Paper I — 2019 July (Supplementary) (2010 Scheme) — Q2
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Paper I
2019 July (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Bacterial spore and its demonstration
Q25 marksShort Essays
Answer
Bacterial spore: a dormant, highly resistant, metabolically inactive form produced by certain Gram-positive bacteria (chiefly Bacillus and Clostridium species) under adverse environmental conditions (nutrient depletion, desiccation, heat) — a survival mechanism, not a means of reproduction (one vegetative cell produces one spore, and one spore germinates back into one vegetative cell).
Structure (outside in): exosporium (outermost, loose covering, not always present), spore coat (keratin-like protein, resistant to chemicals/enzymes), cortex (peptidoglycan layer, important in heat resistance and dehydration), core wall, and the core/protoplast (DNA, ribosomes, very low water content). Resistance derives mainly from a high concentration of calcium dipicolinate in the core (stabilizing DNA/proteins) and the very low core water content.
Location within the cell and whether it distorts (bulges) the cell wall vary by species and are used as identifying features:
Central — e.g., Bacillus anthracis (does not bulge the cell, giving a “box-car” appearance).
Subterminal — e.g., Clostridium tetani variants, C. perfringens.
Ordinary/simple stains do not stain spores well, since the thick, impermeable coat resists dye penetration — spores appear as clear, unstained, refractile areas within an otherwise stained bacterial cell body on a Gram stain, providing an indirect clue to their presence and location.
Special spore stains — e.g., malachite green (Schaeffer-Fulton method): the smear is heated with malachite green (heat helps drive the dye through the resistant spore coat), which stains the spore green; the slide is then counterstained with safranin, which stains the vegetative cell body/background red-pink — spores appear green within red/pink vegetative cells, giving clear visual differentiation.
Spores can also be inferred by resistance to standard heat treatment that kills vegetative cells (a functional/indirect method used in microbiological validation, not routine clinical staining).
