MLT GlobeBacteriology: General bacteriology, stains & sterilization – page 2
61 Bacteriology MCQs on General bacteriology, stains & sterilization with answers and explanations.
During the Gram stain, which cell-wall component traps the crystal violet-iodine complex in Gram-positive bacteria?
A thick, highly cross-linked peptidoglycan layer dehydrates with alcohol and retains the dye complex. Chitin and cellulose are fungal and plant wall materials.
Which stain is the primary differential stain for initial bacterial classification in routine bacteriology?
The Gram stain divides most bacteria into Gram-positive and Gram-negative groups by cell wall structure. ZN is for acid-fast organisms; Giemsa and Leishman are blood stains.
For the most reliable Gram reaction, cultures should be stained during which growth phase?
Young, actively dividing cells have intact walls. Older Gram-positive cultures may lose wall integrity and stain Gram-variable or negative.
Which temperature and holding time is standard for sterilising glassware in a hot air oven?
Dry heat needs higher temperature and longer time than steam: 160 °C for 2 h (or 170 °C for 1 h). 121 °C for 15 min is the autoclave cycle.
While preparing reagents for a Ziehl–Neelsen stain, which one of these would a technologist NOT need?
ZN uses carbol fuchsin (primary stain), acid-alcohol (decolouriser) and methylene blue or malachite green (counterstain). Crystal violet is the primary stain of the Gram method.
Who first described 'animalcules' seen through his own single-lens microscopes in the 1670s?
Van Leeuwenhoek made the first recorded observations of bacteria and protozoa, earning him the title 'father of microbiology'. Pasteur and Koch worked two centuries later on germ theory and causation.
After inoculation into fresh broth, bacteria adapt and synthesise enzymes but do not yet increase in number. This is the:
In the lag phase cells are metabolically active and enlarge but have not started dividing. Exponential division follows in the log phase.
After a correctly performed Gram stain, Escherichia coli cells appear:
Gram-negative cells lose crystal violet during decolourisation because of their thin peptidoglycan and outer membrane; they then take up the red safranin counterstain.
Standard gravity-displacement autoclaving of media and instruments uses:
Saturated steam at 121 °C (15 psi) for 15 minutes kills spores. 160 °C for 1–2 hours is dry heat in a hot-air oven.
Which organism naturally lacks a cell wall and is therefore unaffected by penicillin?
Mycoplasmas have no peptidoglycan cell wall, so beta-lactams have no target. Fungi have walls of chitin and glucan, and typical bacteria have peptidoglycan.
Which bacteria have the thickest peptidoglycan layer in their cell wall?
Gram-positive walls have many peptidoglycan layers (up to 90% of the wall), which retains crystal violet. Gram-negative bacteria have a thin layer; chlamydiae have little or none.
Bacterial ribosomes, the target of aminoglycosides and macrolides, have a sedimentation coefficient of:
Prokaryotic ribosomes are 70S (30S + 50S subunits). Eukaryotic cytoplasmic ribosomes are 80S, which is why these drugs are selectively toxic.
The smallest free-living bacteria, about 0.2–0.3 µm and similar in size to large viruses, belong to the genus:
Mycoplasmas are about 0.2–0.3 µm and can pass through bacterial filters. Poxviruses, the largest viruses, are of similar size.
Most bacteria multiply by:
A bacterial cell copies its chromosome and divides into two equal daughter cells. Endospores are resting forms, not a means of multiplication.
The temperature at which a bacterial species multiplies fastest is its:
Growth rate peaks at the optimum temperature (37 °C for most human pathogens). Minimum and maximum define the limits of growth.
Which process changes bacterial DNA but does NOT transfer genes between cells?
Mutation alters the cell's own DNA. Transformation (free DNA), transduction (phage) and conjugation (pilus) move DNA between bacteria.
A loose, easily washed-off glycocalyx around a bacterium is termed:
A slime layer is loosely attached and unorganised; a capsule is firmly attached and well defined. Both are glycocalyx.
Proteus mirabilis has flagella distributed over its entire surface. This arrangement is:
Peritrichous flagella cover the cell surface. Monotrichous is a single polar flagellum; lophotrichous is a tuft at one pole.
Bacteria that colonise or infect the human body obtain carbon from organic compounds. They are therefore:
Human-associated bacteria are chemoheterotrophs using organic carbon. Autotrophs fix CO2 and chemolithotrophs oxidise inorganic compounds.
Prokaryotes lack mitochondria. Where does their electron transport chain operate?
Respiratory enzymes are embedded in the bacterial cytoplasmic membrane, which generates the proton motive force.