Lecture 8-Salmonella Species

Main Causes of Diarrhoeal Disease

• poor personal hygiene

• poor food hygiene

• lack of clean drinking water

• Lack of education

Killing Organism that Cause Diarrheal disease

• Treating water

• Boiling

How do diarrhoeal organisms end up in the water supply?

secreted in the faeces of infected individuals and ends up in the water supply if sewage is untreated

Centers of Typhoid Fever

• Asia

• Middle East

• Latin America

Salmonella

• Gram-negative rod-shaped bacteria

• Motile

• Enterobacteriacea

• One species with many serotypes

  • serotypes have unnique names

What is the only location of salmonella typhi?

the human intestinal tract

How is S. Typhi ingested?

faecally-contaminated food and water

What types of salmonella cause typhoid in humans?

• S. Typhi

• S. paratyphi

What serotypes of salmonella cause gastroenteritis?

• S. enteritidis

• S. typhimurium

Major Cell Wall Antigen in Enterobacteriaceae

Lipopolysaccharide (LPS or endotoxin)

What components make up LPS?

• O polysaccharide

• common core polysaccharide

• Lipid A

How Enterobacteriaceae serologically classified?

Based on 3 major groups of antigens

• O polysaccharide

• Capsular K antigens (protein/polysaccharide)

• Flagellar H protein

How are antigens detected?

Agglutination by specific antibodies

MacConkey Agar

• Selective between Gram-negative and most Gram-positive bacteria

  • Gram positives are inhibited by bile salts and crystal violet

• Differentiates between lactose-fermenting and lactose nonfermenters

  • Lactose Positive turn pH indicator red

XLD Agar

• Isolation and differentiation of Salmonella and Shigella from other gram negative enterics

• Gram positives inhibited

• Yellow colonies indicate lactose fermentation

Salmonella Disease Process

1. Bacteria ingested

2. Bacteria adhere to epithelial cells or M cells

3. Salmonella induces degeneration of microvilli followed by frpofound membrane ruffling

4. Salmonella multiply in vesicles, lyse the cell, and replicated in the lamina propria

5. Induce inflammatory Response with abdominal pain and inflammatory diarrhoea

6. Diarrhoea caused by ion imbalance from cytokine production

What is the infectious dose of Salmonella?

Greater than 10⁵ bacteria must be ingested.

Are Salmonella bacteria invasive?

Yes, Salmonella are invasive bacteria.

Which intestinal cells do Salmonella adhere to?

Enterocytes and M cells

What is the function of M cells?

They sample antigens from the intestinal lumen and deliver them to macrophages via transcytosis

Where are M cells located?

In the epithelium of Peyer’s patches in the small intestine

What structural change does Salmonella induce upon adhering to epithelial cells?

Degeneration of microvilli

What is membrane ruffling in Salmonella infection?

Localized distortion of the host cell membrane at the site of bacterial contact

What cellular process accompanies membrane ruffling?

Macropinocytosis

What does macropinocytosis lead to in Salmonella infection?

Internalization of bacteria into endocytic vesicles

What happens to Salmonella inside endocytic vesicles?

They multiply within the vesicles

What occurs after Salmonella multiply in the vesicles?

The host cell lyses, releasing bacteria

Where do Salmonella replicate after cell lysis?

In the lamina propria

What immune response is triggered by Salmonella in epithelial cells?

Inflammatory Response

How do Salmonella spread systemically?

Via M cells, surviving and multiplying inside unactivated macrophages before entering the bloodstream

Why can Salmonella survive inside macrophages?

They are resistant to oxidative killing

What role does LPS play in Salmonella infection?

It induces inflammation, causing abdominal pain and inflammatory diarrhea

Is Salmonella diarrhea caused by enterotoxins?

No, it is not caused by enterotoxin release

How do Salmonella disrupt intestinal cells?

By injecting proteins that interfere with host cell functions

What is the result of cytokine release during infection?

Attraction of neutrophils

What do neutrophils release that contributes to diarrhea?

Prostaglandins

How do prostaglandins contribute to diarrhea?

They activate adenylate cyclase, increasing cAMP

What is the final effect of increased cAMP in intestinal cells?

Ion imbalance and water secretion → diarrhea

What may result from the invasion of lymphoid tissue in the intestinal tract?

• Necrosis

• Ulceration

• Intestinal haemorrhage from erosion of blood vessels in the lesions

Salmonella Virulence Factor: Adherence

• Plasmid-encoded fimbriae mediate binding of bacteria to microvilli of enterocytes

• Genes encoding fimbriae are located on virulence plasmid pSLT

Salmonella Virulence Factor: Membrane Ruffling

induce host cell to form pseudopods to engulf bacteria via actin cytoskeletal rearrangements

Salmonella Virulence Factor: Pathogenicity Islands

SPI 1,2,3,4,5 contain many virulence genes

SPI1

• contains the inv genes which cause membrane ruffling

• Encodes type 3 protien secretion system that injects sptP into host cells

sptP

econdes tyrosine phosphatase that affects host signal transduction pathways affecting cell response to outside stimuli

• Causes diarrhoea

SopE

injected and induces cytoskeletal arrangements that induce membrane ruffling

pSLT

carries spv genes which are important for survival and killing of macrophages

Typhoid Fever Pathogenesis

1. S. typhi invade the intestinal mucosa, pass through endothelial cells to the sub-mucosa where they are taken up by macrophages.

2. The organisms survive inside the macrophages and are taken via the lymphatic system to the lymph nodes.

3. The organism replicates and causes a transient bacteraemia before they are cleared and carried to the liver and spleen.

4. The organisms multiply in the macrophages of the liver, spleen and lymph nodes.

5. The organisms are released into the blood, initiating continuous bacteraemia. This results in the start of clinical illness – daily high fevers.

6. Secondary bacteraemia results in invasion of the   gallbladder, kidney and re-invasion of the gut mucosa at Peyer’s patches (a group of lymph   nodes found in the ileum) – inflammation, ulceration, bloody diarrhoea.

7. There may be long-term colonisation of the gallbladder which results in the Typhoid carrier state.  The organism can be continually secreted in the faeces by the convalescent host.

What is the first step in S. typhi infection?

Invasion of the intestinal mucosa

How does S. typhi reach the submucosa?

It passes through endothelial cells into the submucosa

What happens to S. typhi in the submucosa?

It is taken up by macrophages

Can S. typhi survive inside macrophages?

Yes, it survives and replicates inside macrophages

How does S. typhi spread after entering macrophages?

Via the lymphatic system to lymph nodes

What occurs after replication in lymph nodes?

Transient bacteraemia

Where do the bacteria go after transient bacteraemia?

To the liver and spleen

What happens in the liver, spleen, and lymph nodes?

Bacteria multiply inside macrophages

What triggers the onset of clinical illness in typhoid fever?

Continuous bacteraemia

What is a key symptom of continuous bacteraemia in typhoid fever?

Daily high fevers

What is secondary bacteraemia?

Further spread of bacteria to other organs after initial infection

Further spread of bacteria to other organs after initial infection

Gallbladder, kidneys, and intestinal mucosa

What happens when S. typhi reinvades Peyer’s patches?

Inflammation, ulceration, and bloody diarrhoea

Where are Peyer’s patches located?

In the ileum

What is the typhoid carrier state?

Long-term colonisation of the gallbladder by S. typhi

How is S. typhi shed in carriers?

Continuously secreted in faeces

Why are carriers important in disease transmission?

They can spread infection even after recovery

Symptoms of Typhoid Fever: First Week

• Headache

• Increasing fever

• malaise

• Abdominal pain

• constipation

Symptoms of Typhoid Fever: Second Week

• Fever increases

• Diarrhoea becomes prominent and bloody

• Weakness

• Profound fatigue

• Lethargy

• Delirium

• Ill appearance

• Rose Spots

Symptoms of Typhoid Fever: Third Week

• Febrile

• Exhaustion

• Weakness

Rose Spots

Small maculo-papular lesions that blanch on pressure

Diagnosis of Typhoid Fever

• Rose spots on abdomen

• Blood examination: elevated white blood cell count of S. typhi bacteria during first week of fever

• Faecal examination: stool culture of S. Typhi, bloody faeces, leukocytes

• Urine Examination: proteinuria, S. typhi culture

Typhoid Fever Treatment

• Iv fluids and electrolytes

• Antibiotics: ciprofloxacin or chloramphenicol

Typhoid Fever Prevention

Avoid high-risk food and drinking water

Reservoir of Salmonella

• GI tract of wild and domestic animals

  • brids

  • reptiles

  • amphibians

Cause of Salmonellosis

Penetration and passage of Salmonella organisms from gut lumen into epithelium of small intestine where inflammation occurs. In healthy human volunteers, a median dose of 1 million to 1 billion organisms is necessary to produce symptoms.

Salmonellosis Symptoms

• Incubation of 8-48 hours

• nausea and vomiting

• Fever

• Abdominal cramping and diarrhoea

Salmonellosis Diagnosis

based on isolation of the infecting organism from stool

Salmonellosis Treatment

• Replace fluids and electrolytes

• Antibiotics for those with strong symptoms

Salmonellosis Prevention

• Proper hygiene and food storage

• Education

• Cleaning cooking utensils

• Adequate cooking temps to kill bacteria