MLT GlobeBacteriology: Non-fermenters & other GNR
76 Bacteriology MCQs on Non-fermenters & other GNR with answers and explanations.
A wound isolate forms flat colonies with a metallic sheen, a grape-like odour and green diffusible pigment on nutrient agar. It is most likely:
Pseudomonas aeruginosa produces pyocyanin/pyoverdine (green pigment), a grape-like or corn-tortilla odour and often a metallic sheen. Salmonella and streptococci are non-pigmented.
A non-lactose-fermenting Gram-negative rod turns a tetramethyl-p-phenylenediamine strip purple within 10 seconds. Which organism fits?
A rapid purple reaction indicates cytochrome c oxidase, present in Pseudomonas; Enterobacterales such as E. coli and Salmonella are oxidase-negative.
Helicobacter pylori colonises the stomach by producing urease, which generates ammonia around the organism. This allows it to survive in:
Urease converts urea to ammonia and CO2, buffering the organism's microenvironment so it survives gastric acid; this is also the basis of urea breath and rapid urease tests.
An infected burn on the arm yields large, flat, non-lactose-fermenting colonies on MacConkey agar that are oxidase-positive. The likely organism is:
P. aeruginosa is a major burn-wound pathogen; it is a non-lactose-fermenting, oxidase-positive Gram-negative rod with spreading colonies. E. coli ferments lactose; staphylococci are Gram-positive.
A flagellated, oxidase-positive, hospital-acquired pathogen, Pseudomonas aeruginosa, would appear on Gram stain as:
Pseudomonas is a Gram-negative, non-fermenting bacillus with a polar flagellum.
A pneumonia patient fails penicillin therapy; the causative organism has no cell wall and cannot be Gram-stained. Which genus is it?
Mycoplasma lacks peptidoglycan, so beta-lactams that target wall synthesis are ineffective and the organism is not visible on Gram stain.
A patient passes profuse 'rice-water' stools with rapid dehydration. The most likely cause is:
Cholera toxin raises cAMP in enterocytes, driving massive secretion of watery, mucus-flecked stool. Shigella and EIEC cause dysentery.
Vibrio cholerae forms yellow colonies on TCBS agar. What causes the yellow colour?
Sucrose fermentation acidifies the medium, turning bromothymol blue/thymol blue indicators yellow. V. parahaemolyticus does not ferment sucrose and stays green.
The blue-green colour seen in pus and culture media from Pseudomonas aeruginosa infections is mainly due to which pigment?
Pyocyanin (blue) together with pyoverdine (yellow-green fluorescent) gives the typical green colour; pyocyanin is specific to P. aeruginosa. Prodigiosin is the red pigment of Serratia.
Q fever, acquired by inhaling dust contaminated by infected livestock, is caused by:
Coxiella burnetii is an obligate intracellular bacterium shed in birth products of cattle, sheep and goats.
Which of these helical organisms is NOT a spirochaete?
H. pylori is a curved gram-negative rod with external sheathed flagella. Spirochaetes (Borrelia, Leptospira, Treponema) have periplasmic endoflagella.
Most cases of cholera are acquired by:
Vibrio cholerae is transmitted by the faecal–oral route, mainly contaminated water. It colonises the small intestine and is not spread by droplets or vectors.
A burn-wound isolate is an oxidase-positive non-fermenting gram-negative rod with a grape-like odour and blue-green pigment (pyocyanin). Identify it.
P. aeruginosa produces pyocyanin and pyoverdine and is a leading cause of burn-wound infection and otitis externa. Proteus is oxidase-negative; E. faecalis is gram-positive.
A patient passes large volumes of pale, watery stool flecked with mucus ('rice-water'). Which infection is suspected?
Cholera toxin causes massive secretory diarrhoea with rice-water stools. Dysentery gives small-volume stools with blood and pus.
A painful genital ulcer with tender inguinal buboes yields small gram-negative rods in 'school-of-fish' chains. The agent is:
H. ducreyi causes chancroid, a painful ulcer. Syphilis (T. pallidum) gives a painless chancre and the organism is not seen on Gram stain.
Vibrio cholerae on TCBS agar forms colonies that are:
V. cholerae ferments sucrose, turning the bromothymol blue indicator yellow. V. parahaemolyticus does not and forms green colonies.
A gram-negative coccobacillus from an infected dog-bite wound, oxidase-positive and not growing on MacConkey, is likely:
P. multocida is oral flora of dogs and cats and a common cause of bite infections. Toxocara is a nematode, not a bacterium.
Bordet–Gengou or Regan–Lowe charcoal agar is used to isolate the agent of:
These media support Bordetella pertussis, grown from nasopharyngeal swabs. Its colonies resemble mercury drops.
Chronic infection with which organism is linked to gastric MALT lymphoma?
H. pylori drives chronic gastritis and MALT lymphoma; eradication can cause regression. It also predisposes to peptic ulcer.
Which selective medium is recommended to recover Burkholderia cepacia complex from respiratory specimens of patients with cystic fibrosis?
BCSA (or OFPBL agar) contains antibiotics that suppress P. aeruginosa and other CF flora, allowing slow-growing B. cepacia complex to be seen. CIN is for Yersinia, TCBS for Vibrio and Regan-Lowe for Bordetella.