MLT GlobeBacteriology: Enterobacterales – page 5
99 Bacteriology MCQs on Enterobacterales with answers and explanations.
Which member of the Enterobacterales is intrinsically resistant to colistin?
Serratia marcescens, along with Proteus, Providencia and Morganella, is intrinsically resistant to polymyxins. Colistin resistance in E. coli or K. pneumoniae is acquired, for example by mcr genes.
Which is the correct way to write the organism commonly called 'Salmonella Typhimurium'?
Most human Salmonella belong to S. enterica subsp. enterica; serovar names are not species, so they are capitalized and not italicized, often shortened to Salmonella Typhimurium.
Which Klebsiella species is indole positive and linked to antibiotic-associated hemorrhagic colitis?
K. oxytoca is the indole-positive Klebsiella; toxin-producing strains cause hemorrhagic colitis after penicillin therapy. K. pneumoniae and K. aerogenes are indole negative.
Which ESBL enzyme family is now the most common worldwide in Escherichia coli?
CTX-M enzymes (especially CTX-M-15) are the dominant ESBLs worldwide. TEM-1 and SHV-1 are narrow-spectrum beta-lactamases, and KPC is a carbapenemase.
A patient develops septic shock during transfusion of a red cell unit stored for 30 days. A gram-negative rod that multiplies at 4 °C grows from the unit. The most likely organism is:
Y. enterocolitica is psychrotrophic and uses iron from stored red cells, so it multiplies during refrigerated storage and produces endotoxin. It is the classic cause of red cell transfusion sepsis.
A lactose-negative, H2S-positive stool isolate must be screened as possible Salmonella rather than Proteus. Which result supports Salmonella?
Salmonella is urease and PAD negative, while Proteus is urease and PAD positive and swarms. Both genera produce H2S, so H2S does not separate them.
A TSI slant shows acid slant, acid butt with gas and no blackening. Which organism fits best?
E. coli ferments lactose/sucrose (A/A) with gas and no H2S; Shigella gives K/A, while Salmonella and Proteus give K/A with H2S.
A positive methyl red test (red colour) shows that the organism:
Methyl red turns red below about pH 4.4, showing strong mixed-acid fermentation, as in E. coli; acetoin is detected by the VP test.
In typhoid fever, agglutinating antibodies detected by the Widal test usually reach diagnostic (rising) titres from which stage of illness?
Widal antibodies begin to appear at the end of the first week and rise to diagnostic titres in the second week, which is why blood culture is preferred in the first week.
Which E. coli pathotype is often non-motile, lactose non-fermenting or late, lysine decarboxylase negative, and causes dysentery like Shigella?
EIEC invades colonic epithelium like Shigella and shares many of its biochemical features, which can cause misidentification.
An E. coli from chronic diarrhoea in a child adheres to HEp-2 cells in a 'stacked-brick' pattern. The pathotype is:
Enteroaggregative E. coli shows aggregative (stacked-brick) adherence. EPEC shows localized adherence with attaching-and-effacing lesions.
A stool isolate is H2S positive and resembles Salmonella, but is lysine decarboxylase negative, grows in KCN broth and is ONPG positive. It is most likely:
C. freundii is H2S positive but lysine decarboxylase negative and KCN positive; Salmonella is lysine positive and usually KCN negative. Proteus would be urease and PDA positive.
A carbapenem-resistant K. pneumoniae is mCIM positive. In the EDTA-modified test (eCIM), the zone increases by 7 mm compared with mCIM. This indicates:
EDTA chelates zinc and inhibits metallo-beta-lactamases. An eCIM zone increase of ≥5 mm over mCIM indicates a metallo-enzyme; serine carbapenemases are not inhibited.
A neonate has meningitis with a brain abscess. The CSF grows a motile gram-negative rod that is indole positive, citrate positive, H2S negative and urease negative. The most likely organism is:
Citrobacter koseri is indole positive and H2S negative and is classically linked to neonatal meningitis with brain abscess. C. freundii is indole negative and H2S positive; E. coli is citrate negative.
A non-swarming, lactose-negative urine isolate is phenylalanine deaminase positive, strongly urease positive, H2S negative, ornithine decarboxylase positive and citrate negative. The most likely organism is:
Morganella morganii is PAD and urease positive, ODC positive, H2S negative and does not swarm. Proteus species swarm and produce H2S; Providencia stuartii is ODC negative and citrate positive.
A traveller who ate raw oysters has diarrhoea. The stool grows an oxidase-positive, glucose-fermenting gram-negative rod that is lysine and ornithine decarboxylase positive, arginine dihydrolase positive and DNase negative. The most likely organism is:
Plesiomonas shigelloides is the only oxidase-positive member of the Enterobacterales and is positive for all three decarboxylase/dihydrolase tests. Aeromonas is DNase positive; Shigella is oxidase negative.
A Klebsiella pneumoniae producing NDM carbapenemase tests resistant to ceftazidime-avibactam. The reason is that:
Avibactam inhibits class A (including KPC), class C and some class D enzymes, but not metallo-beta-lactamases such as NDM, VIM and IMP, which use zinc.
A wound infection follows an injury while cleaning freshwater fish. The isolate is lactose negative, H2S positive, indole positive, lysine decarboxylase positive and urease negative. The most likely organism is:
Edwardsiella tarda, found in fish and freshwater, is H2S and indole positive and LDC positive. Salmonella is indole negative, P. vulgaris is urease positive and LDC negative, and C. freundii is LDC negative.
A Proteus mirabilis isolate has an imipenem MIC of 2 µg/mL but is susceptible to meropenem and ertapenem, and the mCIM is negative. The most likely explanation is:
Proteus, Providencia and Morganella often have raised imipenem MICs by non-carbapenemase mechanisms (low PBP affinity). Normal meropenem and ertapenem results and a negative mCIM argue against a carbapenemase.