A. Typical Organisms

The Enterobacteriaceae are short Gram-negative rods (Figure 1A). Typical morphology is seen in growth on solid media in vitro, but morphology is highly variable in clinical specimens. Capsules are large and regular in Klebsiella species, less so in Enterobacter species, and uncommon in the other species.

Fig1. A: Gram-stain of E. coli. Original magnification ×1000. (Courtesy of H Reyes.) B: Antigenic structure of Enterobacteriaceae.

B. Culture

E. coli and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges. Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation. The salmonellae and shigellae produce colonies similar to E. coli but do not ferment lactose. Some strains of E. coli produce hemolysis on blood agar.

C. Growth Characteristics

 Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation. Some tests, such as the production of indole from tryptophan, are commonly used in rapid identification systems, but others, such as the Voges-Proskauer reaction (production of acetylmethylcarbinol from dextrose), are used less often. Culture on “differential” media that contain special dyes and carbohydrates (eg, eosin-methylene blue [EMB], MacConkey, or deoxycholate medium) distinguishes lactose-fermenting (colored) from non-lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of enteric bacteria (Table1).

Table1. Rapid, Presumptive Identification of Gram-Negative Enteric Bacteria

Many complex media have been devised to help in identification of the enteric bacteria. 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. The medium contains 0.1% glucose, 1% sucrose, 1% lactose, ferrous sulfate (for detection of H₂S pro duction), tissue extracts (protein growth substrate), and a pH indicator (phenol 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. 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 CO₂ and H2O and released from the slant and as oxidative decarboxylation of proteins continues with formation of amines, the slant turns alkaline (red). If lactose or sucrose is fermented, so much acid is produced that the slant and butt remain yellow (acid). Salmonellae and shigellae typically yield an alkaline slant and an acid butt. 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. Organisms producing acid on the slant and acid and gas (bubbles) in the butt are other enteric bacteria.

1. Escherichia—E. coli typically produces positive test results for indole, lysine decarboxylase, and mannitol fermentation and produces gas from glucose. An isolate from urine can be quickly identified as E. coli by its hemolysis on blood agar, typical colonial morphology with an iridescent “sheen” on differential media such as EMB agar, and a positive spot indole test result. More than 90% of E. coli isolates are positive for β-glucuronidase using the substrate 4-methylumbelliferyl-β-glucuronide (MUG). Isolates from anatomic sites other than urine, with characteristic proper ties (above plus negative oxidase test results) often can be confirmed as E. coli with a positive MUG test result.

2. Klebsiella–Enterobacter–Serratia group—Klebsiella species exhibit mucoid growth, large polysaccharide capsules, and lack of motility, and they usually give positive test results for lysine decarboxylase and citrate. Most Enterobacter species give positive test results for motility, citrate, and ornithine decarboxylase and produce gas from glucose. Enterobacter aerogenes has small capsules. Some species of Enterobacter have been moved into the genus Cronobacter. Serratia species produces DNase, lipase, and gelatinase. Klebsiella, Enterobacter, and Serratia species usually give positive Voges-Proskauer reactions.

3. Proteus–Morganella–Providencia group—The members of this group deaminate phenylalanine, are motile, grow on potassium cyanide medium (KCN), and ferment xylose. Proteus species move very actively by means of peritrichous flagella, resulting in “swarming” on solid media (Figure 2), unless the swarming is inhibited by chemicals, such as phenylethyl alcohol or CLED (cystine-lactose-electrolyte-deficient) medium. Whereas Proteus species and Morganella morganii are urease positive, Providencia species usually are urease negative. The Proteus–Providencia group ferments lactose very slowly or not at all.

Fig2. Proteus mirabilis on sheep blood agar. Proteus species exhibit a characteristic swarming pattern, causing a wave like appearance, and individual colonies are not distinguishable. (Courtesy of S. Riedel.)

4. Citrobacter—These bacteria typically are citrate positive and differ from the salmonellae in that they do not decarboxylate lysine. They ferment lactose very slowly if at all.

5. Shigella—Shigellae are nonmotile and usually do not ferment lactose but do ferment other carbohydrates, producing acid but not gas. They do not produce H₂S. The four Shigella species are closely related to E. coli. Many share common antigens with one another and with other enteric bacteria (eg, Hafnia alvei and Plesiomonas shigelloides).

 6. Salmonella—Salmonellae are motile rods that characteristically ferment glucose and mannose without producing gas but do not ferment lactose or sucrose. Most salmonellae produce H₂S. They are often pathogenic for humans or animals when ingested. Organisms originally described in the genus Arizona are now included as subspecies in the Salmonella group.

 7. Other Enterobacteriaceae—Yersinia species are discussed in Chapter 19. Other genera occasionally found in human infections include Cronobacter, Edwardsiella, Ewingella, Hafnia, Cedecea, Plesiomonas, and Kluyvera.

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مواضيع ذات صلة

Clinical Findings of Enterococci

Streptococcus pneumoniae

Anaerobic Media

Specimen Processing of Anaerobic Bacteriology

Specimen Collection and Transport of Anaerobic Bacteriology

Gonococcal Arthritis

Nongonococcal Bacterial Arthritis

Acute Bacterial Arthritis

Streptococcus pyogenes

Treatment of Staphylococci

Enzymes and Toxins of Staphylococci

Rocky Mountain Spotted Fever