enterobacteriaceae

بكتريا مرضية د- عروبة كطوف البيرماني
Enterobacteriaceae
Classification
This family includes about more than15 different genera, they are
Escherichia Shigella Edwardsiella Salmonella Citrobacter Klebsiella Enterobacter Hafnia Serratia Proteus Providencia Morganella Yaersinia Erwinia Pectinobacterium
Morphology and General Characteristics
1-Gram-negative, non-sporing, rod shaped bacteria
2- Oxidase –
3- Ferment glucose and may or may not produce gas in the process (aerogenic vs anaerogenic)
4- Reduce nitrate to nitrite (there are a few exceptions)
5- Are facultative anaerobes
6- If motile, motility is by peritrichous flagella
7- Many are normal inhabitants of the intestinal tract of man and other animals
8- Some are enteric pathogens and others are urinary or respiratory tract pathogens
9- Differentiation is based on biochemical reactions and differences in antigenic structure
10- Most grow well on a variety of lab media including a lot of selective and differential media originally developed for the the selective isolation of enteric pathogens.
i. Most of this media is selective by incorporation of dyes and bile salts that inhibit G+ organisms and may suppress the growth of nonpathogenic species of Enterobacteriaceae.
ii. Many are differential on the basis of whether or not the organisms ferment lactose and/or produce H2S.
The antigenic structure
The antigenic structure is used to differentiate organisms within a genus or species. Three major classes of antigens are found

1-Somatic O antigens – these are the heat stable polysaccharide part of the LPS. Variation from smooth to rough colonial forms is accompanied by progressive loss of smooth O Antigen.
2-Flagellar H antigens – are heat labile
3-Envelope or capsule K antigens – overlay the surface O antigen and may block agglutination by O specific antisera. Boiling for 15 minutes will destroy the K antigen and unmask O antigens.
The K antigen is called the Vi (virulence) antigen in Salmonella typhi


Escherichia coli
Normal inhabitant of the gastrointestinal tract. Some strains cause various forms of gastroenteritis.Is a major cause of urinary tract infection and neonatal meningitis and septicemia. May have a capsule. Most are motile.May be hemolytic on CBA– more common in pathogenic strains. Antigenic structure - has O, H, and K antigens. K1 has a strong association with virulence, particularly meningitis in neonates.
Virulence factors
1. Toxins
Enterotoxins – produced by enterotoxigenic strains of E. coli (ETEC). Causes a movement of water and ions from the tissues to the bowel resulting in watery diarrhea. There are two types of enterotoxin:
a-LT – is heat labile. Increased cAMP alters the activity of sodium and chloride transporters producing an ion imbalance that results in fluid transport into the bowel
b-ST – is heat stable and binds to specific receptors to stimulate the production of cGMP with the same results as with LT.
Both enterotoxins are composed of five beta subunits (for binding) and 1 alpha subunit (has the toxic enzymatic activity).



Composition of subunits of enterotoxins

Shiga-type toxin – also called the verotoxin -produced by enterohemorrhagic strains of E. coli (EHEC) – is cytotoxic, enterotoxic, neurotoxic, and may cause diarrhea and ulceration of the G.I. tract.
There are two types shiga-like toxin 1 and shiga-like toxin 2. Inhibit protein synthesis by cleaving a 28S rRNA that’s part of the 60S subunit
Enteroaggregative ST-like toxin – produced by enteroaggregative strains of E. coli (EAEC) – causes watery diarrhea.
Hemolysins – two different types may be found: cell bound and secreted.
They lyse RBCs and leukocytes and may help to inhibit phagocytosis when cell bound.
Endotoxin
It represent the lipopolysacharide component of the cell wall , lps activates the complement system and liberates the cytokines and mobilizes the leukocytes and degranulation them.Type III secretion system to deliver effectors molecule directly into the host cells.
Adhesins – are also called colonization factors and include both pili or fimbriae and non-fimbrial factors involved in attachment (e.g. intimin).
There are at least 21 different types of adhesins.
Antibodies to these may protect person from colonization.

Outer membrane proteins - are involved in helping the organism to invade by helping in attachment (acting as adhesin) and in initiating endocytosis
Types of adhesions

Capsule: Virulence factors that protect the bacteria from host defenses
Iron capturing ability aerobactin siderophore system
Clinical picture
Urinary tract infection , Neonatal meningitis and Gastroenteritis – there are several distinct types of E. coli that are involved in different types of gastroenteritis:
? enterotoxigenic E. coli (ETEC),
? enteroinvasive E. coli (EIEC),
? enteropathogenic E. coli (EPEC) ,
? enteroaggregative E. coli (EAEC)
? enterohemorrhagic E. coli (EHEC).
E. coli gastroenteritisETEC – is a common cause of traveler’s diarrhea and diarrhea in children in developing countries.The organism attaches to the intestinal mucosa via colonization factors and then liberates enterotoxin
EPEC –causes Diarrhea with large amounts of mucous without blood or pus occurs along with vomiting, malaise and low grade fever Bundle forming pili are involved in attachment to the intestinal mucosa.

EIEC – The organism attaches to the intestinal mucosa via pili and outer membrane proteins are involved in direct penetration, invasion of the intestinal cells, and destruction of the intestinal mucosa. .
EAEC – Mucous associated autoagglutinins cause aggregation of the bacteria at the cell surface and result in the formation of a mucous biofilm.
EHEC – The organism attaches via pili to the intestinal mucosa and liberates the shiga-like toxin.


















Shigella species
Contains four species that differ antigenically, to
? S. dysenteriae (Group A)
? S. flexneri (Group B)
? S. boydii (Group C)
? S. sonnei (Group D)
Antigenic structure
Differentiation into groups (A, B, C, and D) is based on O antigen serotyping; K antigens may interfere with serotyping, but are heat labile.
Virulence factors
Shiga toxin – is produced by S. dysenteriae and in smaller amounts by S. flexneri and S. sonnei. Acts to inhibit protein synthesis This plays a role in the ulceration of the intestinal mucosa
Outer membrane and secreted proteins
These proteins are expressed at body temperature and upon contact with M cells in the intestinal mucosa they induce phagocytosis of the bacteria into vacuoles. Shigella destroy the vacuoles to escape into the cytoplasm.
From there they spread laterally (Polymerization of actin filaments propels them through the cytoplasm.) to epithelial cells where they multiply but do not usually disseminate beyond the epithelium

Clinical picture
Causes shigellosis or bacillary dysentery. Transmission is via the fecal-oral route.The infective dose required to cause infection is very low (10-200 organisms).There is an incubation of 1-7 days followed by fever, cramping, abdominal pain, and watery diarrhea (due to the toxin)for 1-3 days.

Salmonella
All salmonellae are classified in the species Salmonella enterica with seven subspecies. Nearly all human pathogen salmonellae are grouped under S. enterica, subsp. enterica. Salmonellae are further subclassified in over 2000 serovars based on their O and H antigens(The Kauffman–White scheme)
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Virulence factors
? Endotoxin – may play a role in intracellular survival
? Capsule (for S. typhi and some strains of S. paratyphi)
? Adhesins – both fimbrial and non-fimbrial
? Type III secretion systems and effector molecules – 2 different systems may be found:
? Outer membrane proteins - involved in the ability of Salmonella to survive inside macrophages
? Flagella – help bacteria to move through intestinal mucous
? Enterotoxin - may be involved in gastroenteritis
The organism moves through the intestinal mucosa and adheres to intestinal epithelium. Effector proteins of the type III secretion system mediate invasion of enterocytes and M cells via an induced endocytic mechanism.
Pathogenesis and clinical pictures. Salmonellae are classified as either typhoid or enteric salmonelloses
Typhoid salmonelloses. Attachment of typhoid salmonellae to cells of
the jejunum (M cells). Invasion by means of endocytosis, transfer, and exocytosis. Phagocytosis in the subserosa by macrophages and translocation into the mesenteric lymph nodes. Proliferation occurs. Lymphogenous and hematogenousn dissemination. Secondary foci in the spleen, liver, bone marrow, bilenducts, skin (roseola), Peyer’s patches.
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Enteric salmonelloses. Attachment to enterocytes of the ileum and colon. Invasion of mucosa induced by invasin proteins on the surface of the salmonella cells. Persistence in epithelial cells, possibly in macrophages. Production of Salmonella enterotoxin. Local inflammation manifest illness usually begins suddenly with diarrhea and vomiting, accompanied in some cases by high fever. The symptoms appear after several days without specific
Salmonella multiplies within the endosome


The endosome moves to the basal side of the cell and Salmonella are released and may be phagocytosed by macrophages.





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Diagnosis of typhoid fever
Blood cultures are positive during the first week and after the second week Stool cultures and sometimes urine cultures are positive after the second week The Widal test is a serological test for antibodies against Salmonella typhi. One looks for a 4-fold rise in titer between acute and convalescent stages.