Enterobacteriacae

• All members of Family Enterobacteriaceae are oxidase negative. They do not contain cytochrome c in their electron transport chains.
• All members of Family Enterobacteriaceae ferment glucose. However, two different pathways are used. The mixed acid pathway, used by E. coli, Citrobacter, Salmonella, Proteus, Yersinia and Shigella, converts glucose to mixed acids. The test used to detect the mixed acid pathway is the methyl red test. Klebsiella, Enterobacter and Serratia use the butylene glycol pathway to converts glucose to alcohols by way of acetoin. The test used to detect the butylene glycol pathway is the Voges-Proskauer test.
• All Enterobacteriaceae reduce nitrate.
• EMB agar is designed to test for lactose fermentation. E. coli is a lactose fermenter and hence forms colored colonies on EMB agar. S. flexneri cannot ferment lactose, so it produces colorless colonies when grown on EMB.
• Shigella probably do not enter intestinal epithelial cells but invade the gut through M cells on Peyer's patches. If a person lacked M cells, Shigella should not be able to invade. Remember, invade can either mean the ability to enter and survive inside host cells, or the ability to invade from a mucosal surface and spread systemically. In this case, the former definition is applicable because Shigella almost always stays within the GI tract.
• Shigella and Klebsiella are nonmotile members of Enterobacteriaceae.

AMH