Monday, April 3, 2017

Micro-organism series -Salmonella

Introduction: Salmonella belongs to the type of parasites which infects the intestine of vertebrates and causes infection.It causes following infections  (Mnemonic - "GAS".)
G - Gastro-enteritis
A - Abdominal fever or enteric fever
S - Septicemia

The species Salmonella-typhi was first observed by Eberth in mesenteric lymph nodes and  spleen and was first isolated by Gaffky and hence is also known as Eberth -Gaffky or Eberthella -typhi .
S.cholerae - suis the first organism to be isolated from animals and human beings.

Salmonella is divided into two groups :-
1)Typhoidal Salmonella :Causes typhoid fever eg S.typhi and  S.paratyphi A,B,C
Man is the only reservoir
2)Non-typhoidal : They have many animal reservoir .To remember species name, here is a mnemonic
"Hodor held typhimurium  to enter into port "

Hodor -S.hadar

Held- S.heidelbrurg

typhimurium-S.typhimurium (to)

Enter-S.enteridies

Port-S.new port

Morphology:
Aerobes and facultative anaerobes
Gram negative bacilli
It is motile with peri-trichate flagella except S.Gallinarum and S.Pullorum .
Non-capsulated and non-sporing .

Cultural characteristics:
Can grow on simple media over pH-6-8
Temperature:15-41℃
Colonies are circular ,low-convex and smooth .
1)Mac-Conkey's agar(Differential media) :Non-lactose fermenting colonies

2)Deoxycholate citrate and XLD(Selective media): Black head due to hydrogen sulphide production
(Deoxycholate citrate agar and Mac-Conkey's agar are selective as well as differential media)

3)Wilson & Blair bismuth sulphite medium(Indicator medium):Black colony with metallic sheen due to production of hydrogen sulphide.Only S.typhi produces black colony .Paratyphi A produces green colony as it does not form hydrogen sulphide.

4)Selenite F broth and tetrathionate broth  is the enrichment media used for both shigella and salmonella

Biochemical tests:-
1)It ferments Glucose ,mannitol and maltose producing acid and gas
2)IMViC : - + - +
3)Most salmonella produces hydrogen sulphide in Triple sugar iron(TSI) test except S.parathyphi A and S.Cholerae suis .
4)S.Gallinarum and Pullorum cannot be differentiated but can be identified by biochemical reactions .S Gallinarum ferments dulcitol unlike Pullorum .

Resistance:
Killed in 1 hour at 55℃ or in 15 mins at 60℃
Boiling ,chlorination and pasteurisation destroys bacteria .
Killed in 5 minutes by using mercuric cholride / 5% Phenol .
Survives for weeks in polluted air & water
Survives for months in ice
Survives for years  if prevented from drying .

Antigenic structure:
There are 3 antigens present in salmonella
1)Flagellar antigen H:
- Heat labile
- Present on flagella
- Highly immunogenic and hence form high titre of antibodies
- Destroyed by alcohol or boiling
- When mixed with antisera  , agglutination is rapid producing large ,loose and fluffy clumps.
- H persists longer than O agglutinins

2)Somatic antigen "O"
- Integral part of cell wall
- Identical to endotoxin
- Less immunogenic

3)Vi antigen :
- Heat labile polysaccharide and it prevents the agglutination of O antigen
- Act as virulence factor by preventing phagocytosis.
- Persistance of Vi antigen indicates carrier state

Pathogenesis:
Transmission:Normally food contaminated by the faeces of the animal or humans who carries salmonella

Mode of transmission:Ingestion

Incubation period :7-14 days for enteric fever

Pathogenicity :
Salmonella infection usually causes
Gastroenteritis
Enteric fever
Septicemia

Clinical symptoms:
Clinical course  may vary from mild to undifferentiated pyrexia (Also called as ambulant typhoid)
- Onset :Gradual with headache ,malaise ,anorexia ,a coated tongue and constipation or diarrhea.
- Step ladder pyrexia.
- Palpable spleen
- Rose spot appearence on chest during 3rd or 4th week .
- Complications occurs in 3rd and 4th week causing intestinal perforation and GI hemorrhage (most common )
- Pea soup stools in 3rd week of typhoid fever
- Some degree of bronchitis or bronchopulmonary spasm is always seen
- Osteomyelitis is rarely seen mainly in patients affected with  sickle cell anemia  .

Laboratory diagnosis :
Specimen :
Blood is collected for culture ,as in urine or stool .
Serum for Widal test .
(A very famous mnemonic for collection of sample depending upon time duration is
"BASU")
B - Blood culture in 1st week of infection
A - Antibody (Widal in 2nd week of infection)
S - Stool culture in 3rd week of infection
U - Urine  culture for 4th week of infection.
Best diagnosis is  made by blood culture at any stage .
Detection of Vi antigen indicates carrier state.
If antibiotics is started we must use faeces or bone marrow for laboratory diagnosis .Since antibiotics kills bacteria from the circulation  and if not started then we can use blood culture .

Treatment:
Ceftriaxone is the DOC
Ciprofloxacin is the DOC for susceptible organism .
Other drugs used are azithromycin ,amoxicillin,chloramphenicol
Carrier :Ampicillin or amoxicillin given for six weeks

Salmonellosis:
Non Thyphoidal salmonellosis(NTS) is -commonest type of salmonellosis

Factor causing NTS
Immunosuppressive agents like increase in age ,Disease such as HIV etc.

Clinical manifestation:
Gastroenteritis most common .Diagnosis is done by gastroenteritis .
Localized infection like abscess ,meningitis , Osteomyelitis.

Treatment:
DOC either ceftriaxone or Ciprofloxacin .

That's it :P
Stay awesome and cool:)

~Khushboo and Ojas

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