UPPSC 2024 Prelims GS Paper-I · Question 64 of 150

Correct — B, Pathogens. The trick to this question is noticing that one of the four options is not the name of a group of organisms at all. Archaea, bacteria and protozoa are taxonomic groups, defined by cell structure and evolutionary descent; membership in them says nothing about whether an organism causes disease. 'Pathogen' is a functional label: it means, by definition, a microorganism that causes disease — this is the term school science introduces for exactly this purpose (NCERT's chapter on microorganisms states that the microorganisms which cause diseases are called pathogens). So of the four options, only (b) is disease-causing one hundred per cent of the time, because that is what the word means. Every other option is disease-causing only for a minority of its members. The reasoning is definitional, not statistical, and that is what makes the answer clean. Test it against the alternatives. Most bacteria are harmless or positively useful: Rhizobium fixes atmospheric nitrogen in the root nodules of pulses, Lactobacillus turns milk into curd, gut bacteria help synthesise vitamin K, and decomposer bacteria return nutrients to the soil. Most protozoa are free-living inhabitants of soil and water — Amoeba and Paramecium harm nobody. And if 'bacteria' were the answer, every viral disease (polio, dengue, influenza, COVID-19) and every fungal disease (ringworm, athlete's foot) would have to be excluded from the category of disease-causing microorganisms, which is plainly wrong. Only the word 'pathogen' covers all of them.

• (a)Archaea — The most clearly wrong option in the set. Archaea are single-celled prokaryotes forming a separate domain of life, recognised as distinct from bacteria by Carl Woese in 1977; their cell walls lack peptidoglycan and their membrane lipids are ether-linked, both differences from bacteria. They are best known as extremophiles — methanogens in marshes, paddy fields and the guts of ruminants; halophiles in salt pans; thermoacidophiles in hot springs and volcanic vents. No archaeon has been conclusively established as the causative agent of a human disease, so as a group they are the opposite of a natural answer to a question about disease-causing microbes.
• (c)Bacteria — The most tempting wrong answer, because the diseases a student can name most readily — tuberculosis (Mycobacterium tuberculosis), cholera (Vibrio cholerae), typhoid (Salmonella Typhi), tetanus, plague — are bacterial. But 'bacteria' names a domain of prokaryotic organisms defined by their cell structure, and the great majority of its members are harmless or beneficial. A group whose members merely include some pathogens is not itself 'the disease-causing microorganism'; the disease-causing ones within it are precisely the ones we call pathogenic bacteria.
• (d)Protozoa — Protozoa are single-celled eukaryotes, most of them free-living in soil, fresh water and the sea — Amoeba, Paramecium, Euglena. A minority are pathogens and they cause some of India's most serious diseases: Plasmodium causes malaria, Entamoeba histolytica causes amoebic dysentery, Leishmania donovani causes kala-azar. Once again the group is defined by its biology, not by causing disease, so it fails for the same reason as bacteria.

Microorganisms are organisms too small to be seen without a microscope, and they fall into several quite different groups — bacteria and archaea (prokaryotes), protozoa and many algae and fungi (eukaryotes), with viruses treated separately because they are not cellular and can reproduce only inside a host cell. Cutting across this classification is a functional question: what does the organism do to a host? Those that cause disease are called pathogens, and they occur in almost every group — bacteria, viruses, fungi and protozoa all have pathogenic members. The vast majority of microorganisms, however, are harmless or actively useful: they fix nitrogen, ferment food, produce antibiotics, and decompose dead matter to recycle nutrients.

Whenever an option set mixes taxonomic names with a functional term, the functional term is usually the answer to a functional question. Here the question asks which is 'disease-causing', which is a function; three options answer 'what kind of organism is it?' and only one answers 'what does it do?'. The rigorous version of this idea is Koch's postulates, formulated by Robert Koch in the 1880s: to call a microbe the pathogen of a disease you must find it in every case of the disease, isolate it in pure culture, reproduce the disease by introducing that culture into a healthy host, and recover the same microbe from that host. That set of criteria is the operational definition of a pathogen, and it is worth learning alongside the word because UPSC has repeatedly asked about germ theory, vaccines and disease causation.

• A pathogen is any microorganism that causes disease — a functional category, not a taxonomic group, which is why it can be answered with certainty while 'bacteria' or 'protozoa' cannot.
• Pathogens occur across groups: bacteria (tuberculosis, cholera, typhoid), viruses (polio, influenza, dengue, COVID-19), fungi (ringworm, athlete's foot) and protozoa (malaria, kala-azar, amoebic dysentery).
• Koch's postulates, set out by Robert Koch in the 1880s, are the criteria for establishing that a particular microorganism causes a particular disease — the operational test of pathogenicity.
• Archaea form the third domain of life alongside Bacteria and Eukarya (Carl Woese, 1977); they include methanogens, halophiles and thermoacidophiles, and no archaeon has been conclusively shown to cause human disease.
• Most microorganisms are useful: Rhizobium fixes nitrogen in the root nodules of pulses, Lactobacillus sets curd, yeast leavens bread and ferments alcohol, Penicillium yields penicillin, and decomposers recycle nutrients through the ecosystem.

• Choosing 'bacteria' because most familiar diseases are bacterial. The question asks for the disease-causing category; a taxonomic group that merely contains some pathogens is not that category.
• Assuming all microorganisms are harmful. The overwhelming majority are neutral or beneficial, and exam items on nitrogen fixation, curd and antibiotics test exactly that.
• Forgetting that viruses and fungi are pathogens too. Any option restricted to one taxonomic group leaves out entire classes of disease.

UPPSC asks either the bare definition, as here, or a matching list pairing a disease with its causative agent — its 2023 paper matched peptic ulcer, dengue, elephantiasis and malaria to bacteria, virus, worm and protozoa. UPSC prefers the single-disease form ('Athlete's foot is caused by') or multi-statement items on microbes, antibiotic resistance and vaccines.

• practice — not a real PYQ

  Which one of the following diseases is caused by a protozoan?

  • (a)Tuberculosis
  • (b)Ringworm
  • (c)Malaria
  • (d)Influenza

  Answer(c) Malaria — caused by the protozoan Plasmodium, transmitted by the female Anopheles mosquito. Tuberculosis is bacterial (Mycobacterium tuberculosis), ringworm is fungal and influenza is viral.

• practice — not a real PYQ

  The set of criteria used to establish that a particular microorganism is the cause of a particular disease is known as :

  • (a)Mendel's laws
  • (b)Koch's postulates
  • (c)Pasteur's principle
  • (d)Lamarck's postulates

  Answer(b) Koch's postulates — framed by Robert Koch in the 1880s, they require the microbe to be found in every case, isolated in pure culture, able to reproduce the disease in a healthy host, and recoverable from that host.