Bacteria Questions and Answers

Answer: A

Correct answer: lipid A

Lipid A is the endotoxin. Released when gram-negative cells lyse, it triggers macrophages to pour out cytokines, producing fever, hypotension and, in quantity, septic shock. The O antigen and core polysaccharide are antigenic but not themselves toxic.

Answer: B

Correct answer: bacteria have thin peptidoglycan layer

A gram-negative cell has only a thin layer of peptidoglycan, and its outer membrane is dissolved by the alcohol used to decolourise. The crystal violet-iodine complex therefore washes straight out, and the cell takes up the safranin counterstain instead.

Answer: D

Correct answer: bacteria have thick peptidoglycan layer

Retention is a matter of wall thickness. The many layers of peptidoglycan in a gram-positive wall dehydrate and shrink when alcohol is applied, trapping the crystal violet-iodine complex inside. The teichoic acids and the absence of a periplasm are genuine features of such cells, but they are not what holds the dye.

Answer: A

Correct answer: violet color

Gram-positive cells keep the primary stain, so they appear violet or purple. The safranin counterstain is applied afterwards but is masked by the darker dye already held.

Answer: C

Correct answer: pink-red color

Gram-negative cells lose the crystal violet during decolourisation, so they take up the safranin counterstain and appear pink or red.

Answer: D

Correct answer: Burri

A simple stain uses a single dye and shows shape and arrangement without distinguishing types of cell. Burri’s India-ink method is of that kind - strictly a negative stain, colouring the background rather than the cell. Gram, Neisser and Ziehl-Neelsen are all differential stains, using more than one reagent to separate one group of organisms from another.

Answer: D

Correct answer: all of the above

Fixing serves all three purposes: it kills the organisms, making the slide safe to handle; it makes them stick to the glass so they are not washed off during staining; and it alters the cells so that they take up the dye more readily.

Answer: B

Correct answer: 4 stages

The Gram stain has four steps: the primary stain (crystal violet), the mordant (iodine), decolourisation with alcohol or acetone, and the counterstain (safranin). The third is the critical one - too long, and even gram-positive cells lose the dye.

Answer: C

Correct answer: 3 stages

Ziehl-Neelsen has three steps: carbol fuchsin applied with heat, decolourisation with acid-alcohol, and a methylene blue counterstain. Acid-fast organisms such as mycobacteria keep the fuchsin, because their waxy mycolic acid wall resists the acid-alcohol.

Answer: D

Correct answer: all of the above

The endospore protects its DNA against heat, chemicals and radiation alike. Its defences are the dehydrated core, the calcium dipicolinate that stabilises the DNA, the small acid-soluble proteins that coat it, and the thick cortex and coats outside. This is why boiling will not sterilise and autoclaving is required.