Salmonella and Shigella-Felton

Question 1

What are the characteristics of Salmonella?

Answer 1

oCommonly exist as resident flora in chickens and livestock.
oSource of infection usually from contaminated water or food or direct fecal-oral spread.
oGram- rods.
oOxidase negative
oFerment glucose but can’t ferment lactose.
oMotile.
oProduces gas and hydrogen sulfide.
oNeed large exposures to Salmonella for disease. Mostly found in children.
oNo exotoxins; functions as an intracellular pathogen inside epithelial cells and macrophages.

Question 2

How are Salmonella classified?

Answer 2

Based on species (3):
•S. typhi
•S. choleraesuis
•S. enteritidis
Based on serogroups
•A, B, C1, C2, D and E.

Question 3

What are the virulence factors for Salmonella?

Answer 3

1. Pathogenicity islands SPI1 and SPI2
2. SPI1 involved in invasiveness of epithelium.
3. SPI2 involved in survival inside macrophages.
4. Type III secretion system (encoded on both SPI’s)
   •Inject effector proteins which affect things like cytoskeleton
   •Trigger mechanism: type III secretion injects effector proteins that interact w/ actin of cytoskeleton and cause polymerization and overactivity. Form ruffles in the membrane that brings in the microbe. Used by Salmonella.
   •Zipper mechanism: adhesins on microbe bind to receptors on cell surface. Causes host cell to engulf (zipper up) around the microbe.
5. Endotoxin (LPS)
6. Serum resistance proteins
7. Capsule: Vi antigen; antiphagocytic against PMN’s)

Question 4

What are the clinical syndromes (4 forms)?

Answer 4

1. Enteric fever
2. Septicemia
3. Gastroenteritis
4. Asymptomatic carriage

Question 5

Describe the 2 types of enteric fever.

Answer 5

1. Typhoid fever
   •Caused by S. typhi.
   •High fever, abdominal symptoms and rose spots. (Not primarily a diarrheal disease.)
   •Infection via fecal-oral route.
   •Bacteria are shed in stool at some stages of disease.
   •Systemic infection in blood and several internal organs.
   •Chlorination of water helped to reduce the incidence to about 400.
   •There are vaccines but rarely used now.
2. Paratyphoid fever
   •Caused by S. paratyphi A, S. schottmuelleri and S. hirschfeldii.

Question 6

Describe septicemia.

Answer 6

• Large numbers of organisms in blood w/o concurrent involvement of GI tract.
• Most commonly caused by S. choleraesuis and S. Dublin.
• Secondary symptoms = endocarditis, osteomyelitis and septic arthritis.

Question 7

Describe gastroenteritis.

Answer 7

• Most common type of Salmonella infection.
• Confined primarily to the GI tract (not systemic).
• Appear 6-48 hours after consumption.
• Get nausea, vomiting and diarrhea.
• Stools are loose, moderate volume and w/o blood or mucus.
• Fever and abdominal cramps are also common.
• Last for 2-7 days before spontaneous resolution.
• Usually caused by Salmonella enteriditis (Invasive and can sometimes get to the blood and mesenteric lymph nodes.)
• Treatment should be to maintain fluids and electrolytes (not antibiotics = prolong carrier state, unless in blood or neonate).

Question 8

Describe Asymptomatic carriage.

Answer 8

• People previously infected w/ Salmonellae can continue to excrete bacteria 1 month to 5 months to 1 year after infection.
• Gall bladder can be a reservoir in carriers.
• Carriers = people who excrete Salmonellae after 1 year of infection.
• Usually it’s S. typhi, S. paratyphi A and S. paratyphi B.

Question 9

What are the general characteristics of Shigella?

Answer 9

oCause GI disease called bacillary dysentery or shigellosis.
oClosely related to E. coli.
oNo natural animal host.
oTransferred by contaminated food, water and direct fecal-oral spread.
oVery virulent.
oNon-motile
oLactose negative
oDo not produce gas.
oPossess LPS O antigens, but no H-antigens.
oDoes not get into the blood. Stays only in mucosa.

Question 10

What are the classifications (4 species) of Shigella?

Answer 10

1. Shigella dysenteriae (Group A)
•Unlike the others, Group A can’t ferment mannitol.
2. Shigella flexneri (Group B)
•The second major cause of shigellosis.
3. Shigella boydii (Group C)
4. Shigella sonnei (Group D)
•Some can ferment lactose but slowly.
•Accounts for most of the cases of shigellosis.

Question 11

What are the virulence factors for Shigella?

Answer 11

1. Large pathogenicity plasmid with genes for virulence factors.
2. Type III secretion system (plasmid encoded)
   •Uses the “trigger” mechanism to invade cells.
3. Very low infectious dose
4. Invasiveness (M cells, epithelium) (primary virulence factor)
5. Shiga toxin (phage encoded)
   •Has enterotoxic, cytotoxic, and neurotoxic activities
6. Actin Rocket formation
7. Endotoxin

Question 12

What is the pathogenesis of Shigella?

Answer 12

oBacillary dysentery is the most communicable of the bacterial diarrheas.
oMostly affects children or war-torn places.
oShigella invades the epithelium of the large bowel and then streapds from cell to cell by formation of actin “comets,” similar to Listeria.
oThis leads to micro-abscesses and ultimately ulcers.

Question 13

What are the clinical signs of Shigella?

Answer 13

oFever and scant bloody diarrhea w/ mucus and pus. (sometimes voluminous watery diarrhea)
oSymptoms subside in about a week.
oDoes not make carriers.
oElderly and children most commonly affected.
oNo vaccine.
oAntibiotics can be used to shorten symptoms as well as fluid and electrolyte replacement.