Bacteria Questions and Answers

Answer: A

The statement that is not true is E.coli was the first disease- causing bacterium identified by Robert Koch.

The first bacterium Koch proved to cause a disease was Bacillus anthracis, the agent of anthrax, in 1876, in the work that led to Koch's postulates; he later identified the tubercle bacillus and the cholera vibrio. E. coli was described by Theodor Escherich in 1885.

The other statements are true. E. coli is a normal member of the human gut microbiota, it is beneficial (it competes with pathogens and makes vitamin K), and enterohaemorrhagic strains such as O157:H7 cause bloody diarrhoea.

Answer: B

The master regulator is cyclic dimeric guanosine monophosphate (c-di-GMP).

In Pseudomonas aeruginosa c-di-GMP is made by diguanylate cyclases and broken down by phosphodiesterases. A high level switches the cell from swimming (planktonic) to the sessile state: flagellar motility is turned down and the production of exopolysaccharides such as Pel and Psl, which build the biofilm matrix, is turned up. A low level favours dispersal.

Cyclic thiolactone peptides are quorum-sensing signals of Gram-positive staphylococci, cyclic triadenylate is a second messenger of bacterial antiphage defence systems, and cycloheximide is an antibiotic that blocks eukaryotic protein synthesis. None controls this transition.

Answer: B

Correct answer: Fimbriae

Fimbriae, also called common pili, are short surface filaments whose adhesins bind receptors on host cells, and that attachment is often the first step of infection. Flagella provide motility and the capsule resists phagocytosis.

Answer: C

Correct answer: selective media

Salt (mannitol salt) agar contains about 7.5% sodium chloride, which inhibits most bacteria while staphylococci tolerate it. Suppressing the unwanted flora so that one group can grow is exactly what makes a medium selective.

Answer: D

Correct answer: Corynebacterium diphtheriae

Corynebacterium diphtheriae divides by snapping fission: the cells bend sharply apart but stay joined at one end, giving the V and L shapes described as a Chinese-letter or palisade arrangement. It is a useful diagnostic clue in a stained smear.

Answer: D

Correct answer: all of the above

Lipopolysaccharide has three parts: lipid A, anchored in the outer membrane and responsible for the endotoxic activity; the core polysaccharide; and the O antigen, the variable chain of repeating sugars that projects outwards and determines the serotype.

Answer: C

Correct answer: tube agglutination

The tube method is the quantitative one: the serum is diluted serially and a fixed amount of antigen added to each tube, so the result is a titre - the highest dilution still giving agglutination. Slide agglutination is quick but only qualitative.

Answer: D

Correct answer: Population of the cells decreases

In the decline (death) phase deaths outnumber divisions, so the viable population falls - often exponentially. Nutrients are exhausted and toxic products have accumulated; some cells lyse altogether.

Answer: D

Correct answer: The synthetic machinery of host cell

A virus carries no ribosomes and, in most cases, no metabolism of its own. For biosynthesis it takes over the host cell’s machinery - ribosomes, tRNAs, amino acids and ATP - to make its own proteins and nucleic acid. That total dependence is what makes viruses obligate intracellular parasites, and what makes them so hard to attack with drugs.

Answer: D

Correct answer: All the above

Spirochaetes are too slender to be seen by ordinary bright-field microscopy of a stained film, so all three are used: dark-field microscopy on a fresh preparation, which shows them alive and moving; silver impregnation by Morozov, which thickens them; and the Giemsa and India ink methods.