by the host’s body

Answer 1

Hint: Naturally, bacteria do not have toxins for this disease. When they get infected by a bacteriophage, they synthesize and release the toxin that affects the throat of the host causing diphtheria.

Complete answer:
Though diphtheria is called a disease caused by the infection of bacterium Corynebacterium diphtheriae, the bacterium naturally does not have a toxin that causes this disease. When a bacteriophage infects the bacterium, it integrates the toxin encoding genetic element to the bacterium and then this toxin is produced by the bacterium. Thus, the bacterium is not dead due to the infection of bacteriophage and continues to produce the toxin that causes diphtheria.
There are two peptide chains in the diphtheria toxin and these two are held together with the help of a disulfide bond. The two chains are named as chain A and chain B where the chain B peptide is responsible for providing entry of the toxin to the host cell. It also creates pores in the endosome membrane of the host cell and then A chain is released into the cytoplasm of the host cell.
So, the correct answer is ‘Poisons released by living bacterial cells into the host tissue’.

Note: The other options are inappropriate because if bacteria are dead, it can’t divide and synthesize the toxic peptide. Also, the virus i.e. bacteriophage is not damaging the host cells directly by releasing any toxin, it is just infecting bacteria and forcing it to make toxins as per the information encoded in the viral genome.