ENTEROBACTERIACEAE Flashcards
Biochemically, Enterobacteriaceae species are gram-negative rods that:
Ferment glucose, reduce nitrate to nitrite, and are oxidase negative
The family Enterobacteriaceae consists of more than 100 species and represents the most commonly encountered isolates in clinical specimens. All Enterobacteriaceae species ferment glucose, are nonsporulating, and are oxidase negative (except for Plesiomonas shigelloides, recently added to the family but which is oxidase positive). Most Enterobacteriaceae species are motile, but the genera Shigella and Klebsiella are not.
The ortho-nitrophenyl-β-galactopyranoside (ONPG) test is most useful when differentiating:
Shigella spp. from some strains of Escherichia coli
The ONPG test detects β-galactosidase activity and is most useful in distinguishing late lactose fermenters from lactose nonfermenters. Some strains of E. coli are slow lactose fermenters and may be confused with Shigella spp., which do not ferment lactose. E. coli are ONPG positive, whereas Shigella spp. are ONPG negative.
The Voges-Proskauer (VP) test detects which end product of glucose fermentation?
Acetoin
Acetoin or carbinol, an end product of glucose fermentation, is converted to diacetyl after the addition of the VP reagents (β-naphthol and 40% potassium hydroxide [KOH]). Diacetyl is seen as a red- to-pink–colored complex.
At which pH does the methyl red (MR) test become positive?
4.5
Both MR and VP tests detect acid production from the fermentation of glucose. However, a positive MR test result denotes a more complete catabolism of glucose to highly acidic end products, such as formate and acetate, than occurs with organisms that are VP positive only (e.g., Klebsiella pneumoniae).
A positive Simmons citrate test is seen as a:
Blue color in the medium after 24 hours of incubation at 35°C
The Simmons citrate test determines if an organism can utilize citrate as the sole source of carbon. The medium turns blue, indicating the presence of alkaline products, such as carbonate. Tubes are incubated a minimum of 24 hours at 35°C with a loose cap before reading.
In the test for urease production, ammonia reacts to form which product?
Ammonium carbonate
The test for urease production is based on the ability of the colonies to hydrolyze urea in Stuart broth or Christensen agar to form CO2 and ammonia. These products form ammonium carbonate, resulting in alkalinization. This turns the pH indicator (phenol red) pink at pH 8.0.
Which of the following reagents is added to detect the production of indole?
p-Dimethylaminobenzaldehyde
The indole test detects the conversion of tryptophan (present in the media) to indole by the enzyme tryptophanase. Indole is detected by the reaction with the aldehyde group of p-dimethylaminobenzaldehyde (the active reagent in Kovac and Ehrlich reagents) in acid, forming a red complex.
Decarboxylation of the amino acids lysine, ornithine, and arginine results in the formation of:
Amines
Specific decarboxylases split dibasic amino acids (lysine, arginine, and ornithine), forming alkaline amines. These products turn the pH indicators in the medium (cresol red and bromcresol purple) from yellow to purple.
Lysine iron agar (LIA) showing a purple slant and a blackened butt indicates:
Salmonella spp.
LIA is used as an aid for the identification of Salmonella species. It contains phenylalanine, lysine, glucose, thiosulfate, ferric ammonium citrate, and bromcresol purple. Salmonella spp. produce H2S from thiosulfate. This reduces ferric ammonium citrate, forming ferrous sulfate and causing the butt to blacken. Salmonella also decarboxylate lysine to produce alkaline amines, giving the slant its purple color and differentiating it from Citrobacter spp., which are lysine decarboxylase negative.
Putrescine is an alkaline amine product of which bacterial enzyme?
Ornithine decarboxylase
Putrescine is the amine product of the decarboxylation of ornithine.
Which genera are positive for phenylalanine deaminase?
Morganella, Providencia, and Proteus
Phenylalanine deaminase oxidatively deaminates phenylalanine, forming phenylpyruvic acid. When a solution of ferric chloride is added, the iron reacts with phenylpyruvic acid, forming a green-colored complex. Phenylalanine deaminase is found in the genera Morganella, Providencia, and Proteus and is an excellent test to determine if an organism belongs to this group. Rarely, isolates of Enterobacter may be phenylalanine deaminase positive as well.
Kligler iron agar (KIA) differs from triple-sugar iron agar (TSI) in the:
Use of sucrose in the medium
Both KIA and TSI contain 10-fold more lactose than glucose, peptone, and phenol red to detect acid production (turns yellow) and sodium thiosulfate and ferrous ammonium sulfate to detect H2S production. However, TSI contains sucrose, and KIA does not. Organisms fermenting either sucrose or lactose will turn the slant of the agar tube yellow. Therefore, some organisms (e.g., many species of Cedecea, Citrobacter, Edwardsiella, and Serratia) will produce a yellow slant on TSI but a red slant on KIA.
The malonate test is most useful in differentiating which members of the Enterobacteriaceae family?
Salmonella subgroups 2, 3 (the former Arizona)
The malonate test determines whether an organism can utilize sodium malonate as the sole source of carbon. Malonate is broken down, forming alkaline metabolites that raise the pH of the broth above 7.6. This causes bromthymol blue to turn from green to deep blue (Prussian blue). E. coli, Shigella, and most Salmonella are malonate negative, whereas Enterobacter and Salmonella (formerly Arizona) subgroups 2, 3a, and 3b are positive. Proteus, Providencia, Serratia, and Yersinia are also malonate negative.
Which genera of the Enterobacteriaceae family are known to cause diarrhea and are considered enteric pathogens?
Escherichia, Salmonella, Shigella, and Yersinia
Escherichia, Salmonella, Shigella, and Yersinia are responsible for the majority of enteric diarrhea cases attributable to the Enterobacteriaceae family.
An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea should be tested for which sugar before sending it to a reference laboratory for serotyping?
Sorbitol (fermentation)
An isolate of E. coli (shiga-like producing toxin of E. coli [STEC]) recovered from a stool culture in hemorrhagic colitis can be definitely identified only by serotyping. The isolate is identified as E. coli by the usual biochemical reactions. The strain of E. coli responsible for hemorrhagic colitis is O157:H7 and is usually negative for sorbitol fermentation. Colonies of this strain of E. coli appear colorless on MacConkey agar with sorbitol added.
Care must be taken when identifying biochemical isolates of Shigella because serological cross reactions occur with:
Escherichia coli
Serological confirmation of Shigella isolates is based on O antigen typing. If a suspected Shigella spp. is serologically typed with polyvalent sera before it has been correctly identified biochemically, a false-positive confirmation may occur with an isolate that is E. coli (i.e., anaerogenic, non–gas-producing, lactose-negative or - delayed, and nonmotile strains). These strains were formerly known as the Alkalescens-Dispar serotype.
Which species of Shigella is most commonly associated with diarrheal disease in the United States?
Shigella sonnei
The Shigella spp. are lactose nonfermenters that, for the most part, are biochemically inert and are classified into serogroups A, B, C, and D as a result of their biochemical similarity. S. sonnei is the species most often isolated from diarrhea cases in the United States. It is more active biochemically than the other species because of ornithine decarboxylase and β-galactosidase activity. These enzymes, found in most strains of S. sonnei, distinguish it from other Shigella species.
