Salmonella And Bacterial Toxins

Question 1

The genus Salmonella is divided into two species, what are they?
How many strains are in S. enterica?
This is further divided into how many subgroups based on what?
All strains pathogenic to humans are in which subgroup?
What is the correct taxonomic name of the Salmonella pathogen that causes Typhoid fever.

Answer 1

Salmonella enterica and Salmonella bongori.
There are over 2500 strains grouped into S. enterica.
This species is divided into six subgroups based on their host range specificity, which also involves the immunoreactivity of three surface antigens: O, H and Vi.
All strains pathogenic to humans are in subgroup 1, with the species name for Typhoid fever being: Salmonella enterica ssp. enterica serovar typhi. Simplified version is Salmonella Typhi.

Question 2

What is Salmonella Typhimurium? Describe the symptoms and prevalence.

Answer 2

This is a non-typhoidal salmonella bacterium. It causes enteritis in humans with symptoms including diarrhoea, abdominal cramps (12-72 hours after infection), with the overall symptoms lasting 4-7 days. The illness is generally self-limiting. Salmonella Typhimurium is the second most common food poisoning pathogen, but most deadly. Isolated from 23% of GB pigs in abattoirs.

Question 3

What are chronic complications of Salmonella infection?

Answer 3

Reiters syndrome: joint pain and eye irritation.

Can lead to reactive arthritis.

Question 4

Why was there a large increase in the 1980s/1990s of Salmonella infection?

Answer 4

Microbial adaptation (S. Typhimurium DT104)
Pathogen distribution through global food distribution and international travel.
Growth in susceptible population.
More fast food outlets, less food handling knowledge.

Question 5

The British Lion quality code of practice launched when and results in what?

Answer 5

Launched in 1998. It includes compulsory vaccination against Salmonella enteritidis of all pullets destined for egg producing.

Question 6

How much salmonella bacteria must be ingested before infection is established?
What is the response to the bacteria that causes diarrhoea?

Answer 6

100000 bacteria must be ingested where they then invade mucosal cells.
They induce an inflammatory response, whereby prostaglandin is released into the mucosal cells. This prevents Na2+ uptake and induces Cl2- secretion. This change in water potential causes water to diffuse out of the cells resulting in diarrhoea.

Question 7

Describe how Salmonella adheres to the Lamina propria.

Answer 7

It adheres to the gut epithelium where it then invades. The host senses infection potentially through LPS and recruits phagocytes. Salmonella survives and multiplies inside the vacuole. A pro-inflammatory response (NF-kB) is induced and inflammation of the gut mucosa occurs. This inflammation damages the lamina propria.

Question 8

Describe the basics of the Salmonella life cycle in the human host.

Answer 8

Salmonella are ingested and travel to the small intestine where they adhere to the intestinal lining and begin life cycle.
In severe cases they can break through the intestinal lining into the bloodstream, this can be deadly if not properly treated.

Question 9

What are the symptoms between 12-72 hours?
What occurs between 4-7 days?
What is the treatment for severe cases?

Answer 9

Nausea, vomiting, fever, diarrhoea, abdominal cramps.
Illness ranges from mild to severe, most people recover on their own.
Intravenous administration of antibiotics for 2 weeks.

Question 10

Describe Salmonella Typhi.

Answer 10

S. Typhi is a flagellated gram negative bacteria. The bacteria has several antigens, these being somatic O, flagellar H, envelope K and surface virulence antigens Vi as well as the LPS endotoxin. S. Typhi is host-restricted and only causes typhoid fever in humans. The bacterium can ferment glucose without forming gas and can reduce nitrate to nitrite. The pathogen has a preference for Peyers patches in the intestine, where it can gain access to the blood and lymphatic system.

Question 11

How is S. Typhi transmitted?

Answer 11

Through the stools and urine of infected people.

Question 12

What are the symptoms of S. Typhi throughout the development of Typhoid fever?

Answer 12

First symptoms include: fever, headache, joint pain, abdominal tenderness, constipation (rarely diarrhoea), loss of appetite and sore throat.
As the illness the fever intensifies and the patient becomes delirious. This can be accompanied by extreme exhaustion and a slow heartbeat.
During the second week and last 2-5 days 10% patients will develop pink spots on their chest and abdomen.

Question 13

What symptom complications can form in some people with Typhoid fever?

Answer 13

Intestinal bleeding and perforation can occur in 3%-5% of infected people. Additionally pneumonia can develop as well as the gallbladder and liver becoming infected. At the final stage of infection the blood infection can cause inflammation of the bones (osteomyelitis), heart valves (endocarditis), kidneys (glomerulitis) and tissues covering the spinal cord and brain (meningitis).

Question 14

What is the chronic carrier state?

Answer 14

1-4% of untreated patients for Typhoid fever become chronic carriers. Stool carriage is more frequent in people with pre existing biliary abnormalities and these people have a greater incidence of cholecystitis. When this occurs there is a greater risk of carcinoma of the gall bladder developing, and a 6 fold increase in the risk of death.

Question 15

What are risk factors for people infected with Typhoid fever?

Answer 15

Defects in their immune systems: AIDs patients.
Defects in phagocytic function: malaria and schistomiasis.
Splenectomy or functional asplenia: sickle cell disease.
Low stomach pH: patients of anti-ulcer drugs.
Prolonged use of antibiotics: altered gut flora.
Injured gut barrier: bowel disease.

Question 16

What is the treatment for Typhoid fever?

Answer 16

Medical care includes rehydration, antipyretics and antibiotics. Quinolones were until recently the front line antibiotics. Three commonly prescribed antibiotics were ampicillin, trimethoprim-sulfamethoxalzole and ciprofloxacin.

Question 17

Is there antibiotic resistance in S. Typhi?

Answer 17

Yes. Resistance to quinolone antibiotics was reported back in 1992. This resistance disease is now causing epidemics and limits choices to treatment of typhoid fever.

Question 18

Describe NTS in sub-Saharan Africa. Who are most at risk in both adult and children populations?

Answer 18

NTS is a frequent cause of iNTS (invasive bacterial disease) in Africa. It is common in children afflicted with malnutrition, AIDs, malaria or anaemia. It is also common in adults afflicted with AIDs. iNTS is more clinically frequent that Typhi. Also iNTS is distinct from gastroenteritis and Typhoid fever, the symptoms including a fever (rarely diarrhoea) often indistinguishable from malaria. 22-45% case fatalities.

Question 19

When was the genome of S. Typhimurium sequenced? What is the size of its genome and what does it possess in its genome?

Answer 19

1st November 2000, possesses a genome of around 4857432bp. There are 20 pathogenicity islands, each consisting of 20 genes.

Question 20

What does Salmonella Pathogenicity island 1 encode?

What are the effector proteins released into the epithelial cells?

Answer 20

It is required for bacterial invasion and encodes a type III secretion system.
Effector proteins: SipA: actin-binding protein
SopB: activates GTP-binding proteins CDC42 and Rac
SptB: GTpase activator for CDC42 and Rac + protein tyrosine phosphatase.

Question 21

What does SPI-2 do?

Answer 21

Is required for growth and survival of the bacteria within the host epithelial cells.

Question 22

Salmonella was discovered when and in what organ of what animal?
Salmonella belongs to what family of bacteria?
What are the bacteria’s characteristics?

Answer 22

Discovered by Daniel Salmon in 1880 in a porcine intestine (Pig).
It belongs to the enterobacteriacea family.
It is a gram negative facultatively anaerobic pathogen. Salmonella is a pathogen of both humans and animals and causes gastroenteritis in a range if hosts because this is a non-host adapted symptom. When host adapted it causes a systemic infection.

Question 23

What are exotoxins?

Answer 23

These are secreted proteins, which are soluble as heat-labile. Some are enzymes. They can kill host cells at very low concentrations and play a central role in pathogenesis. Exotoxins are associated with specific diseases and have different modes of action. They are often given the name of the disease they produce.

Question 24

How are humans exposed to exotoxins?

Answer 24

Ingestion of preformed exotoxins (bacteria growing on food eg botulism),
Colonisation of a mucosal surface followed by exotoxins production (cholera toxin),
Colonisation of a wound or abscess followed by local exotoxin production (Clostridium perfringens).

Question 25

If exotoxins enter the blood, what can occur?

Answer 25

The toxins can cause effect away from the site of infection. For example Bordetella pertussis (whooping cough) infects the mucosal lining of the throat however if the toxin enters the bloodstream it can cause systemic disease throughout the body.

Question 26

What are AB toxins? Give two examples.

Answer 26

These are proteins comprised of two protein subunits A (active) and B (binding). The B subunit binds the A subunit to the protein structure within the host cell, normally an enzyme although it can be a structural component. The A subunit then alters the conformation of this bound component. For example the Diphtheria toxin binds to a protein called EF2 and inactivates it, eukaryotes use EF2 to synthesise proteins. Pertussis toxin (Ptx) disrupts the host cells adenylate cyclase system, which mediates intracellular communication.

Question 27

What is Diphtheria? How does its toxin disrupt metabolic functions?

Answer 27

Diphtheria is a very infectious disease caused by the gram positive bacterium Corynebacterium diphtheriae. It affects people in crowded conditions. This disease is an inflammatory condition resulting in the formation of a pseudo membrane on the respiratory mucosal surface. The B component of the of the toxin binds to cell surface receptors and is taken into the cell via a clathrin coated vesicle. Toxin is then cleaved into two parts, the A fragment escaping into the cytosol to disrupt EF2 function.

Question 28

If the diphtheria toxin enters the bloodstream what can occur?
What neautralises this exotoxin?
What antibiotics are given to treat the bacteria?

Answer 28

It can cause destruction of cardiac, kidney and nervous tissue by disrupting protein synthesis.
Diphtheria antitoxin is administered to neutralise any unabsorbed exotoxin in patients tissues.
Penicillin or erythromycin is administered.

Question 29

What is anthrax caused by? Describe the bacteria and the three types of disease it can cause.

Answer 29

Anthrax is caused by Bacillus anthracis. It is a gram positive bacterium capable or producing spores that enable it to survive for many years in the soil until favourable conditions arise. Infection generally occurs through physical wounds leading to cutaneous anthrax. If spores are inhaled, pulmonary anthrax can occur however if spores reach the gastrointestinal tract, gastrointestinal anthrax occurs.

Question 30

The virulence factors of Bacillus anthracis are encoded by what?
Describe these virulence factors.

Answer 30

Encoded by two plasmids. One is involved in the synthesis of a polyglutamyl capsule. The other encodes two genes needed for exotoxin production.

Question 31

Describe the anthrax exotoxin.

Answer 31

This is a complex exotoxin, which is comprised of three proteins. The Protective Antigen (PA), the Edema Factor (EF) and the Lethal Factor (LF).

Question 32

Describe how Bacillus anthracis infection progresses after inhaling spores.

Answer 32

The spores become lodged in the alveolar spaces within the lungs. These are then engulfed by alveolar macrophages. They survive digestion via the polyglutamyl coat and colonise the endosome. The bacteria then spread out and infect the lymph nodes before spreading to two blood stream.

Question 33

Describe how the protective antigen protein of the anthrax exotoxin works.

Answer 33

Macrophages express anthrax toxin receptors on their plasma membrane. Anthrax produces the Protective Antigen Protein which binds to this receptor. Seven in total bind to one receptor and for a ‘doughnut’ configuration around the receptor. This then acts as a syringe, boring into the macrophage.

Question 34

After the binding of seven PA proteins to the macrophage anthrax toxin receptor, what happens?

Answer 34

The PA ring binds the Edema factor and the Lethal factor before the entire complex is engulfed. The PA ring then forms a pore and pierces the endosome membrane allowing access to the cytosol for LF and EF. EF prevents the cell signalling to the immune system for help. The LF prevents expression of a transcription factor called nuclear factor B, that promotes macrophage survival.

Question 35

Describe two types of toxins that effect specific host sites. Give examples for each.

Answer 35

These can be AB type toxins and the two types are neurotoxins and enterotoxins. Neurotoxins are normally ingested as preformed toxins and effect nervous tissue. An example is the botulism toxin.
Enterotoxins effect the intestinal mucosa and cause fluid secretion, an example is the cholera toxin.

Question 36

What are pore forming toxins?

Answer 36

These disrupt the host cell membrane by binding to the cholesterol portion of the membrane and creating a pore which then kills the host cell by allowing the cytoplasmic contents to escape. Due to osmoregulatory imbalances, water rushes into the cell causing it to swell and rupture.

Question 37

Give examples of pore forming toxins.

Answer 37

Leukocidinsm these are produced by streptococci and destroy leukocytes.
Haemolysins are another example, these rupturing red blood cells. Listeria monocytogenes produces this, along with streptococcus.

Question 38

What are phospholipase enzymes?

Answer 38

These enzymes remove the charged polar head group from phospholipid membranes, de-stabilising the membrane and causing the cells to rupture and die. An example of a bacteria that uses this is Clostridium perfringens, this bacteria produces the alpha toxin that is deadly to white blood cells.

Question 39

How can the host immune system neutralise exotoxins?

Answer 39

Through the use of antitoxins, specialised antibodies that bind to toxins. This capacity of the host system has led to the formation of toxin vaccines. The toxin is modified via heat or chemical modification to form a toxoid, this is no longer dangerous to the host system but still activates it, resulting in antitoxin production.

Question 40

How does Penicillin cause bacterial cell death?

Answer 40

Penicillin prevents cell wall production in bacteria, when the bacteria grown in size their walls rupture and they die.

Question 41

What are endotoxins?

Answer 41

These are toxins that are part of the structure of the bacterial cell and are released when the cell is lysed. They are normally part of the LPS, the toxic part being Lipid A.

Question 42

How is Lipid A toxic?

Answer 42

Lipid A stimulates the release of cytokines when the bacterium is lysed. It becomes toxic when large amounts are produced due to mass bacterial death. These cytokines induce inflammation, resulting in a myriad of symptoms that can make recovery worse. Symptoms include: fever, increase or decrease in white blood cells, shock, extreme weakness and even death.

Question 43

Describe the cytotoxin produced by Bordetella pertussis.

Answer 43

Produces a tracheal cytotoxin which is a small fragment of peptidoglycan that kills respiratory cells and stimulates a release of cytokines.

Question 44

How does coagulase benefit the bacteria?

Answer 44

Coagulase splits the serum protein fibrinogen to form fibrin, therefore creating blood clots. Interfering with the blood clotting system might benefit microbes by forming protective clots around themselves or by escaping clots that the body produces.

Question 45

What does the bacterial cag pathogenicity island in Helicobacter pylori encode?

Answer 45

Cag encodes a Type IV Secretion System, H. Pylori with cag have an increased risk of developing gastric cancer.

Question 46

What does the Pasteurella mutocida toxin do?

Answer 46

This toxin has been isolated from chronic infections. PMT enters cells and stimulates cell signal systems that regulate cell growth. One example of a stimulated cell signal system is MAP kinase ERF1/2.

Question 47

What does the Bacteroides fragilis toxin do?

Answer 47

BFT is a metalloprotease that cleaves E-Cadherin, which is a protein important for cell-cell interactions. Degradation of E-Cadherin stimulates β-catenin nuclear localisation and associated c-Myc expression, resulting in cell proliferation.

Question 48

How do bacterial toxins make useful therapeutic agents?

Answer 48

As bacterial toxins alter cell signalling responses they may have some use as cancer treatments. Cytolysin A (ClyA) is a pore forming toxin produced by E. Coli that induces apoptosis. Research in mice has shown that treating them with S. Typhimurium or E. Coli expressing ClyA reduces tumour growth.