GATE 2025 BT Q.11: solution and answer

Official answer: BSource: Official answer key, GATE 2025 BT (IIT Roorkee)

1. Koch's postulates link one specific microbe to one specific disease. The microbe must be present in every case of the disease, be isolated and grown in pure culture, cause the same disease when put into a healthy host, and be re-isolated from that host.

2. Koch's first demonstration of this chain came in 1876 with anthrax, a disease of cattle and sheep. He found rod-shaped bacteria in the blood of diseased animals, grew them in pure culture, and showed that the culture gave anthrax to healthy mice. The organism is $\text{Bacillus anthracis}$. This work is the origin of the postulates, so option (B) is correct.

3. Option (A): $\text{Mycobacterium tuberculosis}$ was identified by Koch in 1882, six years after the anthrax work. The criteria were applied to tuberculosis and stated formally in the 1880s, but the disease on which the approach was first established was anthrax. So (A) is the tempting wrong option.

4. Option (C): $\text{Streptococcus pneumoniae}$ was first isolated in 1881 by Pasteur and, independently, by Sternberg. It is not the organism with which Koch established his method.

5. Option (D): $\text{Bacillus subtilis}$ is a harmless soil bacterium and not a pathogen, so no disease could be tied to it by the postulates.