Microbes in Water Practice Questions
20 free Microbes in Water practice questions for the Microbiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following is the most commonly used indicator organism for fecal contamination in drinking water?
- A Salmonella typhi
- B Escherichia coli
- C Vibrio cholerae
- D Shigella dysenteriae
Correct answer: Escherichia coli
E. coli is the standard indicator organism because it is a natural inhabitant of the intestines of warm-blooded animals. Its presence in water consistently indicates recent fecal contamination and the potential presence of other enteric pathogens.
The 'Membrane Filter Technique' for water analysis involves filtering a water sample through a membrane with a pore size of:
- A 0.22 μm
- B 0.45 μm
- C 1.2 μm
- D 5.0 μm
Correct answer: 0.45 μm
A pore size of 0.45 μm is standard for trapping coliform bacteria while allowing the water to pass through. Once trapped, the membrane is placed on selective media to allow the bacteria to grow into visible colonies.
Which waterborne pathogen is famously resistant to standard chlorination due to its protective oocyst wall?
- A Campylobacter jejuni
- B Cryptosporidium hominis
- C Legionella pneumophila
- D Mycobacterium avium complex
Correct answer: Cryptosporidium hominis
Cryptosporidium produces thick-walled oocysts that are highly resistant to chemical disinfectants like chlorine. Outbreaks often require advanced filtration or UV treatment to ensure safety.
What is the primary characteristic of 'Coliform' bacteria used in water quality testing?
- A Gram-positive cocci that ferment glucose and produce acid
- B Gram-negative bacilli that ferment lactose with gas production
- C Acid-fast bacilli that grow at 4°C and resist decolorization
- D Strictly anaerobic spore-forming rods that reduce sulfate to hydrogen sulfide
Correct answer: Gram-negative bacilli that ferment lactose with gas production
Coliforms are defined as aerobic or facultatively anaerobic, Gram-negative, non-spore-forming bacilli that ferment lactose to produce acid and gas within 48 hours at 35°C. This metabolic profile allows for rapid presumptive testing.
In the 'Multiple-Tube Fermentation Method', the 'Presumptive Test' specifically looks for the fermentation of which medium?
- A Lauryl tryptose broth
- B Brilliant green lactose bile broth
- C Eosin methylene blue agar
- D Blood agar
Correct answer: Lauryl tryptose broth
The presumptive phase uses Lauryl tryptose broth (or MacConkey broth) to screen for organisms that can ferment lactose. If gas is produced in the Durham tubes, the sample is considered presumptive positive for coliforms.
Which organism is the causative agent of 'Pontiac Fever' and 'Legionnaires' Disease', often spread through aerosolized water?
- A Pseudomonas aeruginosa
- B Aeromonas hydrophila
- C Mycobacterium marinum
- D Legionella pneumophila
Correct answer: Legionella pneumophila
Legionella pneumophila thrives in man-made water systems like cooling towers and hot tubs. It is transmitted to humans via the inhalation of contaminated aerosols rather than by drinking the water.
The Presence-Absence (P-A) test is a simplified water quality test that is most appropriate for:
- A Highly polluted river water
- B Estimating the total microbial biomass in soil
- C Identifying specific viral serotypes
- D Routine monitoring of treated drinking water
Correct answer: Routine monitoring of treated drinking water
The P-A test is a qualitative 'yes/no' screen used for samples that are expected to be free of coliforms, such as finished drinking water. It is not suitable for determining the degree of pollution in highly contaminated sources.
Which of the following is an example of an 'autochthonous' microorganism in a freshwater ecosystem?
- A Escherichia coli
- B Salmonella enterica
- C Caulobacter crescentus
- D Clostridium perfringens
Correct answer: Caulobacter crescentus
Autochthonous microbes are indigenous or native to the specific habitat. Caulobacter is a specialized aquatic bacterium with a stalk for attachment, whereas E. coli and Salmonella are transient (allochthonous) organisms introduced via contamination.
Eutrophication in water bodies, often leading to harmful algal blooms, is primarily triggered by an excess of:
- A Dissolved oxygen
- B Sodium chloride
- C Dissolved iron and manganese
- D Nitrogen and phosphorus
Correct answer: Nitrogen and phosphorus
Nutrient enrichment (nitrogen and phosphorus) from agricultural runoff or sewage stimulates the rapid growth of algae and cyanobacteria. As these blooms die and decompose, oxygen is depleted, leading to 'dead zones'.
Which waterborne virus is a leading cause of acute gastroenteritis outbreaks on cruise ships and in institutional settings?
- A Hepatitis A virus
- B Rotavirus
- C Norovirus
- D Adenovirus
Correct answer: Norovirus
Norovirus is highly contagious and stable in water and on surfaces. It requires a very low infectious dose to cause disease and is frequently implicated in waterborne and foodborne outbreaks in crowded environments.
The 'Completed Test' in the standard coliform analysis involves which procedure?
- A Observation of gas in Lauryl tryptose broth
- B Streaking colonies onto Eosin Methylene Blue agar and observing for a diagnostic metallic green sheen
- C Re-inoculation of gas-positive tubes into lactose broth and Gram staining
- D Measuring the turbidity of the sample using a spectrophotometer
Correct answer: Re-inoculation of gas-positive tubes into lactose broth and Gram staining
The Completed Test confirms that the organisms isolated are indeed Gram-negative, non-spore-forming bacilli that ferment lactose. This is the final step to ensure the results of the previous presumptive and confirmed tests were accurate.
Biological Oxygen Demand (BOD) is a measure of:
- A The total amount of oxygen dissolved in a water sample
- B The total oxygen generated by photosynthetic aquatic plants and algae during daylight hours
- C The rate at which oxygen diffuses from the air into the water
- D The amount of oxygen required by microbes to decompose organic matter
Correct answer: The amount of oxygen required by microbes to decompose organic matter
BOD measures the amount of dissolved oxygen consumed by microorganisms as they break down organic material in a water sample over a specific period (usually 5 days). High BOD indicates high levels of organic pollution.
Which protozoan parasite is characterized by 'trophozoites' that look like they have a face with two nuclei and move with a 'falling leaf' motion?
- A Entamoeba histolytica
- B Balantidium coli
- C Acanthamoeba
- D Giardia lamblia
Correct answer: Giardia lamblia
Giardia lamblia (or G. duodenalis) is a flagellated protozoan that causes giardiasis. Its trophozoite stage has a characteristic pear shape and two nuclei, giving it a face-like appearance under the microscope.
Fecal streptococci (Enterococci) are often used as secondary indicators of water quality because they:
- A Are only found in human waste
- B Are much easier to culture than E. coli
- C Produce distinctive visible pigments on all commonly used culture media
- D Survive longer in marine environments than coliforms
Correct answer: Survive longer in marine environments than coliforms
Enterococci are more resistant to environmental stress and salinity than coliforms. This makes them better indicators for assessing fecal contamination in brackish or salt water (marine environments).
What is the primary role of 'Zoogloea ramigera' in the secondary treatment of sewage (activated sludge process)?
- A Producing methane from the anaerobic digestion of organic acids
- B Oxidizing ammonia to nitrate
- C Forming flocs that settle out of the water
- D Killing pathogenic viruses
Correct answer: Forming flocs that settle out of the water
Zoogloea ramigera produces an extracellular polysaccharide matrix that helps bacteria clump together into 'flocs'. These flocs settle to the bottom of the tank, allowing the clarified water to be separated from the microbial biomass.
Which bacteria are used to monitor 'Total Plate Count' (TPC) or Heterotrophic Plate Count (HPC) in water?
- A Broad groups of bacteria that use organic carbon for growth
- B Only those bacteria that are known to cause clinically significant human disease
- C Strictly anaerobic sulfur bacteria
- D Photosynthetic cyanobacteria
Correct answer: Broad groups of bacteria that use organic carbon for growth
HPC/TPC measures the general population of aerobic and facultative anaerobic heterotrophic bacteria in water. While it doesn't indicate fecal contamination directly, high counts suggest poor general water quality or system biofilm growth.
The 'IMViC' series of tests is used to differentiate between:
- A E. coli and Enterobacter aerogenes
- B Vibrio and Aeromonas
- C Salmonella and Shigella
- D Staphylococcus aureus and Streptococcus pyogenes
Correct answer: E. coli and Enterobacter aerogenes
IMViC stands for Indole, Methyl Red, Voges-Proskauer, and Citrate. This suite of tests is essential for distinguishing E. coli (typically ++--) from other coliforms like Enterobacter (typically --++), which may have different environmental significances.
Which of the following describes 'Oligotrophic' water?
- A Water with low nutrient levels and high clarity
- B Water with high nutrient levels and low clarity
- C Stagnant water with high salt concentration
- D Water heavily contaminated with industrial chemicals
Correct answer: Water with low nutrient levels and high clarity
Oligotrophic lakes are nutrient-poor and typically have low primary productivity. These waters are usually very clear and well-oxygenated throughout the water column.
In the MUG test for E. coli, the production of the enzyme beta-glucuronidase results in:
- A Blue-white fluorescence under UV light
- B The formation of a red ring
- C The production of a black precipitate
- D The change of the broth color from blue to yellow
Correct answer: Blue-white fluorescence under UV light
The enzyme beta-glucuronidase, produced by nearly all E. coli, breaks down the substrate MUG (4-methylumbelliferyl-beta-D-glucuronide) into a fluorescent product. This allows for rapid and specific identification of E. coli in water samples.
Which mechanism do 'Stalked' bacteria like Hyphomicrobium use to survive in nutrient-poor aquatic environments?
- A Increasing surface area for nutrient absorption via prosthecae
- B Producing thick endospores
- C Rapidly swimming through the water column using multiple peritrichous flagella
- D Entering a dormant state for several years
Correct answer: Increasing surface area for nutrient absorption via prosthecae
Bacteria with prosthecae (stalks or appendages) increase their surface-to-volume ratio, which enhances their ability to absorb dilute nutrients from the water. This is a common adaptation in oligotrophic aquatic habitats.