MLT GlobeBacteriology: Non-fermenters & other GNR – page 3
76 Bacteriology MCQs on Non-fermenters & other GNR with answers and explanations.
Brucella species grow in culture as:
Brucella are strictly aerobic gram-negative coccobacilli; B. abortus needs added CO2 for primary isolation.
Which feature is NOT typical of human brucellosis?
Brucellosis causes undulant fever, sweats, arthralgia, sacroiliitis and myalgia. Diarrhoea is not a prominent feature.
Cholera toxin produces watery diarrhoea by:
The A subunit ADP-ribosylates Gs protein, keeping adenylate cyclase on. Rising cAMP drives chloride and water secretion without tissue damage.
Legionella can multiply inside free-living amoebae in water systems. Inhaling contaminated aerosols can cause:
Free-living amoebae act as hosts for Legionella pneumophila, and aerosols from cooling towers or showers cause pneumonia. Brucellosis and salmonellosis are mainly foodborne.
A small gram-negative coccobacillus grows on blood agar only around a Staphylococcus aureus streak and needs both X and V factors. It is:
S. aureus lyses red cells (releasing X, hemin) and secretes V (NAD), so H. influenzae grows as satellite colonies next to it. H. parainfluenzae needs only V factor.
A gram-negative coccobacillus from a ventilated ICU patient's tracheal aspirate is oxidase negative, catalase positive and non-motile. It grows on MacConkey agar as a non-lactose fermenter. The most likely organism is:
Acinetobacter is an oxidase-negative, non-motile coccobacillus that grows on MacConkey and causes ventilator-associated pneumonia. S. maltophilia is also usually oxidase negative but is motile and a rod.
An oxidase-positive non-fermenter produces a yellow-green pigment that fluoresces under UV light, but no blue pigment. It does not grow at 42 °C. The most likely organism is:
Fluorescent pseudomonads other than P. aeruginosa produce pyoverdin but not pyocyanin and fail to grow at 42 °C. P. aeruginosa grows at 42 °C and usually makes pyocyanin. Acinetobacter and Stenotrophomonas are oxidase negative.
A patient has watery diarrhoea after eating raw shellfish. The stool grows green (sucrose-negative) colonies on TCBS agar. The isolate is oxidase positive and needs added NaCl for growth. The most likely organism is:
V. parahaemolyticus is halophilic, does not ferment sucrose (green colonies on TCBS) and causes seafood-associated gastroenteritis. V. cholerae ferments sucrose (yellow colonies) and grows without added salt.
A man with liver cirrhosis develops septic shock and haemorrhagic bullae on his legs one day after eating raw oysters. Blood cultures grow a curved, halophilic, lactose-fermenting gram-negative rod. The most likely organism is:
V. vulnificus causes rapidly fatal septicaemia with bullous skin lesions in patients with liver disease or iron overload. It is the lactose-positive vibrio. V. parahaemolyticus mainly causes gastroenteritis and is lactose negative.
An oxidase-positive, glucose-fermenting gram-negative rod is isolated from rice-water stool. Which result best supports Vibrio rather than Aeromonas?
Vibrio cells lyse in 0.5% sodium deoxycholate, releasing DNA that forms a mucoid string; Aeromonas is string-test negative. Both genera are oxidase positive and both may be haemolytic and grow on MacConkey.
Two curved, oxidase-positive gram-negative rods from stool both grow at 42 °C. Which test separates Campylobacter jejuni from C. coli?
C. jejuni hydrolyzes hippurate, while C. coli does not. Both species are oxidase and catalase positive, and nalidixic acid results are no longer reliable because of widespread quinolone resistance.
In a rapid urease test on a gastric biopsy, the phenol red indicator turns from yellow to pink when Helicobacter pylori is present. The colour change is caused by:
H. pylori urease splits urea into ammonia and CO2; ammonia makes the gel alkaline, turning phenol red pink. CO2 alone does not cause this colour change.
A patient with dyspepsia has a negative urea breath test. He has been taking a proton pump inhibitor daily for the past month. What is the most appropriate action?
Proton pump inhibitors suppress H. pylori urease activity and cause false-negative breath and stool antigen tests; they should be stopped about 2 weeks before testing. Serology stays positive after cure, so it cannot confirm current status.
Which carbapenem has no useful activity against Pseudomonas aeruginosa and should not be reported for it?
P. aeruginosa is intrinsically resistant to ertapenem, mainly through efflux and poor penetration. Meropenem, imipenem and doripenem are active against wild-type strains.
A skin flap becomes infected after medicinal leech therapy. The wound grows a beta-hemolytic, oxidase-positive, glucose-fermenting gram-negative rod. The most likely organism is:
Aeromonas species live in the leech gut and are a known cause of infection after leech therapy. They are oxidase positive, beta-hemolytic fermenters; P. aeruginosa does not ferment glucose.
Which growth result separates Vibrio cholerae from most other pathogenic Vibrio species?
V. cholerae (and V. mimicus) can grow without added salt, whereas most other pathogenic vibrios, such as V. parahaemolyticus and V. vulnificus, are halophilic and need NaCl.
A farmer in a tropical region develops a wound infection and sepsis after contact with stagnant water. The isolate is a gram-negative rod producing violet-black colonies. The most likely organism is:
Chromobacterium violaceum lives in tropical soil and water and produces the violet pigment violacein. Infections are rare but often severe. Serratia marcescens produces red pigment.
In Hugh-Leifson oxidation-fermentation (OF) glucose medium, the open tube turns yellow and the oil-sealed tube remains green. The organism is:
Acid only in the open, aerobic tube means glucose is used oxidatively, as by Pseudomonas. Fermenters make acid in both tubes; asaccharolytic organisms leave both tubes green or blue.
A patient treated for Helicobacter pylori needs a test to confirm eradication. The urea breath test or stool antigen test should be done at least:
Antibiotics and bismuth suppress H. pylori and can give false-negative results. Testing at least 4 weeks after therapy (and 2 weeks off proton pump inhibitors) avoids this.
Blood agar containing ampicillin is sometimes used to screen stool for Aeromonas because Aeromonas species:
Most Aeromonas species produce chromosomal beta-lactamases and are resistant to ampicillin, so they grow as beta-hemolytic, oxidase-positive colonies on ampicillin blood agar while much normal flora is inhibited.