Quiz 6: Enterobacterales: LFs, Biochemical Tests for ID

With few exceptions, what are the three general characteristics a gram-negative bacillus must possess to belong to the order Enterobacterales?

Ferment glucose, oxidase negative, usually motile

Somatic antigens in gram-negative bacilli are called _____ antigens

O

The K antigen found in some species of Enterobacterales is

Heat labile

EMB and MAC agar are examples of this type of media

Selective-Differential

The Vogues-Proskauer test identifies organisms capable of

Acetoin formation

The bacterial enzyme tryptophanase breaks down the amino acid tryptophan to

Indole

Ability to utilize this substrate as the sole source of carbon is the basis of this test

Simmon’s citrate

The ONPG test indicates an organism is capable of lactose fermentation by testing for this enzyme

β-galactosidase

A patient is admitted in the ED with fever, low BP, elevated WBC count, thrombocytopenia, and signs of shock. What virulence factor of gram-negative bacilli is responsible for these symptoms?

Endotoxin

Decarboxylation of the amino acids lysine, ornithine, and arginine results in the formation of

Amines

The stock cultures needed for QC testing (positive and negative reaction) of motility media are

Klebsiella pneumoniae and E. coli

This member of Enterobacteriaceae is not a member of the normal GI microbiota, is biochemically inert, and causes bacillary dysentery:

Shigella spp.

Which genera are ALL positive for the Voges-Proskauer test?

Klebsiella, Enterobacter, Serratia

A 20yo female with symptoms of a UTI present to her PCP. A urine culture reveals flat, pink colonies growing on MAC after 18hrs incubation. Given the results below, which is the most likely identification of this isolate?

Indole (+) Urea (-)

LDC (+) VP (-)

ODC (-) β-hemolysis on SBA (+)

Citrate (-)

Escherichia coli

A leg wound culture from a nursing home patient grew mucoid, pink colonies on MAC. Given these test results, which is the most likely identity of this isolate?

Indole (-)

LDC (+) Urease weak (+)

ODC (-) VP (+)

Citrate (+) Motility (-)

Klebsiella pneumoniae

This test is KEY to differentiate Klebsiella pneumoniae and Klebsiella oxytoca

indole

The initial screening of gram-negative rods is done by testing for use of this carbohydrate

Lactose

A blood culture from a post-operative patient becomes positive after 24hrs incubation and reveals flat, pink colonies on MAC. Given these test results, which is the most likely identity of this isolate?

Indole (-)

LDC (-) Urease (-)

ODC (+) VP (+)

Citrate (+) Motility (+)

Klebsiella aerogenes

A urine culture grows colorless colonies on MAC agar after 18hrs incubation. The ONPG test is positive. Explain the test results, include in your answer what types of organisms are being differentiated.

• MAC differentiates LF from NLF

• LF = pink colonies on MAC, clear colonies indicate organism lacks permease (lacY gene) and the lactose was not fermented within incubation period

• SLFs can ferment lactose but lack permease, preventing the fermentation of lactose within shorter timeframes

• ONPG uses β-galactosidase to allow lactose into the cell easier, acting as permease

• Positive ONPG and negative lactose fermentation on MAC indicates SLF

• ONPG test differentiates NLFs from SLFs