Microbiology Interview Questions & Answers

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Microbiology Interview Questions & Answers

Are you B.sc graduate or M.sc microbiology student? Are you curious to know about microorganisms? Are you interested to work as microbiologist? Then log on to www.wisdomjobs.com. Microbiology is the study of microorganisms such as algae, fungi, bacteria, viruses and protozoa. This discipline includes fundamental research on the biochemistry, physiology, cell biology, ecology, evolution and clinical aspects of microorganisms including host response to these agents. The role of microbiologists is to study the living organisms and infectious agents, such as bacteria and viruses that can be only be seen with the microscope. With the new appreciation of the role of microbes in human and environmental health and their potential in microbiology they work in hospitals, clinics, universities, industry and government. They are the cutting edge of science. So, all job seekers go through the given microbiology job interview questions and answers below.

Microbiology Interview Questions

Microbiology Interview Questions
    1. Question 1. How Are Staining Techniques Classified?

      Answer :

      • Simple stain: where only one stain is used and all bacteria are stained similarly. Eg: F1ethylene blue, dilute carbol fuchsin
      • Differential staining: where different bacteria stain differently to a common staining technique depending on their physiological properties. Eg: Gram’s stain and Acid fast staining
      • Special stain: where structures of bacteria like spores. granules. capsule etc are demonstrated. Eg: silver impregnation technique for demonstration of spirochetes. Feulgen stain for demonstration of nucleus. Sudan black stain for demonstration of lipid vacuoles. Ryu’s stain for demonstration of flagella. Albert’s stain for demonstration of metachromatic granules.
      • Negative staining: where the background is stained with an acidic dye such as India ink or Nigrosin. Used for demonstration of capsules.

    2. Question 2. How Are Stains Classified?

      Answer :

      Stains are classified based on the pH of their chromophore (color bearing ion) into acidic, basic and neutral. Acidic dyes have anionic chromophore

      eg.. sodium+ eosinate-. Basic dyes have cationic chromophore eg.. metFiylene blue+ chloride-. Acidic dyes combine more strongly with cytoplasmic components of bacteria, especially the nucleus that is basic in nature. Neutral dyes have both acidic and basic component that nullity each other.

      They are Romanowsky’s stain and are used in staining parasitic forms. Stains can be either natural (eg: carmine and hematoxylin) or coal-tar derivatives /aniline stains (eg: methylene blue. crystal violet). Supravital (cells removed from the body) and intravital (cells still a part of the body).

    3. Question 3. What Is Polychrome Methylene Blue?

      Answer :

      LoetTler’s methylene blue solution treated with Potassium hydroxide turns into Polychrome methylene blue after prolonged storage with shaking. Used in McFadyean’s reaction for Bacillus anthracis in blood films and demonstration of metachromatic granules of Corynebacterium diphtheriae.

    4. Question 4. Who Invented Gram Stain?

      Answer :

      Hans Christian Gram invented this stain in 1884. The original formulation was Aniline Gentian violet. Lugol’s iodine, absolute alcohol and Bismark brown.

    5. Question 5. Which Are The Theories Of Gram Staining?

      Answer :

      Cell wall theory: Cell wall of Gram positive bacteria are 40 times thicker than those of Gram negative cells, hence they are thought to help retain the dye-iodine complex.
      Lipid Content Theory: Cell envelope of Gram negative bacteria contains an additional membrane (outer membrane). hence containing more lipids than Gram positive bacteria. Acetone or alcohol dissolves the lipid thus forming large pores in Gram negative bacteria through which the dye-iodine complex leaks out. Alcohol/acetone dehydrates Gram positive bacteria shrinking the cell wall and the closing the pores.
      Magnesium Ribonucleate Theory: A compound of magnesium ribonucleate and basic protein concentrated at the cell membrane helps Gram positive bacteria retain the primary dye. Gram negative bacteria do not possess this substance.
      Cytoplasmic pH Theory: The cytoplasm of Gram positive bacteria are said to be more acidic (2) than those of Gram negative ones (3). Hence the dye is said to bind with more affinity to Gram positive cells.

    6. Question 6. Which Part Of The Bacteria Actually Gets Stained?

      Answer :

      It is the cytoplasm (especially the nucleic acid) that gets stained and not the cell wall. Presence of an intact cell wall is important for retaining Gram positivity. Cell wall deficient forms such as Mycoplasma and L forms are Gram negative.

    7. Question 7. Which Are The Bacteria Or Bacterial Component That Can’t Be Stained By Gram Stain?

      Answer :

      • Extremely slender bacteria such as Treponema
      • Cells containing waxy substances impermeable to stain such as Mycobacteria
      • Minute intracellular bacteria such as Chlamydia and Rickettsia
      • Cell organelles such as capsule. spore. flagella etc

    8. Question 8. Which Are The Alternatives Used In Gram Stain?

      Answer :

      • Primary stain: Crystal violet. Methyl violet and Gentian violet
      • Mordant: Grams iodine, rarely Lugols iodine
      • Decolorizer: Alcohol, acetone. acteone-alcohol mixture (1:1)
      • Counterstain: Dilute carbol fuchsin. safranin, neutral red. (Sandiford stain ror Gonococcj

    9. Question 9. Which Are The Positive And Negative Controls For Gram Stain?

      Answer :

      • Positive control: Staphylococci
      • Negative control: E.coli. pus cells

    10. Question 10. What Are The Conditions When Gram Positive Bacteria Can Appear Gram Negative?

      Answer :

      • When over-decolourized by either prolonged exposure to decolourizer or using acetone alone.
      • When cell wall gets damaged by exposure to lysozyme or cell wall acting antibiotics such as Penicillin.
      • Old cultures, where cell wall is weakened or action of autolytic enzymes
      • Those bacteria that are phagocytosed. where cell wall is acted upon by lysosomal contents

    11. Question 11. Which Is The More Important Step In Gram Stain?

      Answer :

      Decolourization is the most important step as this step differentiates between Gram positive and Gram negative bacteria. Over-decolourization can result in Gram positive bacteria appearing Gram negative and under-decolourization can result in Gram negative bacteria appearing Gram positive.

    12. Question 12. What Are The Applications Of Gram Staining?

      Answer :

      • Rapid presumptive diagnosis of diseases such as bacterial meningitis
      • Selection of empirical antibiotics based on Gram stain finding
      • Selection of suitable culture media based on Gram stain finding
      • Screening of quality of clinical specimens. such as sputum that should contain many pus cells and few epithelial cells
      • Counting of bacteria
      • Appreciation of morphology and types of bacteria in a clinical specimen 

    13. Question 13. Name A Fungus That Is Gram Positive?

      Answer :

      Candida sps

    14. Question 14. What Are The Various Modifications Of Gram Stain?

      Answer :

      • Kopeloll’ and Beerman’s (Primary stain: Methyl violet. decolourizer: acetone or alcohol-acetone mixture 1:1)
      • Jensen’s (Primary stain: Methyl violet, decolourizer: absolute alcohol. counterstain: Neutral red)
      • Preston and Morrell’s (Primary stain: crystal violet. decolourizer: iodine-acetone)
      • A.igert’s (Primary stain: Carbol gentian violet. decolourizer: Aniline-xylol). This is used to stain tissue sections.

    15. Question 15. What Is Acid Fast Staining?

      Answer :

      Certain bacteria or their structures have the ability to retain the primary dye (strong carbol fuchsin) and resist clecolourization by weak mineral acids such as H2S04. HCI. Such bacteria or their structure are termed acid fast and this property is termed acid fastness. There are two types of acid fast staining, the hot method and the cold method. The hot method (Ziehl-Neelsen) involves heating the slide while the cold methods such as Kinyoun’s and Gabbett’s do not involve heating the slide.

    16. Question 16. Who Introduced Acid Fast Staining?

      Answer :

      Ehrlich in 1882 discovered acid fastness. The original method involved staining with aniline-gentian violet and decolourization with strong nitric acid.

      It was later improved by Ziehl and Neelsen.

    17. Question 17. Why Are Mycobacteria Acid Fast?

      Answer :

      The cell walls of Mycobacterla are made up of waxy substance, Mycolic acid that Is relatively Impermeable to ordinary stainIng techniques. But, by apphcation of heat and a mordant (phenol), the cell can be stained The purpose of heating is to soften the waxy material of the cell wall and allow the stain to enter the cell. Basic fuchsin is more soluble in phenol and phenol is a better solvent for lipids and waxes.

    18. Question 18. What Are The Components Of Ziehi-neelsen Stain?

      Answer :

      • Primary stain: Strong Carbol Fuchsin (contain Basic fuchsin and Phenol)
      • Decolourizer 20% sulphuric acid
      • Counterstain LoelTler’s Methylene blue or 1% Malachite green, Picric acid for color-blind workers

    19. Question 19. What Is Acid-alcohol Decolourizer?

      Answer :

      3% HCI in 95% alcohol (methylated spirit). This is useful in dilrerentiating saprophytic Mycobacteria from pathogenic Mycobacteria Pathogenic Mycobacteria are both acid and alcohol fast but saprophytic Mycobacterla are only acid-fast Saprophytic Mycobacterla can get declourized by alcohol. 95% alcohol can be used as a secondary decolorizer after decolourizing with acid Especially used in staining smears prepared from urine that may contain Mycobacterium smegmatis.

    20. Question 20. Which Are The Various Dilutions Of Sulfuric Acid Used?

      Answer :

      • Mycobacterlum leprae - 5% H2S04
      • Oocysts of Cryptosporidium. Isospora - 1 % H2S04
      • Tissue sections containing Actinomyctes. Nocardia - 1 % H2S04
      • Cultures of Nocardia - 0.5% H2S04
      • Bacterial spores - 0.25-0.5% H2S04

    21. Question 21. What Are Cold Methods Of Acid Fast Staining?

      Answer :

      The two methods namely Kinyoun’s and Gabbetts dont involve heating of slides, hence called cold methods. Heating is substituted by increased concentration of phenol and prolonging the duration of staining. Kinyoun’s method is favoured for detection of Cryptosporidium oocysts in fecal samples. Gabbetts method has decolourizer and counterstain in one solution.

    22. Question 22. Why Should The Slide Be Flooded With Strong Carbol Fuchsin?

      Answer :

      For uniform distribution of heat, or else the slide may break.

    23. Question 23. What Are The Precautions To Be Taken While Preparing Or Observing Smears For Afb?

      Answer :

      • A new slide must be used for every specimen. because scratch marks may give false positive.
      • A uniform smear from thick portion of the sputum must be made.
      • Staining jars should not be used to staining smear as there is risk to cross contamination
      • Fresh blotting paper must be used for each smear for drying the slide to prevent transfer from one slide to another.

    24. Question 24. How To Interpret The Smear?

      Answer :

      At least 100 oIl Immersion fields must be viewed before declaring the smear as negative The sensitivity of smear Is low because It requires the presence of 104 bacillilml to be smear positive. If the number of bacilli is less than this, the chances of detecting them are less In such a case, the sample should be subjected to concentration techniques such as Petroff s method If the smear Es positive for AFB. it should be counted/graded Failure to detect any AFB does not rule tuberculosis Grading of smears has prognostic value.

    25. Question 25. How Is The Smear Graded?

      Answer :

      Smears are graded depending on the number of bacilli seen

      • 3-9 bacilli/entire smear: +
      • 10 bacilli/entire smear ++
      • 10 bacilli/in most oil Immersion fields: +++

    26. Question 26. What Other Methods Are Available For Staining Mycobacteria?

      Answer :

      Sputum smears for Mycobacterla can be stained by fluorescent dyes such as Auramine and Rhodamlne as they have affinity for mycolic acid In their cell walIs The fluorescent microscopy is useful in screening large number of specimens. Large area of smear can be quickly observed that too under high power dry objective.

    27. Question 27. What Is Beaded Appearance Of Mycobacteria?

      Answer :

      Beaded appearance is used to describe the appearance of Mycobacterla when the cell doesn’t stain uniformly. showing stained and unstained regions. These forms are common in Mycobacterium tuberculosis while Mycobacterium bovis stains uniformly. Most saprophytic Mycobacteria stain uniformly.

    28. Question 28. What Are Metachromatic Granules?

      Answer :

      Metachromatic granules are pol’ymetaphosphate reserves produced by Corynebacterium diphtheriae in nutritious medium. These granules are also known as Babes Ernst granules. ‘blutin granules. Polar bodies etc. They are called metachromatic granules because of they exhibit metachromasia.

      a property where the granules appear in a colour different from that of the dye used When stained with polychrome methylene blue, they appear purple They are produced In abundance In serum containing medium such as Loeffler’s serum slope.

    29. Question 29. Which Are The Ways To Demonstrate These Granules?

      Answer :

      Albert’s stain. Neissers stain, Ponder’s stain and Pugh’s stain They can be demonstrated as retractile bodies in wet mount or slightly more gram positive structures in Gram stain.

    30. Question 30. Why Are The Bacilli Arranged At Angles To Each Other?

      Answer :

      The bacdh are arranged at angles to each other resembling English letter V or L or Chinese letter (cuneiform) pattern because the daughter cells doWt separate completely after cell dMslon (binary rission).

    31. Question 31. What Do Albert A(1) And B(2) Solution Contain?

      Answer :

      Solution A(1) contains Toluidine blue, Malachite green, Glacial acetic acid and Alcohol while solution 8(2) contains iodine and potassium iodide In distilled water.

    32. Question 32. What Is Sterilization And Disinfection?

      Answer :

      The process of kdling all hying forms including spores is called sterilization and the process of killing of only the vegetative form of pathogenic bacteria as well as other microbes is disinfection

    33. Question 33. What Is The Temperature And Time Employed To Sterilize The Articles In Hot Air Oven?

      Answer :

      160°C for 60 mInutes

    34. Question 34. What Are The Conditions Of Sterilization In An Autoclave?

      Answer :

      121°C for 15 minutes at 15 pounds per square inch of pressure

    35. Question 35. What Are The Articles Sterilized In A Hot Air Oven?

      Answer :

      Glassware’s, metallic instruments like scissors and forceps, swabs. powder. oils and grease.

    36. Question 36. What Are The Articles Sterilized In An Autoclave?

      Answer :

      Culture media, gloVes. cotton and clothes.

    37. Question 37. How Are Heat Labile Fluids Such As Serum And Antibiotic Solutions Sterilized?

      Answer :

      By filtration.

    38. Question 38. How Is Air Sterilized?

      Answer :

      By High Elliciency Particulate Air (HEPA) filters.

    39. Question 39. Name Some Disinfectants?

      Answer :

      Phenol. Lysol, Formaldehyde. Sodium hypochlorite.

    40. Question 40. What Are Antiseptics?

      Answer :

      Antiseptics are mild disinfectants that can be safely used on skin and mucous membranes.

    41. Question 41. Quaternary Ammonium Compounds?

      Answer :

      Quaternary ammonium compounds are positively charged polyatomic ions, which concentrate at the cell surface and alter the physical and chemical properties of the membrane, thus killing the cefl. Examples inlcude Benzalkonium chloride and Cetrimonium bromide.

    42. Question 42. What Do Popular Brands Of Antiseptics Such As Dettol Or Savlon Contain?

      Answer :

      The active ingredient of Dettol is chloroxylenol whereas Savlon contains a combination of Cetrimide and Chiorhexidine.

    43. Question 43. Which Are The Active Ingredients Of Commercial Mouthwashes?

      Answer :

      The active ingredients include Chlorhexidine. Triclosan. Thymol. Cetylpyridinium Chloride, and alcohol. The composition varies across brands.

    44. Question 44. What Are Chemisterilants?

      Answer :

      These are the chemicals used for sterilization. They are 2% Gluteraldehyde (cidex). Ethylene Oxide (EO). Formaldehyde + steam and Beta — Propiolactone (BPL).

    45. Question 45. Which Iodine Compounds Are Used In Antiseptics?

      Answer :

      • Tincture Iodine - 2% of Iodine in 70% alcohol - lodophore - Povidone Iodine.
      • Name some antiseptics.
      • Chlorhexidine. Chloroxylenol. spirit (70% alcohol), tincture of Iodine. H202.

    46. Question 46. Which Is The Best Disinfectant Used In Serology?

      Answer :

      Sodium hypochlorite or Calcium hypochlorite

    47. Question 47. How Are The Clinical Specimen Disinfected Before Discarding?

      Answer :

      By treating them with disinfectant, boiling or autoclaving and finally by incineration

    48. Question 48. How Are The Articles Commercially Sterilized?

      Answer :

      Gamma rays. Electron beams and Ethylene oxide

    49. Question 49. How Are Hemodialyzers And Endoscopes Disinfected?

      Answer :

      Glutaraldehyde or a combination of peracetic acid and hydrogen peroxide can be used.

    50. Question 50. How Does Ethylene Oxide Sterilize?

      Answer :

      Alkylation (hydrogen atom is replaced with an alkyl group) of protein. DNA. and RNA afFects bacterial metabolism and replication. EQ gas (8.5%) is often mixed with stabilizers such as CO2 (91 5%) or hydrochlorofluorocarbons (HCFC). This requires high humidity (40-80%) and long exposure times (1-6 hrs).

    51. Question 51. What Is Duckering?

      Answer :

      Ducking Is a process of inactivation of Anthrax spores in animal products such as wool, hairs or bristles. It was introduced by Elmhirst Duckering, an enginer at wool factory. This is a live-step process. each lasting for 10 minutes and carried out at 40.5°c.

      • immersion in 0.25-0.3% alkali
      • immersion in soapy water
      • immersion in 2% formaldehyde
      • secondimmersion in 2% formaldehyde
      • rinsinq in water

    52. Question 52. What Are The Various Filters?

      Answer :

      Porcelain filters. Seitz (asbestos) filters. Sintered glass filters. Membrane filters and HEP filters.

    53. Question 53. What Are The Uses Of Inspissator?

      Answer :

      It disinfects and solidifies egg and serum containing media such as U medium and LoelTiers serum slope.

    54. Question 54. How Is Operation Theatre Sterilized?

      Answer :

      By fumigation with formaldehyde.

    55. Question 55. What Is Meant By Cold Sterilization?

      Answer :

      Use of high-energy radiation such as gamma rays to sterilize an article.

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