MLT GlobeBacteriology: Enterobacterales – page 3
99 Bacteriology MCQs on Enterobacterales with answers and explanations.
Thick, bloody 'currant jelly' sputum in an alcoholic patient with pneumonia classically suggests:
K. pneumoniae causes a destructive lobar pneumonia with thick, bloody, mucoid sputum, especially in alcoholics.
Which part of the Gram-negative outer membrane lipopolysaccharide is responsible for endotoxin effects such as fever and shock?
Lipid A is the toxic component of LPS; the O side chain is the antigenic part used in serotyping.
On a TSI slant, blackening of the butt indicates:
H2S reacts with ferrous ions in the medium to form black ferrous sulphide.
On XLD agar, typical Salmonella colonies appear:
Salmonella decarboxylates lysine (red colonies) and produces H2S (black centres); E. coli gives yellow colonies.
In the serotyping of Enterobacterales, the flagellar antigen is designated:
H antigens are flagellar proteins; O is the somatic LPS antigen and K (including Vi) are capsular antigens.
Bubonic plague caused by Yersinia pestis is transmitted to humans mainly by:
Fleas from infected rodents transmit Y. pestis; pneumonic plague can then spread person to person by droplets.
Stool from a patient with bacillary dysentery due to Shigella typically contains:
Shigella invades the colonic mucosa, giving small-volume stools with blood, mucus and neutrophils.
A stool isolate is a non-lactose-fermenting Gram-negative rod that is motile and produces H2S on TSI. Which organism fits?
All Shigella species are non-motile and H2S-negative. Most Salmonella are motile and produce H2S.
Which enteric pathogen can cause disease after ingestion of as few as 10-100 organisms?
Shigella is acid-tolerant and has a very low infectious dose, allowing person-to-person spread. V. cholerae and Salmonella usually need far larger inocula.
On sorbitol-MacConkey agar, typical E. coli O157:H7 colonies appear:
Most E. coli ferment sorbitol and turn pink, whereas O157:H7 does not, giving colourless colonies that are then confirmed with O157 antisera.
A swarming urinary isolate is indole-negative and ornithine decarboxylase-positive. Which species is it?
Both Proteus species swarm, but P. mirabilis is indole-negative and ornithine-positive, whereas P. vulgaris is indole-positive. Morganella and Citrobacter do not swarm.
Which statement about Salmonella Typhi is NOT true?
Like all Enterobacterales, Salmonella is oxidase-negative. S. Typhi typically makes a small H2S 'wisp' on TSI.
Which organism is NOT in the order Enterobacterales?
Campylobacter is a curved, oxidase-positive, microaerophilic rod. Klebsiella, Providencia and Serratia are Enterobacterales.
On CLED agar, which urinary isolate gives green colonies with a matt surface?
P. aeruginosa gives green-blue colonies on CLED. E. coli colonies are yellow (lactose-fermenting); Proteus is blue and does not swarm.
A very mucoid, non-motile, lactose-fermenting, indole-negative, urease-positive rod from sputum is most likely:
The thick capsule makes K. pneumoniae mucoid; it is non-motile and indole negative. K. oxytoca is indole positive, Enterobacter is motile and E. coli is indole positive.
A motile lactose fermenter from a wound has IMViC – – + + and is ornithine decarboxylase positive. It is most likely:
Enterobacter cloacae is VP and citrate positive, motile and ornithine positive. K. pneumoniae shares the IMViC pattern but is non-motile and ornithine negative.
A stool isolate forms red colonies without black centres on XLD agar. It is non-motile, lysine decarboxylase negative and H2S negative. The most likely genus is:
Shigella does not ferment xylose, lactose or sucrose, so colonies stay red, and it lacks H2S, motility and lysine decarboxylase. Salmonella usually gives red colonies with black centres.
Escherichia coli forms yellow colonies on XLD agar because it:
E. coli ferments xylose, lactose and sucrose; the acid turns phenol red yellow. Lysine decarboxylation raises the pH and gives a red colour.
A child with bloody diarrhoea develops hemolytic uremic syndrome. Which laboratory test detects all Shiga toxin-producing E. coli, including non-O157 serotypes?
Non-O157 STEC usually ferment sorbitol and are missed on SMAC. Detecting Shiga toxin or stx genes identifies all STEC serotypes.
A teenager with right lower quadrant pain has a stool isolate forming 'bull's-eye' colonies on CIN agar. It is motile at 25 °C but not at 37 °C. The organism is:
Y. enterocolitica gives red-centred colonies on cefsulodin-irgasan-novobiocin agar and is motile only at 22–25 °C. It can mimic appendicitis (mesenteric lymphadenitis).