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Enterobacteriaceae

  • Gram negative, capsulated or non-capsulated
  • Non-acid fast, non-sporing bacilli
  • Motile (peri-trichous flagella) or non-motile(Klebsiella, Shigella, Y.pestis, S. gallinarum, S. pullorum)
  • Oxidase negative, catalase positive (exception S. dysenteriae Type1)
  • Reduce nitrates to nitrites
  • Aerobes and facultative anaerobes
  • Non-fastidious
  • Ferment glucose with the production of acid/ acid +gas
  1. Escherichia Coli
  • GNR, non-capsulated, motile by peritrichous, 80% strains fimbriae present, some strains capsulated
  • Some strains hemolytic on Blood agar, Lactose fermenting colonies on MacConkey’s agar
  • Inhibited on DCA, SS agar, W&B agar
  1. Biochemical reactions
  1. IMViC: ++--(most constant-indole positivity)
  2. PPA, Urease, H2S, Gelatin Liquefaction, KCN: Negative
  3. Glucose, Mannitol, Maltose, Sucrose, Lactose:  A+G
  1. Antigenic structure:
  1. O : Somatic antigen, Heat stable, LPS, 170
  2. H : Flagellar antigen, thermolabile, 75
  3. K: Capsular antigen, 103
  4. F: Fimbrial antigen, thermolabile
  5. Normal colon strains: “early “O groups (1, 2, 3, 4 etc.); enteropathogenic strains: “ later” O groups
  6. ( 26, 55, 86, 111 etc.) 
  1. Virulence factors:
  1. Surface antigens:
  2. “O” antigen (LPS): endotoxic; inhibits phagocytosis
  3. K antigen: protects from phagocytosis
  4. Fimbriae: Mannose resistant
  5. CFA (I-IV) (colonizing factor antigens) in ETEC
  6. “P” fimbriae in uropathogenic E coli
  1. Toxins:
  1. Hemolysin: exact role is not known
  2. Enterotoxins:
    1. LT (labile)
    2. ST (stable)
    3. VT/ SLT (Shiga like toxin or verocytotoxin) 
  1. LT:
  1. Resembles cholera toxin in structure (A1 +B5), antigenic properties and mode of action (Gm1 ganglioside receptors → activates adenylate cyclase → cAMP →fluid accumulation)
    1. LT1 & LT2
    2. Powerful antigen
  2. Toxigenicity tests- Ligated rabbit ileal loop test (18hrs.), adult rabbit skin, steroid production in YI mouse adrenal   cell culture, morphological changes in Chinese hamster ovary cells, Latex Agglutination, ELISA
  1. ST: (plasmid coded)
  1. Low molecular weight
  2. Poorly antigenic
  3. 2 types: ST-I & ST-II
  4. ST-I: activation of cGMP
  5. Toxigenicity tests- Ligated rabbit ileal loop test (6hrs.), intra gastric in sucking mouse
  6. ST-B: mechanism of Action not known
  1. VT:
  1. E.coli O157: H7
  2. VT1, VT2
  3. VT1: Identical to Shiga toxin, Phage encoded, cytotoxic to Vero cells,
  4. VT is not neutralized by shiga antitoxin
  1. Clinical infections produced by E.coli:
  1. Pyogenic infections
  2. Septicemia
  3. Neonatal meningitis
  4. UTI:
    1. Most common cause of UTI
    2. “early” O groups 1,2,4, 6,7, 18, 75
    3. Ascending infection; pyelonephritis; K antigen
  1. Diarrhoea:
a. EPEC:
  1. Diarrhoea in infants and children; sporadic diarrhoea in adults
  2. O26, 55, 86, 111, 114, 119, 125, 126, 127, 128, 142
  3. Pathogenesis: adherence to intestinal mucosa→disruption of brush border microvilli →attaching   -effacing   lesions
  4. 3 patterns of adhesion; localized, aggregative and diffuse
Diagnosis
  1. Typing
  2. EAE (E. coli attaching & effacing) & EAF (EPEC adherence factor) probes
  3. FAS (Fluorescence actin staining) in HeLa & Hep 2 cell lines for pattern of adherence
b. ETEC:
  1. Acute watery diarrhea in infants and adults; traveller’s diarrhea
  2. O6, 8, 15, 25, 27, 63, 78, 115, 148, 153, 159, 167
  3. Pathogenesis:- LT, ST and Adhesive fimbrial proteins (CFA I, II, III, IV) 
Diagnosis:
  1. Typing
  2. Demonstration of toxins in tissue culture by latex agglutination, ELISA (LT) & RIA (ST),  animal   models
c. EIEC:
  1. Dysentery like disease in all ages
  2. Disease similar to shigella -Atypical E. coli             
  3. O28, 112, 124, 136, 143, 144, 152, 164
  4. Pathogenesis: Epithelial cell invasion
Diagnosis:
  1. Typing
  2. Atypical biochemistry (biochemically similar to Shigella spp. i.e, NLF, anaerogenic and non-motile)
  3. Invasion plasmid probes
  4. HeLa & Hep2 cell invasion assay
  5. Sereny’s test
d. EHEC (VTEC):
  1. O157: H7
  2. Bloody diarrhoea in all ages; Hemorrhagic colitis, ; Hemolytic uremic syndrome
  3. Pathogenesis:
  4. VT1 and/ or VT2
  5. Capillary microangiopathy
Diagnosis:
  1. Typing
  2. Atypical biochemistry (β-glucuronidase negative, donot ferment sorbitol &    rhamnose)
  3. VT1 & VT2 probes
  4. Demonstration of VT1 & VT2 in Vero cell lines
e. EAEC:
  1. Persistent diarrhea especially in developing countries
  2. Most are “O” un-type able but “H” type able
  3. Aggregated in a “Stacked Brick” formation on Hep2 cell lines
  4. EAST 1 (enteroaggregative heat stable enterotoxin)
Example

1. Which of the following is true about Enterotoxigenic E.coli (AIIMS 2010)
A. Infects pediatrics population in developing country          
B. It is not a cause travellers diarrhea
C. It acts by invading intestinal mucosa                        
D. Person to person transmission or fomite borne

Solution

ANS- D

 

 

Extra Edge

Indicator media for E. coli O157:H7 are
1.SMAC agar = Sorbitol MacConkey Agar
2. Rainbow agar


 

B. Shigella
  • Bacillary dysentery: Shigella, E coli, V parahemolyticus, Campylobacter jejuni
  • S. dysenteriae (A), S. flexneri (B), S. boydii (C), S. sonnei (D)
  • Sh sonnei: LLF; more resistant
  • DCA +; Wilson and Blair –
  • Catalase – ve: S. dysenteriae type I
  • Gas producing: Manchester & Newcastle biotypes of S. flexneri type 6
  • Fermentation of mannitol: (S. dysenteriae –ve) 
1. S. dysenteriae :
  1. 12 serotypes
  2. Type 1: Shiga’s bacillus, Catalase – ve, produces Shiga toxin
  3. Shiga toxin has 3 types of toxic activities:
  4. Neurotoxicity, Entero toxicity, cytotoxicity (same as VTEC; A & 5B subunits)
  5. Type 2: Schmitz’s bacillus
  6. Type 3-7: Large and Sachs group
2. S. flexneri
  1. 6 serotypes; X, Y variant
  2. Type 6: 3 biotypes (Boyd 88, Manchester, Newcastle) 
3. S. boydii:
  1. 18 serotypes
  2. Least frequently associated with cases of bacillary dysentery
4. S. sonnei:
  1. Antigenically homogeneous-only one serotype
  2. Phase S/ Form I; Phase R/ Form II
  3. Mildest form of bacillary dysentery
  4. Most common shigellosis in developed countries
  5. India: S. flexneri> S. dysenteriae> S. sonnei> S. boydii
5. Pathogenicity;
  1. 10 – 100 bacilli infective dose
  2. Humans only natural host
  3. Invasive mechanism; demonstrated in HeLa & Hep 2 cell lines and Sereny test
  4. Invasive property coded by plasmid coding for OMP called VMA (virulence marker antigen) 
6. C/ F:
  1. 48 hours incubation period
  2. Scanty feces with blood & mucus, abdominal cramps, tenesmus, fever & vomiting
7. Complications:
  1. Most often in S. dysenteriae type 1
  2. Arthritis, toxic neuritis, conjunctivitis, parotitis, intussusception, HUS
 




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