ملخص

Enterobacteriaceae

inhabits the large bowel of man
called “ enteric bacilli” or “ enterics” These m.o. are gram negative bacilli
responsible for the majority of nosocomial infections ( hospital-acquired ) as UTI , wound infections

Morphology

1-small, gram negative, non-spore forming rods/sometimes coccobacilli
2-motile at body temp. except as Klebsiella, Shigella and Yersinia
3-could be capsulated with well-defined capsule as Klebsiella, ill-defined loose coating “slime layer” as in E. coli, or non-capsulated as Proteus
4-these m.o. have fimbriae (pili) for attachment to the host

Biochemical Properties:

1-facultative anaerobic or aerobic m.o
2-can ferment glucose with acid production
3-reduce nitrate to nitrite
4- no cytochrome oxidase activity.
5- They are divided into 2 groups according to their effect on lactose. The lactose fermenters (LF) are collectively called “coliforms” and include mainly Escherichia, Klebsiella, Citrobacter and Enterobacter. The non-lactose fermenters (NLF) include Salmonella, Shigella and proteu

6-IMViC phenomena (I = Indol, M = Methyl Red, V = Voges Proskauer, C = Citrate Utilization); E.coli = ++--, Klebsiella = --++


Cutural Characteristics:
. Non-differential Non-selective Media:
Examples: Blood agar, Brain-Heart Infusion. The various genera can NOT be distinguished on these media.Proteus shows swarming phenomena (successive waves of translucent growth on the surface of agar). Pathogenic E.coli may produce beta haemolysis on blood agar
II. Differential and/or Selective Media
1. MacConkey’s Medium: Contains lactose, bile pigments and
crystal violet (to prevent the growth of gram positive m.o. and fastidious gram negative m.o.) and neutral red as an indicator (change colour according to pH). The colonies of the lactose fermenter enterics appear rose pink in colour, while the non-lactose fermenter appear pale colonies.
2. Eosin – Methylene Blue Media (EMB):
two sugars; lactose , and sucrose, with eosin and methylene blue which inhibit the growth of gram positive and fastidious gram negative bacteria.
3. Deoxyxholate or Deoxycholate- Citrate Agar (DCA):
contain deoxycholate which prevent the growth of gram + & gram – fastidious m.o. These media favour the growth of Salmonella and Shigella
4. Shigella-Salmonella Agar (SS-Agar):
This medium is a selective medium for Shigella and Salmonella
5. Xylose- Lysine – Deoxycholate (XLD):
This medium contains xylose, lactose, sucrose and lysine with phenol red as an indicator. Also, it contains sodium deoxycholate and sodium thiosulphate. On this medium Salmonella, Shigella and Proteus can grow. Both proteus and Salmonella produce H2S; so their colonies appear black in colour
6. Hektoen Enteric Agar (HE):
Used for the cultivation of enteric bacilli
III. Enrichment Broth:
Examples: Selenite broth. These media can be used for the enrichment of the growth of Salmonella as S. typhi or paratyphi.

Antigenic Structure:

3. Somatic antigens These are lipopolysaccharides of the cell wall and are composed of 3 regions :
1. O- specific antigen; used for serotyping of m.o.
2. Core polysaccharides;
3. Lipid A moiety; which is the toxic portion (endotoxin).
2. Flagellar antigens These
are protein antigens that stimulate IgG production, and are used for motility.They have 2 phases; phase I is specific, while phase II is non-specific
1. Capsular antigens
These are polysaccharides. They stimulate IgM production and could be virulence factors

Determinant of pathogenicity:

1. Endotoxins.
2. Enterotoxins: These are exotoxins that exert their activities on the small intestine causing transduction of fluid into the lumen
3. Other factors:
Invasiveness (penetration) as Shigella; Capsule as Klebsiella pneumoniae; or Vi antigen of Salmonella.

Salmonella, Shigella and Yersinia are always pathogenic m.o. However, other m.o. in the intestine (stool) could part of normal flora.
Outside the intestine they are pathogenic

Endotoxins:

Endotoxins are generally heat stable
1. Fever: Endotoxins can stimulate the liberation of IL-1 (endogenous pyrogen)
2. Leucopenia
3. Hypotension and shock and impaired organ perfusion
4. Impaired organ perfusion and acidosis:
5. Activation of C3 complement component
6. Disseminated intravascular coagulation (DIC): Endotoxins activate factor XII (Hageman)which is the first step of the intrinsic clotting system


N.B.: Naloxone (opiate antagonist) can increase B.P. in endotoxic shock

Anti-Microbial Drugs against Enterics:

1. Broad spectrum penicillins as Ampicillin
2. Cephalosporins
3. Aminoglycosides.
4. Colistin and polymyxin B.
5. Chloramphenicol or tetracyclines.
6. Cotrimoxazole
7. Fluoroquinolones