L12. Enteric Gram-Negative Bacilli: Salmonella, Shigella, Escherichia coli

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43 Terms

1

What are Enterobacteriaceae?

A large family of gram-negative, facultatively anaerobic, rod-shaped bacteria commonly found in the intestines.

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2

What are the biochemical characteristics of Enterobacteriaceae?

Ferment glucose, reduce nitrates to nitrites, oxidase-negative, and most are motile except for Shigella and Klebsiella.

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3

How are Enterobacteriaceae classified based on lactose fermentation?

Lactose fermenters: E. coli, Klebsiella, Enterobacter.

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4

O Antigen

Somatic antigen (heat-stable, used for serotyping)

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5

H Antigen

Flagellar antigen (heat-labile, used for motile species).

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6

K Antigen

Capsular antigen (heat-labile, helps evade phagocytosis).

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7

Which Enterobacteriaceae genus is the most common cause of human infections?

Escherichia coli.

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8

What special virulence factor do some Enterobacteriaceae produce that allows them to resist phagocytosis?

Capsules

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9

What are the key structural differences between gram-negative and gram-positive bacteria?

Gram-negative bacteria have an outer membrane with lipopolysaccharides (LPS), a thin peptidoglycan layer, and porins.

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10

What are the main characteristics of Salmonella species?

Gram-negative bacilli, facultative anaerobes, non-lactose fermenters, oxidase-negative, produce H₂S (except S. paratyphi A).

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11

What are the two main species of Salmonella?

Salmonella enterica and Salmonella bongori.

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12

How many serotypes of Salmonella exist?

More than 2,500 serotypes based on the Kauffmann-White classification.

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13

What is the mode of transmission of Salmonella infections?

Fecal-oral route via contaminated food or water, direct contact with infected animals or people.

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14

What food items are commonly associated with Salmonella outbreaks?

Poultry, eggs, dairy products, raw vegetables, and contaminated water.

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15

What are the virulence factors of Salmonella?

Invasiveness, endotoxin (LPS), enterotoxins, surface antigens, intracellular survival, and type III secretion system.

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16

What is the role of the Vi antigen in Salmonella typhi?

It prevents phagocytosis and is used to detect chronic carriers.

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17

How does Salmonella invade the host?

It attaches to M cells in the intestine, invades Peyer’s patches, and spreads via the bloodstream.

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18

What are the clinical syndromes caused by Salmonella?

Typhoid fever, gastroenteritis (non-typhoidal salmonellosis), bacteremia, and chronic carrier state.

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19

What immune response is critical in clearing Salmonella infections?

Cell-mediated immunity (T-cells and macrophages).

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20

What organism causes typhoid fever?

Salmonella typhi.

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21

How is typhoid fever transmitted?

Fecal-oral route via contaminated food or water.

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22

What is the incubation period of typhoid fever?

7-14 days.

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23

What are the symptoms of typhoid fever?

Prolonged fever, rose spots, hepatosplenomegaly, leukopenia, intestinal perforation, and Peyer’s patch necrosis.

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24

What serious complication can result from intestinal perforation in typhoid fever?

Peritonitis

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25

What is the gold standard test for detecting typhoid fever?

Blood culture during the first week of illness.

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26

What culture media are used for Salmonella?

MacConkey Agar, Salmonella-Shigella (SS) Agar, Hektoen Enteric (HE) Agar, XLD Agar

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27

Results of culture media used for Salmonella (MacConkey Agar)

Colorless colonies (non-lactose fermenter).

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28

Results of culture media used for Salmonella (Salmonella-Shigella (SS) Agar):

Black-centered colonies (H₂S production).

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29

Results of culture media used for Salmonella (Hektoen Enteric (HE) Agar)

Green with black centers.

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30

Results of culture media used for Salmonella (XLD Agar):

Red with black centers.

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31

What test differentiates Salmonella from Shigella?

H₂S production test (Salmonella is H₂S-positive, Shigella is H₂S-negative).

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32

What are the characteristics of Shigella species?

Gram-negative, non-motile, non-lactose fermenters, oxidase-negative.

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33

What is the most virulent species of Shigella?

Shigella dysenteriae (produces Shiga toxin).

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34

How does Shigella invade the intestine?

It invades M cells in Peyer’s patches, escapes phagolysosomes, and spreads intracellularly.

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35

What is the function of Shiga toxin?

Inhibits protein synthesis by targeting the 60S ribosomal subunit, leading to cell death and hemorrhagic colitis.

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36

What are the major diarrheagenic E. coli strains?

ETC, EIEC, EHEC, EPEC, EAEC

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37

ETEC

Traveler’s diarrhea (heat-labile and heat-stable enterotoxins).

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38

EIEC

Dysentery (invades colonic epithelium).

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39

EHEC

O157:H7 (Shiga toxin, causes hemorrhagic colitis and HUS).

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40

EPEC

Infantile diarrhea (adheres to intestinal epithelium).

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41

EAEC

Persistent diarrhea (aggregative adherence).

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42

What laboratory test confirms EHEC O157:H7?

• Sorbitol-MacConkey (SMAC) Agar: EHEC does not ferment sorbitol (colorless colonies).

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43

What is the most common cause of urinary tract infections (UTIs)?

Escherichia coli

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