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Tamil Nadu HSE Second Year Microbiology study materials
Tamil Nadu HSE Second Year Microbiology study resources

Tamil Nadu HSE Second Year Microbiology practice questions

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Question 1History, Scope and Classification of Microbiology

In the five-kingdom classification proposed by R. H. Whittaker, the prokaryotic organisms were placed in which kingdom?

  1. Plantae
  2. Fungi
  3. Monera Correct answer.
  4. Protista

Explanation

Whittaker's 1969 five-kingdom system separated prokaryotes from all eukaryotes by erecting the kingdom Monera, which contained bacteria and cyanobacteria. Protista, Fungi, Plantae and Animalia were reserved for eukaryotic organisms distinguished by nutrition and organisation. Hence the prokaryotes belong to Monera alone.

Question 2History, Scope and Classification of Microbiology

Which scientist is credited with the first observation and description of microorganisms, reporting them as 'animalcules'?

  1. Antonie van Leeuwenhoek Correct answer.
  2. Ferdinand Cohn
  3. Robert Koch
  4. Louis Pasteur

Explanation

Antonie van Leeuwenhoek, a Dutch draper, ground his own simple lenses and in the 1670s described bacteria and protozoa from pond water and dental plaque as 'animalcules'. His observations are the earliest recorded descriptions of microbes. Koch, Pasteur and Cohn worked much later on germ theory and pure culture.

Question 3History, Scope and Classification of Microbiology

The binomial system of nomenclature that gives every organism a genus and species name was introduced by which scientist?

  1. Aristotle
  2. Ernst Haeckel
  3. Charles Darwin
  4. Carl Linnaeus Correct answer.

Explanation

Carl Linnaeus introduced consistent binomial nomenclature in the eighteenth century, giving each organism a two-part Latin name of genus and species. This system, with rules later codified in the International Code of Nomenclature, is still used for bacteria and other microbes. Aristotle and Haeckel proposed earlier classifications but not the binomial format.

Question 4History, Scope and Classification of Microbiology

In a properly written scientific name such as Bacillus subtilis, which convention is followed?

  1. The genus begins with a capital letter and the species with a small letter Correct answer.
  2. The genus is in small letters and the species in capitals
  3. Both words begin with a capital letter
  4. Both words begin with a small letter

Explanation

Under binomial nomenclature the genus name is capitalised and the species epithet is written in lower case, with the whole name italicised in print or underlined when handwritten. This convention is applied uniformly to bacterial names such as Bacillus subtilis and Escherichia coli. It makes the rank of each word immediately clear to the reader.

Question 5History, Scope and Classification of Microbiology

Which statement best describes the scope of industrial microbiology?

  1. It is limited to the microscopic study of plant tissues
  2. It uses microorganisms to produce useful products and processes on a commercial scale Correct answer.
  3. It studies only the diseases caused by bacteria in humans
  4. It deals exclusively with the classification of viruses

Explanation

Industrial microbiology harnesses microbes for large-scale production of substances such as antibiotics, enzymes, alcohols and organic acids, and for processes like sewage treatment. It combines fermentation technology with strain selection and downstream processing. It is therefore an applied branch rather than a purely descriptive or medical one.

Question 6History, Scope and Classification of Microbiology

A student isolates an organism that lacks a nuclear membrane, possesses 70S ribosomes and divides by binary fission. To which group does it most reasonably belong?

  1. Protozoa
  2. Algae
  3. Bacteria Correct answer.
  4. Fungi

Explanation

Absence of a nuclear membrane together with 70S ribosomes and binary fission are defining prokaryotic features, which place the isolate among bacteria. Fungi, protozoa and algae are all eukaryotic and have a true nucleus with 80S cytoplasmic ribosomes. The combination of characters therefore points to bacteria.

Question 7History, Scope and Classification of Microbiology

The experiment in which Pasteur used swan-necked flasks containing boiled broth demonstrated that

  1. boiling destroys all microbial life permanently
  2. spontaneous generation occurs in nutrient broth
  3. microbes arise from pre-existing microbes carried in air Correct answer.
  4. air itself is the sole source of new living cells

Explanation

Pasteur's swan-necked flasks allowed air to enter but trapped dust and microbes in the curved neck, so boiled broth remained sterile. When the neck was broken or the broth tilted into the dust, growth appeared. This supported biogenesis, the principle that microbes come from pre-existing microbes carried in air.

Question 8History, Scope and Classification of Microbiology

Which pair of scientists is most closely associated with the postulates used to prove that a specific microbe causes a specific disease?

  1. Winogradsky and Beijerinck
  2. Fleming and Florey
  3. Pasteur and Jenner
  4. Koch and Henle Correct answer.

Explanation

Robert Koch developed the criteria, building on earlier reasoning by his teacher Jacob Henle, for linking a microorganism causally to a disease. The postulates require consistent isolation of the organism from diseased hosts, pure culture, reproduction of disease and re-isolation. Pasteur, Jenner, Fleming and the soil microbiologists worked on other problems.

Question 9History, Scope and Classification of Microbiology

In the three-domain system of Woese, the domain Archaea is separated from Bacteria mainly because archaeal organisms

  1. reproduce only by sexual means
  2. possess peptidoglycan in their cell walls
  3. have a nuclear membrane and membrane-bound organelles
  4. show distinctive ribosomal RNA sequences and membrane lipid chemistry Correct answer.

Explanation

Woese compared small-subunit ribosomal RNA sequences and found that archaea form a lineage distinct from bacteria. Supporting evidence includes ether-linked branched-chain membrane lipids and cell walls lacking peptidoglycan in most archaea. These molecular features justify separate domain status.

Question 10History, Scope and Classification of Microbiology

Which branch of microbiology applies microbiological principles to the prevention, diagnosis and treatment of infectious diseases in humans?

  1. Food microbiology
  2. Agricultural microbiology
  3. Medical microbiology Correct answer.
  4. Marine microbiology

Explanation

Medical microbiology deals with pathogenic microorganisms, host immunity, laboratory diagnosis and chemotherapy of infectious disease. It draws on bacteriology, virology, mycology and parasitology as they relate to human health. The other branches apply microbiology to crops, seawater or food respectively.

Question 11History, Scope and Classification of Microbiology

A culture is described as a pure culture. This means that the culture

  1. contains cells of only one species or strain Correct answer.
  2. was grown on a solid medium rather than in broth
  3. contains at least two well-characterised species
  4. was incubated at body temperature

Explanation

A pure culture contains the progeny of a single species or strain and no other organisms, which is essential for reproducible study of an organism's properties. The concept was advanced by Koch and by methods such as streak plating and single-colony isolation. Medium type or incubation temperature does not define purity.

Question 12History, Scope and Classification of Microbiology

Which feature is used in Bergey's Manual to place bacteria in major taxonomic groups?

  1. Colour of the colonies on nutrient agar
  2. Cell wall type, Gram reaction and other structural and biochemical characters Correct answer.
  3. Name of the scientist who first isolated the organism
  4. Geographical location of isolation

Explanation

Bergey's Manual classifies bacteria using cell wall composition, Gram reaction, morphology, oxygen requirements, motility and biochemical properties. These phenotypic characters group organisms into sections and families. Colony colour, isolation site and discoverer's name are not taxonomic criteria.

Question 13History, Scope and Classification of Microbiology

Which contribution is correctly matched to the scientist who made it?

  1. Louis Pasteur - discovery of the bacterial endospore
  2. Martinus Beijerinck - development of the Gram stain
  3. Robert Koch - discovery of the first antibiotic
  4. Alexander Fleming - discovery of penicillin from Penicillium notatum Correct answer.

Explanation

Alexander Fleming observed in 1928 that Penicillium notatum inhibited staphylococci and named the active substance penicillin. Koch is linked to the disease postulates, and the Gram stain was developed by Hans Christian Gram. The matching of Fleming with penicillin is therefore the accurate pair.

Question 14History, Scope and Classification of Microbiology

A laboratory technician needs a sterile medium for cultivating a fastidious organism. Which statement about sterilisation is most accurate?

  1. Disinfection is always carried out inside an autoclave
  2. Sterilisation kills all microorganisms including endospores, whereas disinfection reduces pathogens on surfaces Correct answer.
  3. Sterilisation removes only vegetative bacteria and leaves endospores intact
  4. Disinfection and sterilisation are interchangeable terms in microbiology

Explanation

Sterilisation is the complete destruction or removal of all microbial life, including bacterial endospores, and is required for media and surgical supplies. Disinfection reduces the number of pathogens on inanimate surfaces but does not necessarily destroy endospores. The two terms therefore describe different levels of microbial control.

Question 15History, Scope and Classification of Microbiology

Why is the small-subunit ribosomal RNA gene widely used for constructing phylogenetic trees of microorganisms?

  1. It is present in all cellular organisms, is functionally conserved and contains variable regions Correct answer.
  2. It mutates so rapidly that all relationships are obscured
  3. It occurs only in pathogenic bacteria
  4. It is found only in eukaryotic nuclei

Explanation

Small-subunit rRNA genes occur in every cellular organism and combine highly conserved stretches with variable regions, so they allow comparison across wide taxonomic distances. The conserved regions permit alignment and the variable regions carry phylogenetic signal. This combination made rRNA sequencing central to modern microbial taxonomy.

Question 16History, Scope and Classification of Microbiology

An organism grows at 80 degrees Celsius in a hot spring and lacks peptidoglycan in its cell wall. Which classification is most consistent with these observations?

  1. A yeast
  2. A cyanobacterium
  3. An archaeon of the extremophile group Correct answer.
  4. A Gram-positive bacterium

Explanation

Growth near 80 degrees Celsius together with a peptidoglycan-free wall points to a hyperthermophilic archaeon, many of which are isolated from hot springs. Cyanobacteria and Gram-positive bacteria possess peptidoglycan walls and generally do not grow at such temperatures. Yeasts are eukaryotic and also fail both criteria.

Question 17Microbial Cell Structure, Morphology and Staining

A bacterial cell is found to retain crystal violet during Gram staining even after alcohol decolourisation. Which component of the cell envelope is directly responsible for this retention?

  1. The teichoic acid of the capsule
  2. The outer membrane rich in lipopolysaccharide
  3. The waxy mycolic acid of the cell wall
  4. The thick peptidoglycan layer with low lipid content Correct answer.

Explanation

In Gram-positive bacteria the very thick peptidoglycan meshwork loses water during alcohol treatment, closing its pores and trapping the crystal violet-iodine complex inside the cell. The low lipid content of this wall means little of the complex is dissolved out by alcohol. This is why the cells remain purple after decolourisation.

Question 18Microbial Cell Structure, Morphology and Staining

Which staining method would you select to detect a bacterium whose wall contains a high proportion of mycolic acid?

  1. Capsule staining by Maneval method
  2. Spore staining by Schaeffer-Fulton method
  3. Gram staining with safranin counterstain
  4. Ziehl-Neelsen acid-fast staining Correct answer.

Explanation

Walls rich in mycolic acid are waxy and resist penetration, but once heated with strong carbol fuchsin the dye enters and resists decolourisation by acid-alcohol. Such organisms are therefore termed acid-fast and are demonstrated by the Ziehl-Neelsen method. Mycobacteria are the classic example.

Question 19Microbial Cell Structure, Morphology and Staining

A capsule appears as an unstained halo around a stained bacterial cell in a capsule stain. Which explanation correctly accounts for this appearance?

  1. The capsule absorbs nigrosin and therefore appears dark
  2. The capsule is stained by crystal violet along with the cell
  3. The capsule excludes the dye and is seen as a negative image against stained background and cell Correct answer.
  4. The capsule is stained by safranin during counterstaining

Explanation

Capsules are largely polysaccharide and do not take up ordinary basic dyes, so they remain clear. In negative-type capsule staining the background is darkened with an acidic dye such as nigrosin and the cell is stained with a basic dye. The capsule is therefore visible as a transparent halo between the coloured cell and the dark background.

Question 20Microbial Cell Structure, Morphology and Staining

What is the primary purpose of heating the slide during the Ziehl-Neelsen staining procedure?

  1. To fix the bacteria firmly to the slide
  2. To drive carbol fuchsin through the waxy, impermeable cell wall Correct answer.
  3. To counterstain non-acid-fast cells
  4. To decolourise acid-fast cells

Explanation

Mycolic acid makes the wall waxy and poorly permeable, so the primary dye cannot enter by simple diffusion. Gentle heating with carbol fuchsin softens the wax and drives the dye into the wall. Once inside, the dye is difficult to remove with acid-alcohol, which is the basis of acid-fastness.

Question 21Microbial Cell Structure, Morphology and Staining

Which structure gives a bacterial cell its characteristic shape and protects it from osmotic lysis?

  1. The capsule
  2. The flagellum
  3. The cell wall Correct answer.
  4. The cytoplasmic membrane

Explanation

The peptidoglycan cell wall is a rigid meshwork that maintains the shape of the bacterium. It also resists the internal osmotic pressure so that the cell does not burst in hypotonic surroundings. Removal of the wall leaves protoplasts or spheroplasts that lyse easily.

Question 22Microbial Cell Structure, Morphology and Staining

In the Gram stain, what is the function of Lugol's iodine?

  1. It counterstains Gram-negative cells pink
  2. It decolourises Gram-negative cells
  3. It fixes the smear to the slide
  4. It forms a large insoluble complex with crystal violet inside the cell Correct answer.

Explanation

Iodine acts as a mordant: it combines with crystal violet to form a large crystal violet-iodine complex. This complex is too bulky to escape through the intact thick peptidoglycan of Gram-positive cells. It therefore helps to trap the primary dye during decolourisation.

Question 23Microbial Cell Structure, Morphology and Staining

A culture of Gram-positive cocci is over-decolourised with acetone-alcohol. What result should be expected after counterstaining?

  1. The cells appear colourless because safranin cannot enter
  2. The cells appear purple because the wall retains dye permanently
  3. The cells appear green because malachite green is used
  4. The cells appear pink because crystal violet has been removed Correct answer.

Explanation

Even Gram-positive cells can lose the crystal violet-iodine complex if the alcohol is left too long. Once the primary dye is washed out, the cells take up the pink counterstain and mimic Gram-negative organisms. This is a common source of error in reading Gram stains.

Question 24Microbial Cell Structure, Morphology and Staining

Which feature distinguishes a bacterial flagellum from a bacterial fimbria?

  1. The flagellum is made of polysaccharide, whereas the fimbria is made of protein
  2. The flagellum is found only in Gram-negative cells, whereas the fimbria is found in all cells
  3. The flagellum is used for attachment, whereas the fimbria is used for motility
  4. The flagellum is long and used for motility, whereas the fimbria is short and used for attachment Correct answer.

Explanation

Flagella are long filamentous appendages built of flagellin that rotate to propel the cell. Fimbriae are shorter, more numerous protein structures that mediate adhesion to surfaces and host cells. The two differ in length, number and primary function.

Question 25Microbial Cell Structure, Morphology and Staining

During sporulation, which change allows the endospore to survive boiling water for extended periods?

  1. Loss of the peptidoglycan wall
  2. Assembly of a new flagellar apparatus
  3. Formation of a polysaccharide capsule
  4. Dehydration of the core and accumulation of calcium dipicolinate Correct answer.

Explanation

The maturing endospore withdraws water from its core and accumulates calcium dipicolinate, which stabilises DNA and proteins. The dehydrated core is metabolically inactive and highly resistant to heat. Multiple protein coats add further protection.

Question 26Microbial Cell Structure, Morphology and Staining

Which statement correctly describes the arrangement of flagella in a lophotrichous bacterium?

  1. A single flagellum at one pole
  2. One flagellum at each pole
  3. A tuft of flagella at one pole Correct answer.
  4. Flagella distributed over the entire surface

Explanation

The prefix lopho- indicates a tuft, so lophotrichous cells carry a cluster of flagella at one end. This arrangement is contrasted with monotrichous and amphitrichous types. Peritrichous cells, by contrast, have flagella all around the surface.

Question 27Microbial Cell Structure, Morphology and Staining

A bacterium is grown in a medium containing penicillin, which blocks peptidoglycan synthesis. In an isotonic medium the cells survive as rounded bodies. What are these bodies called in a Gram-positive organism?

  1. Spheroplasts
  2. Cysts
  3. L-forms of Gram-negative type
  4. Protoplasts Correct answer.

Explanation

When the wall is completely removed from a Gram-positive cell, the remaining membrane-bound structure is called a protoplast. In an isotonic medium it does not lyse and becomes spherical. Gram-negative cells that lose only part of the wall, retaining the outer membrane, are called spheroplasts.

Question 28Microbial Cell Structure, Morphology and Staining

Which component is found in the cell wall of Gram-negative bacteria but absent from the cell wall of Gram-positive bacteria?

  1. Lipopolysaccharide in an outer membrane Correct answer.
  2. Peptidoglycan
  3. Diaminopimelic acid
  4. N-acetylmuramic acid

Explanation

The Gram-negative envelope has an outer membrane whose outer leaflet contains lipopolysaccharide, which is absent in Gram-positive walls. This outer membrane is also the source of endotoxin. Both groups, however, share peptidoglycan chemistry with N-acetylmuramic acid and often diaminopimelic acid.

Question 29Microbial Cell Structure, Morphology and Staining

A student observes rod-shaped bacteria arranged end to end in chains. Which morphological description is most appropriate?

  1. Streptobacilli Correct answer.
  2. Staphylococci
  3. Diplococci
  4. Vibrios

Explanation

Rod-shaped bacteria that remain attached after division form chains, and this arrangement is termed streptobacilli. The prefix strepto- denotes a chain arrangement. The shape term bacillus indicates the rod form.

Question 30Microbial Cell Structure, Morphology and Staining

Which of the following best explains why mycoplasmas are unaffected by penicillin?

  1. They lack a peptidoglycan cell wall Correct answer.
  2. They grow only inside host cells
  3. They possess an outer membrane that excludes penicillin
  4. They produce beta-lactamase constitutively

Explanation

Penicillin acts on enzymes that cross-link peptidoglycan, so it can only affect cells that build such a wall. Mycoplasmas are the smallest free-living bacteria and naturally lack a peptidoglycan wall, being bounded only by a membrane. They are therefore intrinsically resistant to all beta-lactam antibiotics.

Question 31Microbial Cell Structure, Morphology and Staining

In a negative staining procedure using India ink, what is actually stained?

  1. The bacterial capsule
  2. The bacterial cytoplasm
  3. The background surrounding the cells Correct answer.
  4. The bacterial cell wall

Explanation

India ink is an acidic particulate dye that is repelled by the negatively charged bacterial surface. It therefore cannot enter or coat the cells and instead deposits around them. The cells appear as clear shapes against a dark background, which makes capsules and cell outlines easy to see.

Question 32Microbial Cell Structure, Morphology and Staining

Which structure is responsible for the selective permeability of the bacterial cell and is the site of respiratory electron transport in aerobic bacteria?

  1. The cytoplasmic membrane Correct answer.
  2. The cell wall
  3. The nucleoid
  4. The capsule

Explanation

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins that controls what enters and leaves the cell. In aerobic bacteria, since there are no mitochondria, the membrane also houses the electron transport chain and ATP synthase. It is thus both a permeability barrier and an energy-transducing surface.

Question 33Microbial Cell Structure, Morphology and Staining

A Gram-stained smear shows purple cocci in clusters alongside pink rods. Which conclusion is most defensible?

  1. The sample contains only Gram-negative organisms
  2. The sample contains Gram-positive rods and Gram-negative cocci
  3. The sample contains Gram-positive cocci and Gram-negative rods Correct answer.
  4. The smear was definitely over-decolourised

Explanation

Purple cells retained the crystal violet-iodine complex and are therefore Gram-positive, while pink cells took up the counterstain and are Gram-negative. The shape terms cocci and rods describe the observed morphology. A mixed specimen can legitimately show both reactions, so over-decolourisation is not the necessary conclusion.

Question 34Microbial Cell Structure, Morphology and Staining

Which chemical feature of the bacterial endospore is most directly linked to its resistance to ultraviolet radiation and desiccation?

  1. The absence of dipicolinic acid
  2. The high water content of the spore core
  3. The presence of lipopolysaccharide in the spore coat
  4. Small acid-soluble spore proteins binding and protecting the DNA Correct answer.

Explanation

Small acid-soluble proteins saturate the spore chromosome and alter its conformation, shielding it from UV damage and desiccation. They also serve as a carbon source during germination. The dehydrated core and calcium dipicolinate add further protection.

Question 35Microbial Cell Structure, Morphology and Staining

What is the main structural difference between the flagellum of a bacterium and the cilium of a eukaryotic cell?

  1. The bacterial flagellum is a rigid helical filament of flagellin, while the eukaryotic cilium has a microtubule core Correct answer.
  2. Both are made of flagellin but differ only in length
  3. The bacterial flagellum is made of microtubules, while the eukaryotic cilium is made of flagellin
  4. The bacterial flagellum is enclosed by a membrane sheath in all species

Explanation

Bacterial flagella are protein filaments of flagellin that rotate like a propeller and are not covered by a membrane in most species. Eukaryotic cilia and flagella contain a 9+2 arrangement of microtubules and beat with a whip-like motion. The two structures are therefore analogous in function but entirely different in construction.

Question 36Microbial Cell Structure, Morphology and Staining

Which reagent acts as the primary stain in the Gram staining procedure?

  1. Crystal violet Correct answer.
  2. Acetone-alcohol
  3. Safranin
  4. Lugol's iodine

Explanation

Crystal violet is applied first and imparts the initial purple colour to all cells. Iodine then mordants the dye, alcohol decolourises Gram-negative cells, and safranin counterstains them pink. The primary stain is thus crystal violet.

Question 37Microbial Nutrition, Growth and Culture Techniques

A culture medium contains peptone, beef extract and sodium chloride but no added carbohydrate and no agar. How is this medium best described?

  1. Selective differential medium
  2. Defined solid medium
  3. Synthetic transport medium
  4. Complex liquid medium Correct answer.

Explanation

Peptone and beef extract are digests of biological material whose exact chemical composition is unknown, so the medium is complex rather than defined. The absence of agar keeps it liquid (broth), and no selective or differential agents are present. Hence it is a complex liquid medium.

Question 38Microbial Nutrition, Growth and Culture Techniques

Which category of organism obtains both its carbon and its energy from preformed organic compounds such as glucose?

  1. Chemoheterotroph Correct answer.
  2. Photoautotroph
  3. Photoheterotroph
  4. Chemoautotroph

Explanation

The prefix indicates the energy source and the suffix the carbon source. A chemoheterotroph uses organic compounds as both energy source and carbon source, which is the pattern of most medically important bacteria and fungi.

Question 39Microbial Nutrition, Growth and Culture Techniques

In the standard streak plate method, why is the loop flamed and cooled between successive sectors?

  1. To increase the number of organisms deposited in each sector
  2. To dilute the inoculum progressively so isolated colonies can develop Correct answer.
  3. To sterilise the agar surface before each set of streaks
  4. To prevent condensation forming inside the Petri dish

Explanation

Flaming between sectors kills organisms remaining on the loop, so each new sector receives a smaller fraction of the previous one. This progressive dilution eventually deposits single cells that grow into isolated colonies.

Question 40Microbial Nutrition, Growth and Culture Techniques

A bacterial population increases from 1,000 to 8,000 cells during exponential growth with no lag phase. How many generations have occurred?

  1. Three Correct answer.
  2. Eight
  3. Two
  4. Four

Explanation

Each generation doubles the population, so the final count equals the initial count multiplied by 2 raised to the number of generations. Here 1,000 × 2^n = 8,000, giving 2^n = 8 and therefore n = 3.

Question 41Microbial Nutrition, Growth and Culture Techniques

Which term describes a microorganism that grows optimally at 15 degrees Celsius and cannot grow at 37 degrees Celsius?

  1. Psychrophile Correct answer.
  2. Hyperthermophile
  3. Mesophile
  4. Thermophile

Explanation

Psychrophiles have low optimum growth temperatures, generally below 20 degrees Celsius, and fail to grow at body temperature. Mesophiles prefer 20 to 45 degrees Celsius, while thermophiles and hyperthermophiles prefer much higher temperatures.

Question 42Microbial Nutrition, Growth and Culture Techniques

What is the main purpose of adding agar rather than gelatin to solidify a bacteriological medium?

  1. Agar resists microbial digestion and sets firmly at incubation temperatures Correct answer.
  2. Agar is digested by most bacteria, supplying extra carbon
  3. Agar melts at a lower temperature than gelatin
  4. Agar is a defined pure compound of known formula

Explanation

Agar is a sulfated polysaccharide from red algae that very few bacteria can degrade, so it stays solid and does not add nutrients. It also melts near 95 degrees Celsius and solidifies around 42 degrees Celsius, remaining firm at normal incubation temperatures.

Question 43Microbial Nutrition, Growth and Culture Techniques

A batch culture is inoculated and the viable count stays almost unchanged for four hours before rising. Which phase of the growth curve does this interval represent, and what is occurring in the cells?

  1. Log phase, with balanced binary fission
  2. Lag phase, with metabolic adaptation and enzyme synthesis Correct answer.
  3. Stationary phase, with division equal to death
  4. Death phase, with autolysis exceeding division

Explanation

The lag phase is a period of adaptation in which cells enlarge, synthesise ribosomes, enzymes and metabolites, and adjust to the new medium without net increase in viable number. Once this preparation is complete, exponential division begins.

Question 44Microbial Nutrition, Growth and Culture Techniques

Why does the viable plate count of a culture usually give a lower figure than the direct microscopic count?

  1. The plate count uses larger sample volumes than the counting chamber
  2. The plate count includes only cells able to form colonies, while microscopy counts living and dead cells alike Correct answer.
  3. Microscopy cannot detect cells smaller than one micrometre
  4. The plate count is performed at a temperature above the growth optimum

Explanation

A viable plate count records only cells that survive and multiply to form a visible colony, so dead cells and cells that fail to grow under the test conditions are excluded. Direct microscopic counting with a counting chamber records all cells or particles present, whether viable or not.

Question 45Microbial Nutrition, Growth and Culture Techniques

A medium contains glucose, ammonium sulfate, magnesium sulfate, potassium phosphate and a trace of biotin. Which nutritional requirement does this formulation satisfy?

  1. An undefined requirement for growth factors
  2. A differential indicator of lactose fermentation
  3. A selective inhibition of Gram-positive bacteria
  4. A defined source of carbon, nitrogen, sulfur and phosphorus Correct answer.

Explanation

Every ingredient is a known pure chemical, so the medium is defined. Glucose supplies carbon and energy, ammonium sulfate supplies nitrogen and sulfur, and phosphate supplies phosphorus, while biotin covers the growth-factor requirement.

Question 46Microbial Nutrition, Growth and Culture Techniques

A facultative anaerobe is inoculated into a tube of thioglycollate broth. Which distribution of growth is expected?

  1. No growth anywhere in the tube
  2. Growth only as a band a few millimetres below the surface
  3. Dense growth throughout the tube, heaviest near the top Correct answer.
  4. Growth only at the very top of the medium

Explanation

Thioglycollate lowers the redox potential and an oxygen gradient forms from the surface downwards. A facultative anaerobe grows aerobically near the top and fermentatively in the deeper, anaerobic region, so turbidity appears throughout with the densest growth near the surface.

Question 47Microbial Nutrition, Growth and Culture Techniques

A culture in log phase has a generation time of 30 minutes. Starting with 10,000 cells, what is the approximate population after three hours of unrestricted growth?

  1. 320,000 cells
  2. 80,000 cells
  3. 160,000 cells
  4. 640,000 cells Correct answer.

Explanation

Three hours contain six 30-minute intervals, so the population doubles six times. The factor is 2^6 = 64, and 10,000 × 64 gives approximately 640,000 cells.

Question 48Microbial Nutrition, Growth and Culture Techniques

Why is the stationary phase reached in a closed batch culture even when the medium was initially rich in nutrients?

  1. An essential nutrient becomes limiting and toxic waste products accumulate Correct answer.
  2. The cells switch permanently from binary fission to sporulation
  3. All cells simultaneously lose their cell walls
  4. The incubation temperature falls below the minimum for growth

Explanation

In a closed system nothing is added or removed, so growth slows as a limiting nutrient is exhausted. At the same time acidic and other inhibitory waste products accumulate, so the division rate falls until it equals the death rate.

Question 49Microbial Nutrition, Growth and Culture Techniques

Which method would you choose to obtain an accurate viable count of a low-density water sample without waiting for colonies to grow?

  1. Measurement of turbidity in a colorimeter
  2. Direct counting in a Neubauer chamber
  3. Membrane filtration followed by culture of the filter Correct answer.
  4. Determination of dry weight of the sample

Explanation

Membrane filtration concentrates organisms from a large volume onto a small filter, which is then placed on a medium so that colonies can be counted. This gives a viable count even when the original density is too low for direct plating.

Question 50Microbial Nutrition, Growth and Culture Techniques

A bacterium grows in a mineral medium containing only glucose and inorganic salts, with no vitamins added. Which conclusion is most defensible?

  1. It requires an organic growth factor supplied by peptone
  2. It is an obligate intracellular parasite
  3. It is unable to use glucose as a carbon source
  4. It can synthesise all its essential growth factors from the supplied nutrients Correct answer.

Explanation

Growth without added vitamins or other organic supplements shows that the organism can biosynthesise its own growth factors from glucose and inorganic salts. Such organisms are nutritionally undemanding and are described as prototrophic for those factors.