The enteric bacteria generally do not cause disease, and in the intestine, they may even contribute to normal function and nutrition. The enteric bacteria are sometimes found in small numbers as part of the normal microbiota of the upper respiratory and genital tracts. Other enteric bacteria ( Proteus, Enterobacter, Klebsiella, Morganella, Providencia, Citrobacter, and Serratia species) are also found as members of the normal intestinal microbiota but are considerably less common than E coli. Organisms producing acid on the slant and acid and gas (bubbles) in the butt are other enteric bacteria.Į coli are members of the normal intestinal microbiota (see Chapter 10). Although Proteus, Providencia, and Morganella species produce an alkaline slant and acid butt, they can be identified by their rapid formation of red color in Christensen’s urea medium. Salmonellae and shigellae typically yield an alkaline slant and an acid butt. If lactose or sucrose is fermented, so much acid is produced that the slant and butt remain yellow (acid). If only glucose is fermented, the slant and the butt initially turn yellow from the small amount of acid produced as the fermentation products are subsequently oxidized to CO 2 and H 2O and released from the slant and as oxidative decarboxylation of proteins continues with formation of amines, the slant turns alkaline (red). It is poured into a test tube to produce a slant with a deep butt and is inoculated by stabbing bacterial growth into the butt. The medium contains 0.1% glucose, 1% sucrose, 1% lactose, ferrous sulfate (for detection of H 2S production), tissue extracts (protein growth substrate), and a pH indicator (phenol red). One such medium is triple sugar iron (TSI) agar, which is often used to help differentiate salmonellae and shigellae from other enteric gram-negative rods in stool cultures. Many complex media have been devised to help in identification of the enteric bacteria. This technology is unable to differentiate Shigella from E coli. This new technology seems to work quite well for identification of most of the common Enterobacteriaceae encountered in clinical material except for Shigella species. The implementation of matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF MS) for identification of culture isolates is replacing the more traditional panels of biochemicals currently in use in most clinical microbiology laboratories. ![]() However, these are largely being replaced by other methods. In the United States, commercially prepared kits or automated systems are used to a large extent for this purpose. They can be differentiated to species level by a vast array of biochemical tests. Members of the family Enterobacteriaceae have the following characteristics: They are gram-negative rods, either motile with peritrichous flagella or nonmotile grow on peptone or meat extract media without the addition of sodium chloride or other supplements grow well on MacConkey agar grow aerobically and anaerobically (are facultative anaerobes) ferment rather than oxidize glucose, often with gas production are catalase positive, oxidase negative (except for Plesiomonas) and reduce nitrate to nitrite and have a 39–59% G + C DNA content.
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