MLT GlobeBacteriology: Enterobacterales
99 Bacteriology MCQs on Enterobacterales with answers and explanations.
A urine isolate spreads as a thin, wave-like film over the entire blood agar plate and has a characteristic 'burnt chocolate' odour. Which organism is most likely?
Swarming motility across blood agar is characteristic of Proteus species, especially P. mirabilis, a common urease-positive urinary pathogen.
A urine culture on MacConkey agar grows large, pink, very mucoid colonies that string when touched with a loop. Which genus is most likely?
Klebsiella ferments lactose (pink) and has a large polysaccharide capsule giving mucoid, stringy colonies. Salmonella is non-lactose fermenting; Gram-positive cocci are inhibited on MacConkey.
A lactose-fermenting Gram-negative rod produces a red ring after Kovac's reagent is added to tryptone broth. The organism is most likely:
Kovac's reagent detects indole from tryptophan breakdown. E. coli is indole-positive; K. pneumoniae and most Salmonella are indole-negative.
A stool isolate forms flat, pink colonies with a hazy bile precipitate on MacConkey agar. Which organism is most likely?
E. coli ferments lactose, lowering pH so neutral red turns pink and bile salts precipitate. Salmonella forms pale non-lactose-fermenting colonies; the others do not grow on MacConkey.
Which organism is NOT an important cause of urinary tract infection?
B. fragilis is an anaerobe of intra-abdominal abscesses and rarely causes UTI. E. coli, Klebsiella and Proteus are leading uropathogens.
A 30-year-old woman has had watery, non-bloody diarrhoea for 14 hours. Which organism is least likely to be responsible?
S. pyogenes causes pharyngitis and skin infections, not enteritis. The others are recognised enteric pathogens, though Shigella often becomes bloody.
On a urine dipstick, which pad most specifically suggests the presence of Enterobacterales such as E. coli?
Gram-negative uropathogens reduce dietary nitrate to nitrite. Leukocyte esterase indicates pyuria; protein and glucose are not infection markers.
Which serological test detects agglutinating antibodies to Salmonella Typhi O and H antigens?
The Widal test measures anti-O and anti-H agglutinins; it supports but cannot replace culture. VDRL and RPR screen for syphilis; ASO indicates recent streptococcal infection.
Which genus is NOT in the order Enterobacterales?
Pseudomonas is an oxidase-positive, non-fermenting gram-negative rod. Enterobacterales are oxidase-negative glucose fermenters that reduce nitrate.
Typical biochemical profile of Escherichia coli is:
Most E. coli are motile, produce indole from tryptophan and ferment lactose (pink on MacConkey). Klebsiella is non-motile; Proteus is lactose-negative.
A 5-day-old neonate has suspected E. coli meningitis. Which specimen gives the definitive diagnosis?
Meningitis is confirmed by Gram stain and culture of the infant's CSF. Maternal swabs only show colonisation.
The principal indicator of faecal contamination in drinking water is:
E. coli is abundant in faeces and rarely grows in the environment, so its presence shows recent faecal pollution.
Suspected Salmonella or Shigella colonies are quickly confirmed by mixing them with specific antisera on a slide. This is:
Visible clumping with polyvalent or specific O antisera confirms the serogroup. The other tests detect antibodies in patient serum.
On MacConkey or EMB agar, Salmonella and Shigella usually form colonies that are:
These pathogens do not ferment lactose, so colonies remain pale. Pink (MAC) or green sheen (EMB) indicates lactose fermenters such as E. coli.
The commonest cause of community-acquired urinary tract infection is:
Uropathogenic E. coli causes about 70–80% of uncomplicated UTIs. Proteus and Pseudomonas are more common with catheters or stones.
Which organism is a non-lactose fermenter on MacConkey agar?
Proteus forms pale colonies. E. coli, Klebsiella and Enterobacter ferment lactose and form pink colonies.
Which gram-negative organism is a leading cause of neonatal meningitis?
E. coli with K1 capsule and group B streptococcus are the main causes of neonatal meningitis. Group A streptococcus is not.
Which of these is an enrichment broth for Salmonella from stool?
Selenite F suppresses coliforms while Salmonella multiplies, then it is subcultured. The others are plated media.
Which property is typical of Shigella?
As Enterobacterales, Shigella is oxidase-negative. It is non-motile, does not ferment lactose rapidly and is urease-negative.
On MacConkey agar, which organism typically forms pink (lactose-fermenting) colonies?
Lactose fermentation produces acid, turning the neutral red indicator pink. E. coli ferments lactose quickly; Salmonella, Shigella and Proteus are non-fermenters and form colourless colonies. S. sonnei is a late fermenter.