MLT GlobeBacteriology: Enterobacterales – page 2
99 Bacteriology MCQs on Enterobacterales with answers and explanations.
Which set of characteristics is typical of members of the order Enterobacterales?
Enterobacterales ferment glucose, reduce nitrate to nitrite and are oxidase negative (Plesiomonas is the main oxidase-positive exception). Oxidizers such as Pseudomonas are non-fermenters.
The expected IMViC pattern (indole, methyl red, Voges-Proskauer, citrate) for Escherichia coli is:
E. coli is indole and methyl red positive, VP and citrate negative. The reverse pattern (– – + +) is typical of Klebsiella and Enterobacter.
A traveller has sustained fever and relative bradycardia in the first week of illness. Which specimen gives the best chance of isolating Salmonella Typhi?
Blood culture is most sensitive in the first week of enteric fever. Stool and urine cultures become positive more often from the second or third week.
Watery travellers' diarrhoea caused by heat-labile and heat-stable enterotoxins is typical of which E. coli pathotype?
ETEC produces LT and/or ST toxins causing secretory diarrhoea and is the leading cause of travellers' diarrhoea. EHEC produces Shiga toxin and bloody diarrhoea.
A lactose non-fermenter from a respiratory specimen produces a red pigment at room temperature, is DNase positive and hydrolyzes gelatin. It is most likely:
S. marcescens produces prodigiosin (red pigment, best at 22–25 °C) and is DNase and gelatinase positive. Many strains are not pigmented at 37 °C.
On Hektoen enteric (HE) agar, a typical Salmonella colony appears:
Salmonella does not ferment lactose, sucrose or salicin, so colonies are blue-green, and H2S gives black centres. Salmon-orange colonies are fermenters such as E. coli.
Which Shigella produces Shiga toxin and causes the most severe dysentery, sometimes followed by hemolytic uremic syndrome?
S. dysenteriae type 1 carries the stx gene for Shiga toxin, which damages the colon and kidney endothelium. S. sonnei causes milder illness and is the main species in many high-income countries.
Chronic carriers of Salmonella Typhi most often harbour the organism in the:
Chronic typhoid carriage occurs mainly in the gallbladder, especially when gallstones are present, and organisms are shed in the stool for long periods.
Which virulence factor allows uropathogenic E. coli to bind uroepithelial cells and ascend to cause pyelonephritis?
P fimbriae (pyelonephritis-associated pili) bind to receptors on kidney and ureter epithelium. The K1 capsule is linked to neonatal meningitis, not upper urinary tract adhesion.
Most cases of E. coli neonatal meningitis are caused by strains that carry which antigen?
The K1 polysialic acid capsule resists phagocytosis and complement and is found in most E. coli strains that cause neonatal meningitis. O157:H7 is linked to hemorrhagic colitis.
In the rapid spot indole test using p-dimethylaminocinnamaldehyde (DMACA) reagent, a positive result is shown by:
DMACA reacts with indole to give a blue to blue-green colour within seconds. A red ring at the top of a broth is the positive reaction with Kovac's reagent in the tube test.
After incubation in MR-VP broth, which reagents are added to perform the Voges-Proskauer test?
The VP test detects acetoin; alpha-naphthol and 40% KOH (Barritt's reagents) are added, and a red colour is positive. Methyl red is used for the separate MR test of mixed acid fermentation.
An organism is inoculated on Simmons citrate agar. After 24 hours the slant has turned deep blue. This means the organism:
Organisms that use citrate as the only carbon source also use ammonium salts, which makes the medium alkaline; bromthymol blue turns from green to blue.
On eosin methylene blue (EMB) agar, Escherichia coli typically produces:
Strong lactose fermentation by E. coli produces a lot of acid, which precipitates the dyes and gives a green metallic sheen. Klebsiella usually gives pink, mucoid colonies without sheen.
A toddler whose family keeps a pet turtle develops fever and bloody diarrhoea. Which organism is most likely?
Reptiles such as turtles, lizards and snakes commonly carry nontyphoidal Salmonella, and young children are often infected through contact with them.
Which Enterobacterales genus is classically linked to osteomyelitis in patients with sickle cell disease?
Patients with sickle cell disease have bone infarcts and reduced splenic function, which make them prone to Salmonella osteomyelitis.
Shigella flexneri belongs to which Shigella serogroup?
Shigella serogroups are A (S. dysenteriae), B (S. flexneri), C (S. boydii) and D (S. sonnei).
In the tube indole test, a red ring after adding Kovac's reagent shows that the organism broke down:
Tryptophanase splits tryptophan into indole, pyruvate and ammonia; indole reacts with p-dimethylaminobenzaldehyde in Kovac's reagent to form a red colour.
Proteus mirabilis urinary infections can lead to kidney stones because the organism produces:
Urease splits urea to ammonia, making urine alkaline so that struvite stones form.
Salmonella Typhi is usually acquired by:
Typhoid spreads by the faecal-oral route, often from carriers or contaminated water.