Microbiology - MCQ Practice Questions
Microbiology sits behind a lot of applied biology, so the fundamentals here carry into medicine, food technology and biotechnology alike. Practice covers bacterial structure and growth, viruses, fungi and parasites, sterilisation and culture techniques, immunology, and microbial genetics. Technique based questions explain the purpose of each step, because that is usually what separates a memorised protocol from a usable one.
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Which of the following processes describes the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) by microorganisms?
Understanding:
We need to identify which microbial process converts atmospheric nitrogen gas into ammonia.
Step 1: Define each option
Nitrification is the oxidation of ammonia to nitrite and then nitrate, carried out by bacteria such as Nitrosomonas and Nitrobacter.
Step 2: Evaluate denitrification
Denitrification is the reduction of nitrate (NO₃⁻) back to nitrogen gas (N₂) under anaerobic conditions, carried out by organisms like Pseudomonas denitrificans.
Step 3: Evaluate biological nitrogen fixation
Biological nitrogen fixation is the process by which diazotrophic microorganisms (e.g., Rhizobium, Azotobacter, cyanobacteria) use the enzyme nitrogenase to reduce atmospheric N₂ to NH₃, making nitrogen available to living organisms.
Step 4: Evaluate ammonification
Ammonification (mineralisation) is the decomposition of organic nitrogen-containing compounds (proteins, nucleic acids) into ammonia by decomposer microorganisms.
Answer:
The conversion of atmospheric N₂ into NH₃ by microorganisms is called biological nitrogen fixation.
Quick Tip:
The enzyme nitrogenase, which is irreversibly inactivated by oxygen, is the key enzyme in biological nitrogen fixation — a common exam fact.
Thiobacillus thiooxidans is used in the bioleaching of metal sulfide ores. Which characteristic of this bacterium makes it uniquely suited for this environment?
Understanding:
We need to identify the physiological trait of Thiobacillus thiooxidans that enables it to participate in bioleaching of metal sulfide ores.
Step 1: Understand bioleaching
Bioleaching involves the use of microorganisms to solubilise metal ions from insoluble ores. Metal sulfide ores (e.g., pyrite, chalcopyrite) are oxidised to release soluble metal ions.
Step 2: Characteristics of Thiobacillus thiooxidans
This bacterium is an obligate acidophile, growing optimally at pH 2–3.5. It is a chemolithoautotroph that derives energy by oxidising reduced sulfur compounds such as elemental sulfur (S⁰) and thiosulfate to sulfuric acid. The acid produced further dissolves the ore, releasing metal ions.
Step 3: Eliminate incorrect options
It is not an anaerobe — it requires oxygen for sulfur oxidation. It is not a heterotroph nor a nitrogen fixer, and it does not thrive at neutral pH.
Answer:
Thiobacillus thiooxidans is an acidophilic chemolithoautotroph that oxidises reduced sulfur compounds, generating sulfuric acid that facilitates metal solubilisation.
Quick Tip:
In bioleaching, Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans) oxidises both Fe²⁺ and S⁰, while Thiobacillus thiooxidans specialises in sulfur oxidation — distinguishing them is a common exam point.
In the nitrogen cycle, which group of bacteria is responsible for the oxidation of nitrite (NO₂⁻) to nitrate (NO₃⁻)?
Understanding:
We need to identify which bacterial group carries out the second step of nitrification — the oxidation of nitrite to nitrate.
Step 1: Understand nitrification
Nitrification is a two-step aerobic process:
Step 2: Evaluate each option
Nitrosomonas performs the first step (NH₃ → NO₂⁻). Azotobacter is a free-living nitrogen-fixing aerobe. Clostridium is an anaerobic nitrogen fixer/denitrifier. Nitrobacter performs the second step (NO₂⁻ → NO₃⁻) using nitrite oxidoreductase.
Step 3: Confirm
Nitrobacter is the classic example of a nitrifying bacterium responsible for nitrite oxidation to nitrate in the soil.
Answer:
Nitrobacter oxidises nitrite to nitrate in the second stage of nitrification.
Quick Tip:
A useful mnemonic: Nitrosomonas → 'so' refers to the starting step; Nitrobacter → 'b' for the 'bigger' oxidation product, nitrate.
Biochemical Oxygen Demand (BOD) is a key parameter used to assess water quality. Which of the following statements about BOD is correct?
Understanding:
We need to identify the correct statement describing Biochemical Oxygen Demand (BOD) and its significance in water quality assessment.
Step 1: Define BOD
BOD is defined as the amount of dissolved oxygen (in mg/L) consumed by microorganisms during the aerobic decomposition of organic matter present in a water sample over a specified time period.
Step 2: Evaluate each option
A high BOD indicates heavy organic pollution, meaning microorganisms are consuming large amounts of oxygen — this depletes dissolved oxygen, harming aquatic life. So option A is incorrect.
Step 3: Standard BOD measurement conditions
The standard BOD test (BOD₅) is conducted at 20°C (not 37°C) for 5 days. Option C is incorrect regarding the temperature.
Step 4: Scope of BOD
BOD measures oxygen consumed during biological (microbial) decomposition of organic compounds only. It does not measure inorganic pollutants. Option D is therefore incorrect.
Step 5: Confirm option B
BOD correctly measures the oxygen demand of microorganisms decomposing organic matter aerobically, and is a direct indicator of the level of organic pollution in water.
Answer:
BOD measures the oxygen consumed by microorganisms during aerobic decomposition of organic matter in water.
Quick Tip:
Standard BOD₅ test: 20°C, 5 days. Clean drinking water has BOD < 1 mg/L; heavily polluted water can have BOD > 200 mg/L.
Mycorrhizal associations between fungi and plant roots are crucial in ecosystem nutrient cycling. Which of the following correctly describes ectomycorrhizae?
Understanding:
We need to identify the correct structural description of ectomycorrhizal associations.
Step 1: Distinguish ecto- from endomycorrhizae
Endomycorrhizae (arbuscular mycorrhizae, AM) involve fungal hyphae that penetrate the root cortical cells and form branched structures called arbuscules inside the cells. These fungi belong to the phylum Glomeromycota.
Step 2: Describe ectomycorrhizae
Ectomycorrhizae are characterised by:
Step 3: Eliminate incorrect options
Option A describes endomycorrhizae. Option C is incorrect — ectomycorrhizae are associated with woody perennial trees, not herbaceous crops. Option D is incorrect — ectomycorrhizal fungi are Basidiomycetes/Ascomycetes, not Zygomycetes.
Answer:
Ectomycorrhizae form a fungal mantle around the root and a Hartig net between root cells without intracellular penetration.
Quick Tip:
Key distinction: Ecto = OUTSIDE and BETWEEN cells (mantle + Hartig net); Endo = INSIDE cells (arbuscules and vesicles).
Which of the following best describes the role of sulfur-reducing bacteria such as Desulfovibrio in the sulfur cycle?
Understanding:
We need to identify the specific metabolic role of sulfate-reducing bacteria (SRB) like Desulfovibrio in the sulfur cycle.
Step 1: Role of Desulfovibrio
Desulfovibrio is an anaerobic bacterium that uses sulfate (SO₄²⁻) as a terminal electron acceptor in its anaerobic respiration (dissimilatory sulfate reduction). Organic compounds or hydrogen gas serve as electron donors, and SO₄²⁻ is reduced to hydrogen sulfide (H₂S).
Step 2: Significance
This process is critical in anoxic environments (waterlogged soils, marine sediments) and contributes to:
Step 3: Eliminate incorrect options
Option A describes the role of phototrophic sulfur bacteria (e.g., Chlorobium) or chemolithotrophs. Option C is not a recognised biological process. Option D describes Thiobacillus thiooxidans, not Desulfovibrio.
Answer:
Desulfovibrio reduces sulfate to hydrogen sulfide under anaerobic conditions using sulfate as a terminal electron acceptor.
Quick Tip:
Sulfate-reducing bacteria are important in diagenesis and are responsible for the characteristic smell of marsh mud and anaerobic sediments.
The Multiple Tube Fermentation (Most Probable Number, MPN) method is commonly used to detect fecal contamination in water. Which organism serves as the primary indicator of fecal contamination in this method?
Understanding:
We need to identify the standard bacterial indicator used in the MPN method to detect fecal contamination in water samples.
Step 1: Principle of indicator organisms
An ideal indicator organism for fecal contamination must be:
Step 2: Why Escherichia coli
E. coli is the gold standard fecal indicator organism because:
Step 3: Eliminate other options
Salmonella typhi and Vibrio cholerae are actual pathogens, not routine indicators. Clostridium perfringens is sometimes used as a secondary indicator for older or treated contamination but is not the primary indicator in the standard MPN method.
Answer:
Escherichia coli is the primary fecal indicator organism used in the MPN method for assessing water quality.
Quick Tip:
The MPN method provides a statistical estimate of bacterial density. Three stages: Presumptive (lactose broth) → Confirmed (brilliant green bile broth) → Completed (EMB agar for E. coli).
In composting, the thermophilic phase is critical for pathogen destruction and decomposition. Which temperature range defines the thermophilic phase of composting?
Understanding:
We need to identify the temperature range that characterises the thermophilic phase in the composting process.
Step 1: Phases of composting
Composting progresses through distinct microbial phases:
Step 2: Importance of the thermophilic phase
Temperatures of 45°C – 70°C are sufficient to:
Step 3: Eliminate incorrect options
10°C–25°C is below mesophilic range. 25°C–40°C is the mesophilic range. 80°C–100°C is too high and would kill most beneficial decomposer microorganisms.
Answer:
The thermophilic phase of composting occurs between 45°C and 70°C.
Quick Tip:
For compost to be certified as sanitised (safe for agricultural use), it must reach at least 55°C for a minimum of 3 consecutive days — a regulatory standard worth remembering.
Biofilms are structured communities of microorganisms encased in a self-produced extracellular polymeric substance (EPS). Which of the following statements about biofilms is correct?
Understanding:
We need to identify the correct statement regarding the biology and properties of microbial biofilms.
Step 1: Stages of biofilm formation
Biofilm formation follows a sequential process:
1. Reversible attachment of planktonic cells to a surface.
2. Irreversible attachment — cells produce EPS (polysaccharides, proteins, nucleic acids) anchoring them firmly.
3. Microcolony formation and maturation into a structured community.
4. Dispersal — cells detach and resume planktonic lifestyle.
Step 2: Evaluate each option
Option A is incorrect — biofilm bacteria are 10–1000 times MORE resistant to antibiotics than planktonic cells due to the EPS barrier, slow growth rates, and presence of persister cells.
Step 3: Role of quorum sensing
Option C is incorrect — quorum sensing (cell-to-cell chemical signalling using autoinducers) plays a central role in regulating biofilm formation, maturation, and dispersal.
Step 4: Taxonomic scope
Option D is incorrect — biofilms are formed by both Gram-positive (e.g., Staphylococcus) and Gram-negative (e.g., Pseudomonas aeruginosa) bacteria, as well as fungi and mixed-species communities.
Step 5: Confirm option B
Irreversible attachment to a surface followed by EPS production is correctly described as the initiating committed step of biofilm formation.
Answer:
Biofilm formation is initiated by irreversible surface attachment of planktonic cells followed by EPS production.
Quick Tip:
Pseudomonas aeruginosa is the model organism for biofilm research. Its biofilms in the lungs of cystic fibrosis patients are notoriously resistant to antibiotic therapy.
Which of the following correctly describes the process of bioremediation using microorganisms in contaminated environments?
Understanding:
We need to identify the correct statement describing the process and types of bioremediation.
Step 1: Define bioremediation
Bioremediation is the use of living microorganisms (bacteria, fungi, algae) to degrade or detoxify pollutants in contaminated soil, water, or air, converting them to less harmful or harmless products.
Step 2: Types of bioremediation
Step 3: Evaluate each option
Option A is incorrect — bioremediation uses biological agents (microorganisms), not physical/chemical methods alone.
Option B is incorrect — phytoremediation uses plants (sometimes in conjunction with rhizosphere microbes), and bioremediation uses microorganisms; they are distinct processes.
Option D is incorrect — bioremediation is highly effective for organic pollutants (e.g., petroleum hydrocarbons, pesticides, chlorinated solvents). For heavy metals, microbes facilitate biotransformation or biosorption, not degradation.
Step 4: Confirm option C
Intrinsic bioremediation correctly describes the natural, unassisted degradation of contaminants by indigenous microbial communities.
Answer:
In intrinsic bioremediation, naturally occurring indigenous microorganisms degrade contaminants without human intervention.
Quick Tip:
Pseudomonas putida was the first patented organism (1980, Chakrabarty case) — it was engineered to degrade petroleum hydrocarbons, a landmark in bioremediation history.