Enteric Gram-Negative Rods (Enterobacteriaceae):
The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera (escherichia, shigella, salmonella, enterobacter, klebsiella, serratia, proteus, and others). Some enteric organisms, eg, Escherichia coli, are part of the normal flora and incidentally cause disease, while others, the salmonellae and shigellae, are regularly pathogenic for humans. The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and other virulence factors.Classification
The Enterobacteriaceae are the most common group of gram-negative rods cultured in the clinical laboratory and along with staphylococci and streptococci are among the most common bacteria that cause disease. The taxonomy of the Enterobacteriaceae is complex and rapidly changing since the introduction of techniques that measure evolutionary distance, such as nucleic acid hybridization and sequencing. More than 25 genera and 110 species or groups have been defined
The family Enterobacteriaceae have the following characteristics: They are gram-negative rods, either motile with peritrichous flagella or nonmotile; they grow on peptone or meat extract media ; MacConkey's agar; grow aerobically and anaerobically (are facultative anaerobes); ferment rather than oxidize HYPERLINK "javascript:showDrugInfo(2458);" glucose, often with gas production; are catalase-positive, oxidase-negative, and reduce nitrate to nitrite; and have a 3959% G + C DNA content .
Morphology & Identification
Typical Organisms
The Enterobacteriaceae are short gram-negative rods. 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, less so in enterobacter, and uncommon in the other species.
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.Growth Characteristics
Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation. Some tests, eg, the production of indole from HYPERLINK "javascript:showDrugInfo(2362);" tryptophan, are commonly used in rapid identification systems, while others, eg, 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's, or deoxycholate medium) distinguishes lactose-fermenting (colored) from non-lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of 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.
Escherichia
E coli typically produces positive tests for indole, lysine decarboxylase, and HYPERLINK "javascript:showDrugInfo(363);" mannitol fermentation and produces gas from HYPERLINK "javascript:showDrugInfo(2458);" 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 , oxidase negative and methyl red positive .
Klebsiella-Enterobacter-Serratia Group
Klebsiella species exhibit mucoid growth, large polysaccharide capsules, and lack of motility, and they usually give positive tests for lysine decarboxylase and citrate. Most enterobacter species give positive tests for motility, citrate, and ornithine decarboxylase and produce gas from HYPERLINK "javascript:showDrugInfo(2458);" glucose. Enterobacter aerogenes has small capsules. Serratia produces DNase, lipase, and gelatinase. Klebsiella, enterobacter, and serratia usually give positive Voges-Proskauer reactions.
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 unless the swarming is inhibited by chemicals, eg, phenylethyl alcohol . Proteus species and Morganella morganii are urease-positive, while providencia species usually are urease-negative. The proteus-providencia group ferment lactose very slowly or not at all. Proteus mirabilis is more susceptible to antimicrobial drugs, including HYPERLINK "javascript:showDrugClass('penicillins');" penicillins, than other members of the group.
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.
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.
Salmonella
Salmonellae are motile rods that characteristically ferment HYPERLINK "javascript:showDrugInfo(2458);" 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. Arizona is included in the salmonella group.
Other Enterobacteriaceae
Yersinia Eedwardsiella Ewingella, Hafnia,Cedecea, and kluyvera.
Antigenic Structure
Enterobacteriaceae have a complex antigenic structure. They are classified by more than 150 different heat-stable somatic O (lipopolysaccharide) antigens, more than 100 heat-labile K (capsular) antigens, and more than 50 H (flagellar) antigens
O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. Some O-specific polysaccharides contain unique sugars. O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination. Antibodies to O antigens are predominantly IgM.
K antigens are external to O antigens on some but not all Enterobacteriaceae. Some are polysaccharides, including the K antigens of E coli; others are proteins. K antigens may interfere with agglutination by O antisera, and they may be associated with virulence .Klebsiellae form large capsules consisting of polysaccharides (K antigens) covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera.
H antigens are located on flagella and are denatured or removed by heat or alcohol. They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG.
Diseases Caused by Enterobacteriaceae Other Than Salmonella & Shigella
Causative Organisms
E coli is a member of the normal intestinal flora .Other enteric bacteria (proteus, enterobacter, klebsiella, morganella, providencia, citrobacter, and serratia species) are also found as members of the normal intestinal flora but are considerably less common than E coli. The enteric bacteria are sometimes found in small numbers as part of the normal flora of the upper respiratory and genital tracts. The enteric bacteria generally do not cause disease, and in the intestine they may even contribute to normal function and nutrition. When clinically important infections occur, they are usually caused by E coli, but the other enteric bacteria are causes of hospital-acquired infections and occasionally cause community-acquired infections. The bacteria become pathogenic only when they reach tissues outside of their normal intestinal or other less common normal flora sites. The most frequent sites of clinically important infection are the urinary tract, biliary tract, and other sites in the abdominal cavity, but any anatomic site (eg, bacteremia, prostate gland, lung, bone, meninges) can be the site of disease. Some of the enteric bacteria (eg, Serratia marcescens, Enterobacter aerogenes) are opportunistic pathogens. When normal host defenses are inadequateparticularly in infancy or old age, in the terminal stages of other diseases, after immunosuppression, or with indwelling venous or urethral catheterslocalized clinically important infections can result, and the bacteria may reach the blood stream and cause sepsis.
Pathogenesis & Clinical Findings
The clinical manifestations of infections with E coli and the other enteric bacteria depend on the site of the infection and cannot be differentiated by symptoms or signs from processes caused by other bacteria.
E Coli
Urinary Tract Infection
E coli is the most common cause of urinary tract infection and accounts for approximately 90% of first urinary tract infections in young women. The symptoms and signs include urinary frequency, dysuria, hematuria, and pyuria. Flank pain is associated with upper tract infection. None of these symptoms or signs is specific for E coli infection. Urinary tract infection can result in bacteremia with clinical signs of sepsis .Nephropathogenic E coli typically produce a hemolysin. (80% community and most nosocomial) Originate from gastrointestinal tract Important virulence factors Resistance to serum-killingProduction of toxins e.g hemolysins Pili-mediated binding (not demonstrated in vivo) Production of slime layer that participates in cell adhesion .
E coli-Associated Diarrheal Diseases
E coli that cause diarrhea are extremely common worldwide. These E coli are classified by the characteristics of their virulence properties and each group causes disease by a different mechanism. the toxins often are plasmid- or phage-mediated..
Enteropathogenic E coli (EPEC) is an important cause of diarrhea in infants, especially in developing countries. EPEC adhere to the mucosal cells of the small bowel. The result of EPEC infection is watery diarrhea .
Enterotoxigenic E coli (ETEC) is a common cause of "traveler's diarrhea" and a very important cause of diarrhea in infants in developing countries. ETEC colonization factors specific for humans promote adherence of ETEC to epithelial cells of the small bowel. Some strains of ETEC produce a heat-labile exotoxin (LT)(MW 80,000) , which results in intense and prolonged hypersecretion of water and chlorides and inhibits the reabsorption of sodium. The gut lumen is distended with fluid, and hypermotility and diarrhea ensue, lasting for several days.Some strains of ETEC produce the heat-stable enterotoxin STa (MW 15004000), ST stimulates fluid secretion. Many STa-positive strains also produce LT. The strains with both toxins produce a more severe diarrhea.
Enterohemorrhagic E coli (EHEC) produces verotoxin , There are at least two antigenic forms of the toxin. EHEC has been associated with hemorrhagic colitis, a severe form of diarrhea, and with hemolytic uremic syndrome, a disease resulting in acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia. Verotoxin has many properties that are similar to the Shiga toxin produced by some strains of Shigella dysenteriae type 1; however, the two toxins are antigenically and genetically distinct. Of the E coli serotypes that produce verotoxin, O157:H7 is the most common and is the one that can be identified in clinical specimens. ETEC O157:H7 does not use HYPERLINK "javascript:showDrugInfo(1565);" sorbitol, unlike most other E coli, and is negative on sorbitol MacConkey agar sorbitol is used instead of lactose .
Enteroinvasive E coli (EIEC) produces a disease very similar to shigellosis. The disease occurs most commonly in children and in travelers . Like shigella, EIEC strains are nonlactose or late lactose fermenters and are nonmotile. EIEC produce disease by invading intestinal mucosal epithelial cells.
Enteroaggregative E coli (EAEC) causes acute and chronic diarrhea (> 14 days in duration) . These organisms also are the cause of food-borne illnesses . They are characterized by their characteristic pattern of adherence to human cells. EAEC produce ST-like toxin and a hemolysin.
Sepsis
When normal host defenses are inadequate, E coli may reach the bloodstream and cause sepsis. Newborns may be highly susceptible to E coli sepsis because they lack IgM antibodies. Sepsis may occur secondary to urinary tract infection.
Meningitis
E coli and group B streptococci are the leading causes of meningitis in infants. Approximately 75% of E coli from meningitis cases have the K1 antigen.
Klebsiella-Enterobacter-Serratia; Proteus-Morganella-Providencia; and Citrobacter
The pathogenesis of disease caused by these groups of enteric gram-negative rods is similar to that of the nonspecific factors in disease caused by E coli.
Klebsiella
K pneumoniae is present in the respiratory tract and feces of about 5% of normal individuals. It causes a small proportion (about 1%) of bacterial pneumonias. K pneumoniae can produce extensive hemorrhagic necrotizing consolidation of the lung. It occasionally produces urinary tract infection and bacteremia with focal lesions in debilitated patients. Other enterics also may produce pneumonia. K pneumoniae and Klebsiella oxytoca cause hospital-acquired infections. Two other klebsiellae are associated with inflammatory conditions of the upper respiratory tract .
Enterobacter aerogenes
This organism has small capsules, may be found free-living as well as in the intestinal tract, and causes urinary tract infections and sepsis.
Serratia
S marcescens is a common opportunistic pathogen in hospitalized patients. Serratia (usually nonpigmented) causes pneumonia, bacteremia, and endocarditisespecially in narcotics addicts and hospitalized patients. Only about 10% of isolates form the red pigment
Proteus
Proteus species produce infections in humans only when the bacteria leave the intestinal tract. They are found in urinary tract infections and produce bacteremia, pneumonia, and focal lesions in debilitated patients or those receiving intravenous infusions. P mirabilis causes urinary tract infections and occasionally other infections. Proteus vulgaris and Morganella morganii are important nosocomial pathogens.
Proteus species produce urease, resulting in rapid hydrolysis of urea with liberation of ammonia. Thus, in urinary tract infections with proteus, the urine becomes alkaline, promoting stone formation and making acidification virtually impossible. The rapid motility of proteus may contribute to its invasion of the urinary tract.
Strains of proteus vary greatly in antibiotic sensitivity. P mirabilis is often inhibited by HYPERLINK "javascript:showDrugClass('penicillins');" penicillins; the most active antibiotics for other members of the group are HYPERLINK "javascript:showDrugClass('aminoglycosides');" aminoglycosides and HYPERLINK "javascript:showDrugClass('cephalosporins');" cephalosporins.
Providencia
Providencia species (Providencia rettgeri, Providencia alcalifaciens, and Providencia stuartii) are members of the normal intestinal flora. All cause urinary tract infections and occasionally other infections and are often resistant to antimicrobial therapy.
Citrobacter
Citrobacter can cause urinary tract infections and sepsis.
Diagnostic Laboratory Tests
Specimens
Urine, blood, pus, spinal fluid, sputum, or other material, as indicated by the localization of the disease process.
Smears
The Enterobacteriaceae resemble each other morphologically. The presence of large capsules is suggestive of klebsiella.
Culture
Specimens are plated on both blood agar and differential media. With differential media, rapid preliminary identification of gram-negative enteric bacteria is often possible .
Immunity
Specific antibodies develop in systemic infections, but it is uncertain whether significant immunity to the organisms follows.
Treatment
No single specific therapy is available. The HYPERLINK "javascript:showDrugClass('sulfonamides');" sulfonamides, HYPERLINK "javascript:showDrugInfo(34);" ampicillin, HYPERLINK "javascript:showDrugClass('cephalosporins');" cephalosporins, fluoroquinolones, and HYPERLINK "javascript:showDrugClass('aminoglycosides');" aminoglycosides have marked antibacterial effects against the enterics, but variation in susceptibility is great, and laboratory tests for antibiotic sensitivity are essential. Multiple drug resistance is common
Treatment of gram-negative bacteremia and impending septic shock requires rapid institution of antimicrobial therapy, restoration of fluid and electrolyte balance, and treatment of disseminated intravascular coagulation. Administration of antiglycolipid antibody is experimental but can prevent shock and death.
Various means have been proposed for the prevention of traveler's diarrhea, including daily ingestion of HYPERLINK "javascript:showDrugInfo(69);" bismuth subsalicylate suspension (bismuth subsalicylate can inactivate E coli enterotoxin in vitro) and regular doses of HYPERLINK "javascript:showDrugClass('tetracyclines');" tetracyclines or other antimicrobial drugs for limited periods. Because none of these methods are entirely successful or lacking in adverse effects, it is widely recommended that caution be observed in regard to food and drink in areas where environmental sanitation is poor and that early and brief treatment (eg, with HYPERLINK "javascript:showDrugInfo(127);" ciprofloxacin or HYPERLINK "javascript:showDrugInfo(631);" trimethoprim- HYPERLINK "javascript:showDrugInfo(581);" sulfamethoxazole) be substituted for prophylaxis.
Epidemiology, Prevention, & Control
The enteric bacteria establish themselves in the normal intestinal tract within a few days after birth and from then on constitute a main portion of the normal aerobic (facultative anaerobic) microbial flora. E coli is the prototype. Enterics found in water or milk are accepted as proof of fecal contamination from sewage or other sources.
Control measures are not feasible as far as the normal endogenous flora is concerned. Enteropathogenic E coli serotypes should be controlled like salmonellae . Some of the enterics constitute a major problem in hospital infection. It is particularly important to recognize that many enteric bacteria are "opportunists" which cause illness when they are introduced into debilitated patients. Within hospitals or other institutions, these bacteria commonly are transmitted by personnel, instruments, or parenteral medications. Their control depends on hand washing, rigorous asepsis, sterilization of equipment, disinfection, restraint in intravenous therapy, and strict precautions in keeping the urinary tract sterile .drainage).