The Enterobacteriaceae form a large family of gram-negative rods found primarily in the colon of human and other animals, many as part of the normal flora. They are the major facultative anaerobes in the large intestine but are present in relatively small numbers compared with anaerobes such as Bacteroides. The family is traditionally divided into two subcategories. The coliforms include Escherichia coli and other gram-negative normal enteric flora that ferment lactose rapidly (within 48 hours). Noncoliforms are generally non-lactose-fermenting or slow lactose-fermenting bacteria that are either normal flora or regular pathogens.
Enteric pathogens are the most frequent cause of diarrheal illnesses , which account for an annual mortality rate of 3 million people and an estimated 4 billion infections worldwide. Counting the total number of cases that receive medical attention, along with estimates of unreported, subclinical cases, enteric illness is probably responsible for more morbidity than any other disease. Prominent pathogenic enterics include Salmonella, Shigella, and strains of Escherichia. Other important enteric opportunists are Klebsiella, Proteus, Enterobacter, Serratia, and Citrobacter. Interestingly, the diseases caused by these agents usually involve systems other than the gastrointestinal tract, such as the lungs and the urinary tract.
This group is probably the most common one isolated in clinical specimens – both as normal flora and as agents of disease.
E.Coli ferments lactose, a property that distinguishes it from the 2 major intestinal pahtogens, Salmonella and Shigella. Salmonellae do not ferment lactose, form H2S and do not form indole. S.typhi differs from other salmonellae in that it does not produce gas from glucose
The gram-negative enterics have complex surface antigens that are important in pathogenicity and are also the basis of immune responses. They are designated H, the flagellar antigen; K, the capsule and/or fimbrial antigen; and O, the somatic or cell wall antigen. Not all species carry the H and K antigens, but all have 0, the lipopolysaccharide implicated in endotoxic shock. Most species of gram-negative enterics exhibit a variety of subspecies or serotypes caused by slight variations in the chemical structure of the HKO antigens.
The modern classification of E. is based on antigenic structure. It has 3 major antigens that are used to identify the microorganism in epidemiologic investigations: the O Ag, the H Ag and the K Ag. Because there are 171 O, 97 K and 57 H antigens, the various combinations result in more than 1000 antigenic types of E.coli. Specific serotypes are associated with certain diseases; eg, O 55 and O 111 cause outbreaks of neonatal diarrhea.
After attachment by pili, the bacteria synthesize enterotoxins (exotoxins that act in the enteric tract), which act on the cells of intestine to cause diarrhea. Heat-labile enterotoxin acts by stimulating adenylate cyclase. The resultant increase in intracellular cyclic AMP concentration stimulates cAMP-dependent protein kinase, causing an autpouring of fluid, potassium, and chloride from the enterocytes. This toxin is similar to cholera toxin (choleragen) in action and antigenicity. Heat-stable toxin stimulates guanylate cyclase. Both enterotoxins cause profuse, watery diarrhea.
We know E.coli as one of the members of normal intestinal flora of humans. But mistaken view is that E.coli is a harmless commensal. Some strains of E.coli can cause a variety of diseases both within and outside the intestinal tract. Most clinical diseases of E. coli are transmitted exclusively among humans. Pathogenic strains of E. coli are frequent agents of infantile diarrhea, the greatest single cause of mortality among babies. In some areas of the world, about 15% to 25% of children 5 years or younger die of diarrhea as either the primary disease or a complication of some other illness. Traveler's diarrhea cases are due to an enterotoxigenic strain of E. coll with profuse, watery diarrhea, low-grade fever, nausea, and vomiting. Escherichia coli often invades sites other than the intestine. For instance, it causes 50% to 80% of urinary tract infections in healthy people.
Enterotoxigenic E. coli causes a severe diarrheal illness brought on by two enterptoxins, heat-labile toxin and heat-stable toxin, that stimulate heightened secretion and fluid loss. In this way it mimics the pathogenesis of cholera. This strain of bacteria also has fimbriae that provide adhesion to the small intestine. The enterotoxin-producing strains do not invade the intestinal mucosa and cause a wattery, non-bloddy diarrhea. Enteroinvasive E. coli causes an inflammatory disease similar to Shigella dysentery* that involves invasion and ulceration of the mucosa of the large intestine. Enteropathogenic strains of E. coli are linked to a wasting form of infantile bloody diarrhea accompanied by inflammatory cells in the stool. These strains cause disease not by enterotoxin formation by invasion of the epithelium of the large intestine. When enteroinvasive strains invade and penetrate enterocytes, enterohemorrhagic strains do not penetrate intestinal epithelial cells and sause the death of enterocytes by cytotoxin that interfering with protein synthesis.
The salmonellae are distinguished from the coliforms by having well-developed virulence factors, being primary pathogens, and not being normal flora of humans. The illnesses they cause—called salmonelloses .
There are 2 methods of classification of Salmonella: modern by Ewing and designed by Kaufman and White in 1934. S.enteritidis includes around 1700 different serotypes, based on variations on the major O, H, and Vi antigens.
According to O Ag S. are divided on serogroupes A, B, C, D…… According to H Ag every serogroup is divided on the serotypes or species (K W classification)
Kaufman and White assign different species names to each serotypes, there are about 1500 different species.
Typhoid fever occurs only in human. In the world, typhoid fever is still a serious health problem responsible for 25,000 deaths each year and probably millions of cases. Source of infection is ill human or carrier. Infectious dose is between 1000 and 10000 bacilli. Incubation period – 7-21 days (pink).
Bohomolets Microbiology Lecture #18
Enterobacteria Escherichia Salmonella
Family Enterobacteriaceae ( more than 2 0 genuses , 100 species ). <ul><li>The most important genuses : </li></ul><ul><li> </li></ul><ul><li>Escherichia </li></ul><ul><li>Salmonella </li></ul><ul><li>Shigella </li></ul><ul><ul><li>Citrobacter </li></ul></ul><ul><ul><li>Enterobacter </li></ul></ul><ul><ul><li>Hafnia </li></ul></ul><ul><ul><li>Klebsiella </li></ul></ul><ul><ul><li>Serratia </li></ul></ul><ul><ul><li>Edwardsiella </li></ul></ul><ul><ul><li>Yersinia </li></ul></ul><ul><ul><li>Proteus </li></ul></ul><ul><ul><li>Morganella </li></ul></ul><ul><ul><li>Providentia </li></ul></ul>* Coliforms ** Noncoliforms
Properties of Enterobacteriaceae family members <ul><li>They are residents of colon, some of them inhabit respiratory tract. </li></ul><ul><li>Fecal-oral (sometimes airborn) rout of transmission. </li></ul><ul><li>Resistant to environment factors. </li></ul><ul><li>They are small, non-spore forming gram-negative rods. </li></ul><ul><li>They are facultative anaerobes </li></ul><ul><li>Not fastidious to nutrient medium. </li></ul><ul><li>They all ferment glucose to acid or to acid and gas. </li></ul><ul><li>Do not have proteolitic ferments </li></ul><ul><li>None have cytochrome oxidase. </li></ul><ul><li>Catalase-positive </li></ul><ul><li>They reduce nitrates to nitrites. </li></ul>
Biochemical activities of E.coli and Salmonella a – acid g - gas - + - - + (ag) + (ag) Other salmonella - + - - + (ag) + (ag) S. paratyphi - + - - + (a) + (a) S. typhi + - + + (ag) + (ag) + (ag) E. coli Indole H 2 S Sucrose Lactose Manitol Glucose Fermentation Species
Antigenic structure of enterobacteria <ul><li>O antigen . Sometic heat-stabile polysaccharide Ag, situated in cell wall. </li></ul><ul><li>H antigen. Heat-labile protein Ag located in flagella. Only flagellated microorganisms, such as E.coli and Salmonella, have H Ag, whereas nonmotile ones, such as Shigella, do not. </li></ul><ul><li>K antigen . Heat- labile polysaccharide Ag located in capsule or in fimbria. </li></ul>
Modern classification of Escherichia 171 О- antigens 97 К- antigens 57 Н- antigens Instance : E . coli O 26:K60: H 2 Serotypes E.coli Species Escherichia Genus Enterobacteriaceae Family
Virulence factors of E.coli <ul><li>Pili </li></ul><ul><li>Capsule </li></ul><ul><li>Endotoxin </li></ul><ul><li>Exotoxins </li></ul><ul><ul><li>2 enterotoxins </li></ul></ul><ul><ul><ul><ul><li>Heat-labile toxin (LT) acts by stimulating adenylate cyclase </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Heat-stable toxin (ST) acts by stimulating guanylate cyclase </li></ul></ul></ul></ul><ul><ul><li>Cytotoxin </li></ul></ul>
Enteropathogenic stains of E.coli <ul><li>Enterotoxigenic E.coli ( О6:Н16; О8:Н9; О78:Н11 serotypes ) cause a severe diarrheal illness brought on by two enterotoxins (HL and HS) that stimulate heightened secretion and fluid loss ( enterotoxins, endotoxin ) </li></ul><ul><li>Enteroinvasive E.coli ( О124; О144; О152 serotypes ) cause an inflammatory disease similar to Shigella dysentery that involves invasion and ulceration of the mucosa of the large intestine ( invasion , cytotoxin, endotoxin ) </li></ul><ul><li>Enteropathogenic E.coli ( О26: K 6; О55: K 5; О111: K 4 ) are linked to a wasting form of infantile diarrhea ( endotoxin) </li></ul><ul><li>Enterohemorrhagic E.coli ( О157:Н7; О126:Н11 ) causes a bloody diarrhea and hemorrhagic syndrome that can cause permanent damage to the kidney ( endotoxin, cytotoxin ) </li></ul>
Treatment and prevention <ul><li>Replacing the fluid loss resulting from vomiting and diarrhea </li></ul><ul><li>Antibacterial drugs (antibiotic or sulfonamide) </li></ul><ul><li>There is no specific prevention, such as active or passive immunization </li></ul>
Laboratory diagnosis <ul><li>Bactoriologic method. Isolation bacteria from stool by inoculation on differential medium (MacConkey’s agar). E.coli which ferments lactose, forms pink colonies, whereas lactose-negative microorganisms are colorless. </li></ul><ul><li>Important features that help to distinguish E.coli from other lactose-fermenting gram-negative rods are as: </li></ul><ul><li>indole production </li></ul><ul><li>nonproduction of H 2 S </li></ul><ul><li>it is motile </li></ul><ul><li>fermentation of lactose to acid and gas </li></ul><ul><li>antigen structure </li></ul>
Disease caused by Salmonella <ul><li>Typhoid fever is caused by S.typhi </li></ul><ul><li>Enterocolitis is caused by S.paratyphi A, B, C, S.typhimurium </li></ul><ul><li>Salmonella food poisoning ( different species of Salmonella ) </li></ul><ul><li>Septicemia is most often caused by S.cholerae-suis </li></ul>
Modern classification of Salmonella (by Ewing) <ul><li>The genus Salmonella is divided into 3 species: </li></ul><ul><li>S.typhi ( 1 serotype ) </li></ul><ul><li>S.cholerae-suis ( 1 serotype, causes zoonosis of swine ) </li></ul><ul><li>S.enteritidis ( approximately 1700 serotypes ) </li></ul>
Antigenic structure of Salmonella <ul><li>O antigen </li></ul><ul><li>The H antigen of certain Salmonella species are unusual because the microorganisms can reversibly alternate between 2 types of H Ag called phase 1 and phase 2. </li></ul><ul><li>In S.typhi K antigen is called the Vi antigen (Vi being the vi rulence or capsule Ag.) </li></ul>
Kaufman – White classification of Salmonella (1934) - (1,7) D g, m 9,12 1,9,12 Serogroup D S . typhi S. enteritidis 1,5 - ( c ) m, s, (p) 6, 7 6, 7 Serogroup C S . choleraesuis S. montevideo 1,2 1,2 1,2 B i R 1, 4, (5), 12 1, 4, (5), 12 1, 4, (5), 12 Serogroup B S. schottmuelleri S. typhimurium S. heidelberg (1,5) A 1, 2, 12 Serogroup А S. paratyphi A Phase 2 Phase 1 H antigen O antigen Serogroup (or species)
Modern classification of Salmonella Salmonella Species S. bongori (10 serotypes ) – all salmonellae parasite in cold-blood animals Species S. enterica ( 6 subspecies , > 2000 serotypes – all are agents of diseases in warm-blood animals and human)
Stage of pathogenesis of typhoid fever <ul><li>Digestive. The typhoid bacillus usually enters the alimentary canal along with water or food contaminated by feces. </li></ul><ul><li>Invasive. Bacteria penetrate in lymphoid system of small intestine (Peyer’s patches, mesenteric lymph nods) and reproduce in these cells. </li></ul><ul><li>Bacteremia . With infected phagocytes bacteria get to the bloodstream where they are destructed, release endotoxin and cause toxemia. Since this stage manifestation begins. </li></ul><ul><li>Parenchymal diffusion. Bacteria with blood and infected cells spread to the liver, gallbladder, spleen, bone marrow, where father multiplication takes place. </li></ul><ul><li>Excretive-allergic. S.typhy again get to the enterocytes where induces allergy reaction that lead to intestinal rupture and hemorrahage. Bacteria in great amount excrete from the organism with feces and urine. </li></ul><ul><li>Immunity formation </li></ul><ul><li>End of disease ( death, complete convalescence or transformation in bacteria carring ) </li></ul>
Damage of intestinal wall (excretive-allergic stage of typhoid fever) Perforations Ulcers
Laboratory diagnosis of typhoid fever Serum Serological (indication of rise antibody to agent) Urine, feces, bile, bone marrow Bacteriological. Since 2 st week to end of disease Blood Bacteriological. Since 1 st week to end of disease (bacteremia) Material from the patient Method Time of investigation
Treatment and prevention <ul><li>Treatment. Antibiotics </li></ul><ul><li>Specific prophylaxis. There are 2 vaccine: </li></ul><ul><ul><li>Inactivated vaccine from acetone-killed S.typhi organisms </li></ul></ul><ul><ul><li>Live oral vaccine from attenuated S.typhi </li></ul></ul>