MLT GlobeBacteriology: General bacteriology, stains & sterilization
61 Bacteriology MCQs on General bacteriology, stains & sterilization with answers and explanations.
In the Gram stain, Gram's iodine forms a complex with crystal violet that resists decolourisation in Gram-positive cells. Iodine therefore acts as a:
A mordant fixes a dye in the cell; the crystal violet-iodine complex is trapped by thick peptidoglycan. Safranin is the counterstain.
After decolourisation in the Gram stain, which reagent colours Gram-negative cells pink-red?
Safranin (or dilute carbol fuchsin in some modifications) is the counterstain that makes decolourised Gram-negatives visible. Crystal violet is the primary stain; iodine is the mordant.
Glassware is sterilised by dry heat. Which piece of equipment is used?
The hot air oven uses dry heat (typically 160 °C for 2 h or 170 °C for 1 h). The autoclave uses moist heat under pressure; an inspissator coagulates egg/serum media.
A disinfectant that kills bacteria, rather than merely stopping their growth, is described as:
Bactericidal agents cause irreversible cell death; bacteriostatic agents inhibit multiplication, which resumes when the agent is removed.
Which everyday microorganism is a Gram-positive bacterium?
Lactobacilli are Gram-positive rods that ferment milk to lactic acid. Rhizobium, E. coli and Neisseria are Gram-negative.
After a correctly performed Gram stain, what colour are Gram-positive bacteria?
Thick peptidoglycan traps the crystal violet-iodine complex, so Gram-positive cells stay purple; decolourised Gram-negatives take up safranin and appear pink-red.
Treponema pallidum, the cause of syphilis, belongs to which bacterial order?
Treponema is a helical spirochete in the order Spirochaetales (phylum Spirochaetota), with Borrelia; Leptospira is now placed in the separate order Leptospirales. Enterobacterales includes E. coli; Bacillales includes Bacillus and Staphylococcus.
Most human pathogenic bacteria grow best at which pH range?
Most pathogens are neutrophiles with an optimum around pH 7.2-7.6, which is why culture media are typically adjusted to about pH 7.4.
During a Gram stain, immediately after the alcohol-acetone step and before the counterstain, Gram-negative cells appear:
The decolouriser removes crystal violet-iodine from the thin Gram-negative wall, leaving the cells colourless until safranin is applied.
A thermophilic bacterium isolated from a hot spring would show optimum growth at about:
Thermophiles have optima above 45 °C, typically 50-60 °C (some higher). 25-40 °C is mesophilic, the range of most human pathogens.
Psychrophilic bacteria that spoil refrigerated food grow best at around:
True psychrophiles grow at 0 °C and have optima at or below about 15 °C. Psychrotrophs (e.g. Listeria) grow in the fridge but prefer higher temperatures.
Most human pathogens, which grow well at body temperature, are classified as:
Mesophiles grow best between about 25 and 40 °C, including 37 °C, which is why clinical cultures are incubated at 35-37 °C.
Which technique is used specifically to demonstrate bacterial capsules?
Hiss's method (crystal violet with copper sulphate) and India ink show capsules as clear halos. Albert's and Ponder's stains show volutin granules.
Fontana's silver impregnation stain is used to demonstrate which bacteria?
Spirochetes are too thin for ordinary stains; silver deposition thickens them so they appear dark brown-black.
For which organism does a routine Gram stain give a reliable, characteristic result?
E. coli stains clearly as a Gram-negative rod. Mycobacteria resist Gram reagents, T. pallidum is too thin, and Mycoplasma lacks a wall.
A wet preparation on a cavity slide (hanging drop) is examined under the microscope. What bacterial property does it assess?
The hanging drop lets live bacteria move freely so true directional motility can be distinguished from Brownian movement.
Which process achieves sterility, i.e. killing all microbes including spores?
Moist heat under pressure kills vegetative cells and endospores. Pasteurisation and disinfection reduce pathogens but do not kill spores; asepsis only prevents contamination.
Which component of the Gram-negative outer membrane acts as endotoxin?
Lipid A of LPS triggers fever and septic shock. Teichoic acids are Gram-positive; cellulose and pectin are plant cell wall components.
Which of these organisms stains Gram-negative?
E. coli is a Gram-negative rod; the other three are Gram-positive.
Milk pasteurised at 72 °C for 15 seconds is considered:
Pasteurisation kills vegetative pathogens such as Mycobacterium bovis, Brucella and Salmonella but not endospores, so it is not sterilisation.