Lec 7 Enterobacteriacea I - E coli PDF

Medical Bacteriology Lec – 8 Enterobacteriacea I Dr. Sahand K Arif Enterobacteriacea I More than 25 genera and 110 species clinically significant 20-25 species Escherichia Shigella Edwardsiella...

Medical Bacteriology Lec – 8 Enterobacteriacea I Dr. Sahand K Arif Enterobacteriacea I More than 25 genera and 110 species clinically significant 20-25 species Escherichia Shigella Edwardsiella Salmonella Citrobacter Klebsiella Enterobacter Hafnia Serratia They are gram-negative rods, either motile with peritrichous flagella or nonmotile grow well on MacConkey's agar; grow aerobically and anaerobically (are facultative anaerobes); ferment rather than oxidize glucose, often with gas production; are catalase-positive, oxidase-negative, and reduce nitrate to nitrite.  The family is numerically important to the medical microbiologist, as they may account for 80% of clinically significant Gram-negative bacilli ~50% of isolates from cases of septicaemia. 50 are definitely or probably associated with human disease Other genera within the family Enterobacteriaceae (Klebsiella, Enterobacter, Serratia and Citrobacter), which share this ability to ferment lactose rapidly (typically within 24 h), are collectively termed coliform bacteria.  Slowly Rapidly Not Fermented Lactose Fermented Lactose Fermented Lactose E coli: metallic sheen Edwardsiella, serratia, Shigella species: on differential media; citrobacter, arizona, nonmotile; no gas from motile; flat, nonviscous providencia, erwinia dextrose colonies Enterobacter Salmonella species: aerogenes: raised motile; acid and usually colonies; often motile; gas from dextrose more vescous growth K pneumoniae : very Proteus species: viscous, Mucoid growth; "swarming" on agar; urea nonmotile Pseudomonas species rapidly hydrolyzed (smell soluble pigments, blue- of ammonia) green and fluorescing; sweetish smell  Antigenic Structure They are classified by >150 different heat-stable somatic O (lipopolysaccharide) antigens, >100 heat-labile K (capsular) antigens, >more than 50 H (flagellar) antigens. In Salmonella typhi, the capsular antigens are called Vi antigens. 1- O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination. a single organism may carry several O antigens. Thus, most shigellae share one or more O antigens with E coli. E coli may crossreact with some providencia, klebsiella, and salmonella species. Occasionally, O antigens may be associated with specific human diseases, eg, specific O types of E coli are found in diarrhea and in urinary tract infections.  2- K antigens are external to O antigens on some but not all Enterobacteriaceae. K antigens may interfere with agglutination by O antisera, and they may be associated with virulence (eg, E coli strains producing KI antigen are prominent in neonatal meningitis, and K antigens of E. coli cause attachment of the bacteria to epithelial cells prior to gastrointestinal or urinary tract invasion). 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. Human infections of the respiratory tract are caused particularly by capsular types 1 and 2; those of the urinary tract, by types 8, 9, 10, and 24. 3- H antigens are located on flagella and are denatured or removed by heat or alcohol.  Colicins (Bacteriocins) Many gram-negative and Gram-positive organisms produce - bacteriocins. These bactericidal substance are produced by certain strains of bacteria active against other strains of the same or closely related Bacteria Their production is controlled by plasmids. Colicins are produced by E coli, Marcescins by serratia, and Pyocins by pseudomonas. Bacteriocin producing strains are resistant to their own bacteriocin; thus can be used for "typing" of organisms.  E coli E COLI is a member of the normal intestinal flora. > 700 antigenic types (serotypes) are recognized based on O, H, and K antigens. Serotyping is still important in distinguishing the small number of strains that actually cause disease. E. coli is responsible for three types of infections in humans: 1-urinary tract infections (UTI), 2-neonatal meningitis, and 3-intestinal diseases (gastroenteritis). These three diseases depend on a specific array of pathogenic (virulence) determinants. E coli typically produces positive test 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 as EMB agar, and a positive spot indole test.  Biochemistry Most are motile. May be hemolytic on BA KEY tests for the normal strain: TSI is A/A + gas Urea – Indole + Citrate – Motility + There is an inactive biotype that is anaerogenic, lactose – , and nonmotile.  Nevertheless, E. coli is one of the most common Gram-negative bacteria responsible for bacteraemia in humans. Isolates of E. coli that infect the bloodstream often possess virulence factors that enable the organisms to circumvent the normal clearance mechanisms and evade the host immune response. These include : a range of adhesins (P, S and M), the siderophore aerobactin and haemolysin which are found in other ExPE  A-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 adhesin that has been most closely associated with uropathogenic E. coli is the P fimbria (or pyelonephritis-associated pili [PAP]). The letter designation is derived from the ability of P fimbriae to bind specifically to the P blood group antigen which contains a D-galactose-D-galactose residue. The fimbriae bind not only to red cells but to a specific galactose dissaccharide that is found on the surfaces uroepithelial cells in approximately 99% of the population.  B- 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. Six distinct groups have been defined within IPEC commonly associated with intestinal disease: EIEC ETEC EPEC EHEC, enteroaggregative (EAggEC) and the diffusely adherent E. coli (DAEC).  1- 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. EPEC induce a watery diarrhea similar to ETEC, but they do not possess the same colonization factors and do not produce ST or LT toxins. They produce a non fimbrial adhesin designated intimin, an outer membrane protein, that mediates the final stages of adherence. Although they do not produce LT or ST toxins, there are reports that they produce an enterotoxin similar to that of Shigella. strains are identified by O antigen and occasionally by H antigen typing. usually infantile diarrhea; inflammation, no fever; symptoms probably result mainly from invasion rather than toxigenesis (moderately invasive not as invasive as Shigella or EIEC) One of the most important characteristics of EPEC is attaching and effacing (A/E) demonstrated histologically, characterized by the local effacement of the microvilli and intimate adherence between the bacterium and the host’s epithelial cell membrane.  B-Enterotoxigenic E coli (ETEC) a common cause of "traveler's diarrhea" and a very important cause of diarrhea in infants in developing countries. ETEC are acquired by ingestion of contaminated food and water, The disease requires colonization and elaboration of one or more enterotoxins. Both traits are plasmid-encoded. ETEC colonization factors specific for humans promote adherence of ETEC to epithelial cells of the small bowel. Some strains of ETEC produce a 1- heat-labile exotoxin (LT) (MW 80,000) that is under the genetic control of a plasmid. Its subunit B attaches to the Gi ganglioside at the brush border of epithelial cells of the small intestine and facilitates the entry of subunit A (MW 26,000) into the cell, where the latter activates adenyl cyclase. This markedly increases the local concentration of cyclic adenosine monophosphate (CAMP), 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. LT is antigenic and cross-reacts with the enterotoxin of Vibrio cholerae. LT stimulates the production of neutralizing antibodies in the serum (and perhaps on the gut surface) of persons previously infected with enterotoxigenic E coli.  2-heat-stable enterotoxin ST: (MW 1500-4000), which is under the genetic control of a heterogeneous group of plasmids. ST" activates guanylate cyclase in enteric epithelial cells and stimulates fluid secretion. 3-A second heatstable enterotoxin. STh, stimulates cyclic nucleotideindependent secretion with a short onset of action in vivo. Many ST,positive strains also produce LT. The strains with both toxins produce a more severe diarrhea. fimbrial adhesins e.g. CFA I, CFAII, non invasive produce LT and/or ST toxin watery diarrhea in infants and travelers; no inflammation, no fever  C-Enterohemorrhagic E coli (EHEC) a single strain (serotype O157:H7), which causes a diarrheal syndrome distinct from EIEC (and Shigella) in that there is copious bloody discharge and no fever. A frequent life-threatening situation is its toxic effects on the kidneys (hemolytic uremia). produces verotoxin, named for its cytotoxic effect on Vero cells, a line of African Green monkey kidney cells. There are at least two antigenic forms of the toxin. EHEC has been associated with a-hemorrhagic colitis, a severe form of diarrhea, and b-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. EHEC 0157:H7 does not use sorbitol, negative on sorbitol MacConkey agar (sorbitol is used instead of lactose) negative on MUG tests. Specific antisera are used to identify the 0157:H7 strains. Assays for verotoxin are done in reference laboratories. Many cases of hemorrhagic colitis and its associated complications can be prevented by thoroughly cooking ground beef.  D-Enteroinvasive E coli (EIEC) The disease occurs most commonly in children in developing countries and in travelers to these countries. EIEC closely resemble Shigella in their pathogenic mechanisms and the kind of clinical illness they produce. EIEC penetrate and multiply within epithelial cells of the colon causing widespread cell destruction. The clinical syndrome is identical to Shigella dysentery and includes a dysentery-like diarrhea with fever. EIEC apparently lack fimbrial adhesins but do possess a specific adhesin that, as in Shigella, is thought to be an outer membrane protein. Also, like Shigella, EIEC are invasive organisms. Do not produce LT or ST toxin and, unlike Shigella, they do not produce the shiga toxin. invading intestinal mucosal epithelial cells.  E-Enteroaggregative E coli (EAEC) The distinguishing feature of EAggEC strains is their ability to attach to tissue culture cells in an aggregative manner. These strains are associated with persistent diarrhea in young children. They resemble ETEC strains in that the bacteria adhere to the intestinal mucosa and cause non-bloody diarrhea without invading or causing inflammation. This suggests that the organisms produce a toxin of some sort. Recently, a distinctive heat-labile plasmid-encoded toxin has been isolated from these strains, EAST (EnteroAggregative ST) toxin. They also produce a hemolysin related to the hemolysin produced by E. coli strains involved in urinary tract infections. The role of the toxin and the hemolysin in virulence has not been proven. The significance of EAggEC strains in human disease is controversial.  ETEC fimbrial adhesins e.g. CFA I, CFAII, non invasive produce LT and/or ST toxin watery diarrhea in infants and travelers; no inflammation, no fever EIEC nonfimbrial adhesins, possibly outer membrane protein invasive (penetrate and multiply within epithelial cells) does not produce shiga toxin dysentery-like diarrhea (mucous, blood), severe inflammation, fever EPEC non fimbrial adhesin (intimin) moderately invasive (not as invasive as Shigella or EIEC) does not produce LT or ST; some reports of shiga-like toxin usually infantile diarrhea; watery diarrhea similar to ETEC, some inflammation, no fever; symptoms probably result mainly from invasion rather than toxigenesis EAggEC adhesins not characterized non invasive produce ST-like toxin (EAST) and a hemolysin persistent diarrhea in young children without inflammation, no fever EHEC adhesins not characterized, probably fimbriae moderately invasive does not produce LT or ST but does produce shiga toxin pediatric diarrhea, copious bloody discharge (hemorrhagic colitis), intense inflammatory response, may be complicated by hemolytic uremia  C. Sepsis When normal host defenses inadequate, E coli may reach the bloodstream and causes sepsis. Newborns may be highly susceptible to E coli sepsis because they lack IgM antibodies. Sepsis may occur secondary to urinary tract infection. D. Meningitis- E coli and group B streptococci are the leading causes of' meningitis in infants. The K-1 antigen is considered the major determinant of virulence among strains of E. coli that cause neonatal meningitis. This antigen cross-react with group B capsular polysaccharide of N meningitidis, The mechanism of virulence associated k1 antigen is not understood.