ICL 2.21: Shigella & Listeria

Question 1

what’s the microbiology of listeria?

Answer 1

gram (+) rod

Question 2

what is the microbiology of shigella?

Answer 2

gram (-) rod

Question 3

what pathogen caused this?

infant V., a 22-month-old girl living in Texas, became febrile, lost her appetite, and developed watery diarrhea

by the next day, her diarrhea had abated, but her parents noticed that her stools contained mucus and were blood-tinged

the number of stools and the bloody appearance increased, and the baby began to vomit. The parents brought her to a hospital emergency room. Temperature was 40 C. Shortly after arrival, she had a generalized seizure.

PE revealed a sick-looking, somnolent infant with mild dehydration and hyperactive bowel sounds.

laboratory results showed leukocytosis and a mild decrease in serum sodium and glucose.

child was given fluids and antibiotics and was fine after a few days

Answer 3

shigella flexneri

Question 4

what does shigella cause?

Answer 4

bloody diarrhea and dysentery

they invade and damage the mucosa leading to bloody diarrhea

these infections are serious and life threatening

antibiotics are often required; rehydration therapy has little impact

they’re a problem in developing countries.

Question 5

what is dysentery?

Answer 5

dysentery is characterized by frequent passage of stools (~30 per day) that typically contain small amounts of blood, mucus and pus

Question 6

what are the symptoms of a shigella infection?

Answer 6

1. dysentery = 30 stools a day containing blood, mucus and pus
2. cramps
3. pain from straining to pass stools

Question 7

which bacteria often causes dysentery?

Answer 7

shigella

entamoeba histolytica can also cause it

Question 8

which shigella species are most common among which populations?

Answer 8

S. dysenteriae and S. flexneri, associated with poverty and poor hygiene, are the predominant species in resource limited settings

S. sonnei is more common in affluent regions

Question 9

what are the 4 shigella species? how are they distinguished?

Answer 9

1. S. dysenteriae = serogroup A (the most serious*)
2. S. flexneri = serogroup B
3. S. boydii = serogroup C
4. S. sonnei = serogroup D (least serious)

Question 10

shigella is a pathotype of which other bacteria?

Answer 10

E. coli

Shigella is an invasive form of E. coli resulting from a large plasmid and other imported genes

all Shigella and EIEC have pINV which is a large plasmid

Question 11

is the connection between E. coli and Shigella divergent or convergent?

Answer 11

divergent

divergent evolution refers to a single genetic population yielding distinct subpopulations

so ancestral e. coli yielded EIEC and Shigella

however, E. coli –> shigella has happened more than once

so that means shigella consists of at least 2 distantly related subclades implying that the subclades evolved into a single clade via convergence

Question 12

how is shigella transmitted?

Answer 12

direct person-person contact

sometimes transmitted through food and water

most major outbreaks can be traced to polluted water supplies in areas with high concentrations of people like cities, refugee camps, etc.

Question 13

what population is more likely to get shigella infection?

Answer 13

children under 5 are 10x more likely to get the disease

Question 14

can shigella tolerate acid?

Answer 14

yes; they’re acid-resistant

this allows them to pass through the stomach without much cell death

it also explains why a small dose can cause disease

once they reach the small bowel (neutral-to-alkaline pH) they begin to grow and invade

Question 15

where do shigella replicate?

Answer 15

once they reach the small bowel (neutral-to-alkaline pH) they begin to grow and invade

in the colon bacterial multiplication occurs within intestinal epithelial cells

Question 16

how does shigella enter cells?

Answer 16

shigella approaches the mucosal surface via diffusion (nonmotile)

intestinal epithelial cells are resistant to invasion on their lumenal surface, but susceptible on their basolateral surfaces

so isntead there are microfold (M) cells that are adjacent with enterocytes –> the M cell binds bacteria; the lack of a brush border facilitates this binding

several pInv plasmid genes are required for M cell invasion

Question 17

how does shigella spread?

Answer 17

shigella pass through the M cell and is released into the lamina propria while others are ingested by macrophages

macrophages start an inflammatory response and release IL-1 and IL-8

the bacteria are now near the basal surface of the epithelial cells, where their invasin binds α5β1 integrin

the inflammatory response loosens the tight junctions, allowing Shigella to penetrate

the Shigella invasin is what induces phagocytosis since epithelial cells are not professional phagocytes

Shigella also produces a “hemolysin” that lyses the phagosome, releasing bacteria into the cytoplasm

Shigella then use actin-based motility to spread into adjacent cells

Question 18

what damage does shigella do to the body?

Answer 18

an ulcer is formed when a sufficient number of epithelial cells are invaded and slough off

neutrophils that accumulate in large numbers in the mucosa are shed in the stool where they are visible by light microscopy

bloody and pus-containing stools, and bowel pain (tenesmus), are characteristic of dysentery

Question 19

does shigella cause bacteremia?

Answer 19

it’s uncommon except for S. dysenteriae type 1

S. dysenteriae type 1 produces Shiga toxin = ~10% risk of HUS

Question 20

what is a common symptom of shigellosis?

Answer 20

in up to 40% of children with severe shigellosis, neurological symptoms are seen

1. febrile seizures
2. confusion
3. lethargy
4. headache
5. nuchal rigidity

Question 21

what kind of stools are seen with dysentery?

Answer 21

low volume but with blood and mucus

may or may not see WBCs

happens in the ileum and colon

there is fever, severe pain and tissue invasion involved

Question 22

which type of acute infectious diarrhea does shigella cause?

Answer 22

dysentery

Question 23

what is shiga toxin?

Answer 23

a cytotoxin that kills intestinal epithelial and endothelial cells

destruction of endothelial cells contributes to the bloody diarrhea

it irreversibly inactivates the mammalian 60S ribosome, halting protein synthesis

it also targets the villus cells causing decreased Na+ adsorption and excess water in the lumen

inactivation of Shiga toxin gene decreases the severity of the disease (monkeys)

Question 24

how do you diagnose shigella?

Answer 24

shigella should be considered when a patient has bloody diarrhea or dysentery

leukocytes in the stool is diagnostic of an invasive pathogen

but to diagnose shigellosis specifically, Shigella must be cultured

however, Shigella die rapidly in the feces so samples must be fresh and processed rapidly

PCR methods are being developed to identify it

Question 25

how do you treat Shigella?

Answer 25

FQs are the most effective but contraindicated in kids under 17

new B-lactams and cephalosporins are also effective

antibiotics reduce the severity, duration and infectivity of the disease but Shigella develop antibiotic resistance rapidly

dehydration is usually not a problem, so I.V. fluids are rarely needed

Question 26

FLASHCARD: shigella

Answer 26

gram (-) acid resistant

4 major serotypes; A is the most serious

closely related to E. coli, especially EIEC

shiga toxin damages rRNA

causes dysentery

vacterial pathogens can undergo convergent and divergent evolution

selection for survival, growth and/or spread in different environments can result in divergence from parental strain

ex. EHEC/commensal E. coli and Shigella/EIEC diverged from one another

Question 27

which pathogen caused this?

64 yr women has 5 day history of fever, headache, confusion

recent history of diarrhea which resolved before current symtpoms appeared

unable to answer simple questions, agitated, no nuchal rigidity, no focal neurological deficit

high WBC, elevated %PMNs, low Hct

normal head CT, MRI shows meningeal enhancement

CSF has WBCs but no bacteria

culture grew gram + rods

Answer 27

listeria monocytogenes

Question 28

which pathogen causes bacterial meningitis in 0-4 weeks old and people older than 50?

Answer 28

listeria monocytogenes

Question 29

what are PPIs and what do they do?

Answer 29

PPI = proton pump inhibitor

PPIs increase susceptibility to listeria infections!!

severe hypochlorhydria generated by PPI use leads to bacterial colonisation and increase susceptibility to enteric bacterial infections

also increase susceptibility to salmonella, vibrio cholerae, invasive E. coli, and vegetative C. diff

Question 30

what diseases can listeria monocytogenes cause?

Answer 30

1. listeriosis

2. intrauterine or cervical infections in pregnant women

Question 31

what is listeriosis?

Answer 31

caused by listeria monocytogenes

invasion of intestinal epithelium then spread via growth inside macrophages

can cause septicemia, meningitis and/or encephalitis, pneumonia, corneal ulcers

7-10 days fever, myalgia, vomiting, diarrhea

Question 32

what are the intrauterine/cervical infections in pregnant women from listeria monocytogenes?

Answer 32

can cross endothelial layer of placenta and cause spontaneous abortion or still birth

or there can be granulomatosis infantiseptica = pyogenic granulomas distributed over the infant’s body

80% perinatal/neonatal mortality

Question 33

describe the motility of listeria?

Answer 33

actin based motility

Question 34

what is the lifestyle of listeria vs. shigella?

Answer 34

LISTERIA
1. InIA invasin induces target cell to take up bacterium

2. hemolysin HlyA disrupts phagosomal membrane
3. bacteria grow in cytoplasm, move on ActA actin tails and infect adjacent cells
4. released or incoming bacteria use FbpA adhesin to attach to target cell

then this cycle just starts over

SHIGELLA
literally the same thing except:

1. IpaD
2. IpaB
3. IcsA
4. IcsB

none of these proteins are related to one another

Question 35

what is convergent evolution?

Answer 35

convergent evolution refers to distinct genetic populations coming to resemble one another

bacteria infecting a given site all face the same host defenses, and have often independently developed similar evasion or subversion strategies

ex. gram +s and gram-s converged to listeria and shigella/EIEC

Question 36

how does listeria evade the immune system?

Answer 36

there is N-deacetylation of the PG so it’s no longer recognized by TLR2

pdgA is the enzyme responsible for this

Question 37

how do you diagnose listeria?

Answer 37

1. microscopy will show short, gram + rods; sometimes in chains
2. culturing
3. cell properties = motile at room temperature, weakly hemolytic

Question 38

what are the culturing conditions that listeria grows in?

Answer 38

wide temperature range; gows at 4 C

grows in high salt

grows over a wide pH range (but not as acid resistant as shigella)

aka these are basically the conditions most often used in food preservation!

Question 39

what other part of the body can listeria infect?

Answer 39

the heart

cardiac infections caused by L. monocytogenes represent a significant but poorly studied facet of listeriosis

subpopulations of L. monocytogenes have acquired an enhanced ability to target and invade the myocardium

Question 40

where is listeria found in nature?

Answer 40

1. soil and water
2. vegetation
3. variety of animals

Question 41

how do you get listeria infections?

Answer 41

usually acquired by food poisoning

Question 42

what are the risk factors for listeria infection?

Answer 42

1. age extremes = babies and elderly
2. pregnancy
3. immunosuppression

Question 43

how do you treat listeria?

Answer 43

synergistic combination therapy = B-lactam (amoxicillin) with an aminoglycoside (gentamicin)

a person in a higher-risk category
(pregnant woman, older adults, and
people with weakened immune systems)
who experiences fever and other non-specific symptoms, such as fatigue and aches, within 2 months of eating contaminated food should seek medical care and tell the physician or health care provider about eating the contaminated food

Question 44

how do you prevent listeria infection?

Answer 44

at-risk patients should avoid raw or partially-cooked:

1. milk and soft cheeses
2. processed meats
3. raw vegetables

Question 45

is there antibiotic resistance with listeria?

Answer 45

it’s seen but it’s pretty rare

a small fraction show antibiotic resistance to tetracycline, clindamycin, and trimethoprim

Question 46

FLASHCARD: listeria in general

Answer 46

listeria is a serious Gram+ human pathogen

causes variety of diseases, including meningitis

like Shigella, uses an invasin, escapes phagosome, moves between cells via actin polymerization

unlike Shigella, is sensitive to low pH

most often spread via contaminated food (grows @ 4°C)

bacterial pathogens can undergo convergent and divergent evolution

selection for survival, growth and/or spread in different environments can result in convergence with species already in a given niche

example: Shigella/EIEC and Listeria converged with one another

Question 47

FLASHCARD: micro, path, epidemiology, clinical, diagnosis and treatment of listeria monocytogenes

Answer 47

MICROBIOLOGY: Gram-positive rod (differs from Bacillus in that Listeria does NOT form endospores), motile via flagella at room temp (at 37°C only motile via actin polym).

PATHOLOGY: Invades intestinal epithelium, grows in macrophages, can spread among cells via actin polymerization, deacetylates its peptidoglycan.

EPIDEMIOLOGY: A leading cause of neonatal meningitis (age 0-4 weeks), more common in those >50y old. Not acid resistant, so use of PPIs is a risk factor, as is immunosuppression and pregnancy.
Environmental pathogen, most commonly spread via raw produce, or dairy and meat products. Grows under conditions normally used to preserve food (4°C, high salt, mildly acidic conditions).

CLINICAL: Invasive pathogen
septicemia, meningitis/encephalitis, pneumonia, corneal ulcers
can cross placenta, leading to spontaneous abortion or granulomatosis infantiseptica

DIAGNOSIS: Culturing

TREATMENT: Antibiotic resistance appearing but still relatively rare; treatment usually synergistic combination of aminoglycoside (like gentamicin) and b-lactam (often amoxicillin)

Question 48

FLASHCARD: micro, path, epidemiology, clinical, diagnosis and treatment of shigella

Answer 48

MICROBIOLOGY: Gram-negative enteric rod, very closely related to E. coli (especially Entero-Invasive, or EIEC)

PATHOLOGY: Causes dysentery; serotype A (S. dysenteriae) produces Shiga toxin, which kills host cells by removing same rRNA base as does the plant toxin ricin. Can spread among cells via actin polymerization.

EPIDEMIOLOGY: Outbreaks particularly common among concentrations of young children (e.g., daycare centers) due to limited handwashing; low infectious dose as Shigella species are acid resistant

CLINICAL: Depends on serotype:
Serotype A causes most damage, in part due to production of Shiga toxin and bacteremia. Can lead to hemolytic-uremic syndrome (HUS) with kidney failure.
All four (A-D) cause dysentery, with intestinal ulcers

DIAGNOSIS: Blood, mucus and pus in the low-volume, frequent stools. Tenesmus (bowel pain). In severe cases, neurological symptoms in ~~half of cases (febrile seizures, confusion, etc.). Confirmation requires culturing, which requires fresh samples.

TREATMENT: Antibiotics essential (hydration not usually a problem). Resistance is widespread and changing, but in general fluoroquinolones (if patient >17y old), or newer b-lactams. [Vax candidates in trials, but none yet approved.]