Bacillus thuringiensis is used in agriculture as
Answer: A
Bacillus thuringiensis forms a crystalline protein toxin during sporulation that kills specific insect larvae after ingestion while sparing most other organisms. Its genes are also the source of the Bt trait in transgenic crops. It fixes no nitrogen.
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Phosphate solubilising bacteria make soil phosphorus available mainly by
Answer: B
Much soil phosphorus is locked in insoluble calcium, iron or aluminium compounds, and these bacteria release gluconic and other organic acids that lower the local pH and chelate the cations, freeing the phosphate. There is no phosphorus in the atmosphere to fix.
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Plant growth promoting rhizobacteria may help a plant by
Answer: C
Siderophore production is one of several documented mechanisms, alongside hormone production, phosphate solubilisation and antibiotic suppression of pathogens. Bacteria do not photosynthesise for the plant, and one that digested the cortex would be a pathogen rather than a helper.
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The Winogradsky column is used to
Answer: D
Sealed mud, a carbon source and a sulphur source in a glass column develop opposing gradients of oxygen and sulphide over weeks, so distinct layers of phototrophs, sulphur oxidisers and sulphate reducers establish themselves. It is an enrichment device, not a physical or sterilising one.
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The serial dilution and plating method estimates
Answer: A
Counting colonies on a plate from a known dilution gives colony forming units per gram, but only for organisms that grow on the chosen medium. Since most soil organisms do not, the method is known to understate the true population by a very large margin.
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The great plate count anomaly refers to the observation that
Answer: B
Direct microscopic counts of soil routinely exceed plate counts by two or three orders of magnitude, because the great majority of soil organisms will not grow on standard laboratory media. It is the central reason molecular methods displaced cultivation for community surveys.
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The most probable number technique estimates population size from
Answer: C
A set of tubes is inoculated at several dilutions and scored simply for growth or no growth, and the pattern of positives is referred to a statistical table. It suits organisms that are awkward to count on plates, such as nitrifiers, which grow slowly and form no visible colony.
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Soil is usually sterilised in the laboratory by
Answer: D
Bacterial endospores survive boiling, drying and freezing, so only moist heat under pressure reliably destroys them. Soil is commonly autoclaved more than once with intervals between, so that spores which germinate after the first cycle are killed in the next.
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Most soil bacteria grow best at a soil pH that is
Answer: A
Bacterial activity, including nitrification and most nitrogen fixation, peaks close to neutrality. As soil acidifies, fungi come to dominate because they tolerate low pH far better, which is why acidic forest soils are fungally driven.
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As a soil becomes strongly acidic, the balance of the microbial community shifts towards
Answer: B
Fungi tolerate low pH much better than most bacteria, so they take over decomposition in acid soils. Actinomycetes are among the least acid-tolerant groups and nitrifiers slow markedly, which is why acid soils accumulate undecomposed organic matter.
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