MLT GlobeBacteriology: General bacteriology, stains & sterilization – page 3
61 Bacteriology MCQs on General bacteriology, stains & sterilization with answers and explanations.
The oxidation of ammonia to nitrite by soil bacteria such as Nitrosomonas is called:
Nitrification converts ammonia to nitrite then nitrate. Denitrification reduces nitrate to nitrogen gas.
Peptidoglycan is built from:
Chains of alternating N-acetylglucosamine and N-acetylmuramic acid are cross-linked by peptide bridges.
Which component is found in gram-positive but not gram-negative cell walls?
Teichoic and lipoteichoic acids are unique to gram-positive walls. Both types have peptidoglycan and a cell membrane; LPS is gram-negative.
Incineration in the laboratory is best suited to:
Incineration burns material to ash, so it is used for disposal. Reusable instruments and glassware are autoclaved or hot-air sterilised.
Pre-packed disposable plastic syringes are sterilised commercially by:
Ionising radiation penetrates packaging and heat-sensitive plastics at room temperature. Heat methods would melt them.
Surgical linen and gowns for the operating theatre are best sterilised by:
Steam penetrates porous fabric quickly and leaves no toxic residue. Ethylene oxide is reserved for heat-sensitive items.
Harmless microorganisms normally living on skin, in the upper respiratory tract and in the gut are best described as:
Normal microbiota are commensals: they live on the host without causing disease. Saprophytes feed on dead matter in the environment.
Serological typing of bacteria depends on surface or structural antigens. Which component is least useful for this purpose?
Capsular (K), cell-wall (O) and flagellar (H) antigens are exposed and strain-specific, forming the basis of serotyping; ribosomes are internal and highly conserved.
A distinct periplasmic space lying between the cytoplasmic membrane and an outer membrane is a characteristic feature of:
Gram-negative cells have an outer membrane, enclosing a periplasm containing thin peptidoglycan and enzymes such as beta-lactamases. Gram-positives lack an outer membrane; fungi have chitin walls.
A Gram-negative bacterium treated with lysozyme and EDTA loses part of its wall but retains some outer membrane. The resulting osmotically fragile form is a:
Spheroplasts retain wall remnants (typically Gram-negatives); protoplasts have completely lost their wall (typically Gram-positives treated with lysozyme in hypertonic medium).
Which spore strip is used as a biological indicator to monitor hot air oven (dry heat) sterilisation?
B. atrophaeus (formerly B. subtilis var. niger) spores are highly resistant to dry heat. G. stearothermophilus is the indicator for steam autoclaves.
Dark-field microscopy of urine from a febrile farm worker shows thin, tightly coiled motile organisms with hooked ends. Which genus is this?
Leptospira are fine spirochetes with hooked ends, spread by animal urine. They are too thin for Gram stain and are seen by dark-field or detected by serology/PCR.
A heat-sensitive endoscope from a TB clinic needs high-level disinfection. Which agent is suitable?
Glutaraldehyde is a high-level, tuberculocidal disinfectant for heat-sensitive instruments with adequate contact time. Quats and chlorhexidine are low-level and not tuberculocidal.
Bacterial flagella are too thin to see with routine stains. Which method makes them visible under the light microscope?
Flagella stains use tannic acid mordants to build up the flagellar thickness so it becomes visible. Albert's and Ponder's stains show volutin granules.
A flexible bronchoscope must be sterilised, but it cannot withstand heat. Which method is appropriate?
Heat-labile instruments are sterilised by low-temperature processes such as ethylene oxide or hydrogen peroxide plasma, or given high-level disinfection. Heat methods would damage the scope.
Biological indicators used to check steam autoclave performance contain spores of:
G. stearothermophilus spores are highly resistant to moist heat. B. atrophaeus spores are used for dry heat and ethylene oxide.
L-forms are wall-deficient bacterial variants. Which statement about them is FALSE?
L-forms can be cultured on osmotically protective (hypertonic) media. Loss of the wall makes them pleomorphic and variable in size.
Which statement about bacterial plasmids is INCORRECT?
Many toxins are plasmid-encoded, e.g. anthrax toxin and some E. coli enterotoxins. Plasmids also carry resistance genes.
Which statement about bacterial capsules is INCORRECT?
Capsular polysaccharides are the basis of pneumococcal, meningococcal, Hib and typhoid Vi vaccines. Capsules block phagocytosis and aid adherence.
Phenol is bactericidal (though not sporicidal) at a concentration of about:
Phenol at around 1% kills vegetative bacteria and is the reference in the phenol coefficient test. Lower concentrations are merely bacteriostatic.