Infectious diarrhoea II

Question 1

Broadly describe the types of IC pathogens with examples and the advantages of being IC

Answer 1

• Intracellular pathogens invade host cells and proliferate inside them:
  
  • Facultative intracellular pathogens benefit from intracellular location, but it is not obligatory.
    
    • e.g., Salmonella spp.
  • Obligate intracellular pathogens cannot survive outside human host cell, require host products for growth, can only reproduce when inside cell host.
    
    • e.g., Chlamydia spp.
• Advantages of intracellular lifestyle:
  
  • Access to host nutrient supply
  • Escape from host defences
  • Protection from antibiotics

Question 2

Broadly describe the mechanism of invasion fo IC patogens

Answer 2

• How do intracellular pathogens gain entry into cells?
  
  • Intracellular pathogens gain access to IECs via M cells (recall from [[Microbiology B5 - Lecture 4]], take up antigens via endo/phagocytosis).
  • Intracellular pathogens gain access to macrophages via M cells.
    
    • Macrophages used for dissemination via lymphatic ducts of intestines to other regions of the body.
    • Dissemination can also occur via intercellular spread.
  • Pathogen-mediated internalisation (non-phagocytic cells)【Murphy et al. (2008)】

Question 3

Describe the general characteristics of shigella

Answer 3

• Non-motile, Gram-negative rod
• Facultative anaerobes
• Resistant to gastric acid and bile salts
• Non-lactose/xylose fermenting
  
  • Inability to ferment xylose can be used to distinguish Shigella spp. from Salmonella spp. (black colonies) from clinical samples and from food using Xylose Lysine Desoxycholate (XLD) agar (red colonies)

Question 4

Describe shigella taxonomy

Answer 4

• Four species:
  
  • Shigella sonnei (most infections: mild form of disease)
  • Shigella flexnerii (more severe form of the disease)
  • Shigella boydii (more severe form of the disease)
  • Shigella dysenteriae (most serious form of the disease)
• Human pathogens exclusively – no animal reservoirs.
• No environmental reservoirs.
• Faecal-oral spread, contaminated foods, *flies act as mechanical vectors.

Question 5

Describe shigellosis

Answer 5

• Primarily a paediatric disease in developing countries.
  
  • Acute onset: Incubation 1–4 days, Duration 2–3 days.
• Small infective dose (10-100 organisms).
• Epidemics occur due to overcrowding and insufficient sanitation.
  
  • Refugee camps, nurseries, day-care centres and other residential facilities
• Symptoms:
  
  • Spectrum: Mild to severe depending on the species (As well as host factors)
  • (+/-) Fever, abdominal cramps, severe and bloody diarrhoea - especially with dysentery, often with pus (PMNs) and mucus ^[stool MCS shows leucocytes]
  • Inflammatory diarrhoea: due to invasion
  • Osmotic diarrhoea: due to loss of absorptive surface

Question 6

Describe shigella virulence factors

Answer 6

• Invasiveness:
  
  • Intracellular survival, multiplication, and intercellular spread
  • Shigella can lyse the phagocytic vacuole (phagosome) and replicate in the cytoplasm
    
    • in this way it can escape host defences
• Type III secretion system and effector proteins required for invasion and escape from the vacuole
• AB5 exotoxin (Shiga toxin) and other toxins (e.g., SigA)
• IcsA autotransporter:
  
  • Outer membrane protein
  • Localised at pole of bacterium
  • Facilitates host-actin polymerisation, brings pathogen closer to epithelial cell, thereby faciltiating intracellular spread【Sansoneh (2004)】

Question 7

Describe EIEC general featuers

Answer 7

• EIEC and Shigella are genotypically nearly identical.
  
  • Have same virulence factors and strategies
  • Cause a syndrome identical to Shigellosis
• EIEC can be differentiated from Shigella only by a very limited number of tests based on utilisation of different nutrients, including utilisation of serine, xylose and/or sodium acetate, and mucate fermentation
  
  • EIEC isolates may be positive for one or more of the tests but Shigella are generally negative
  • Do not fit the full definition for the “genus” Shigella

Question 8

Describe salmonella genearl acharacterisitcs

Answer 8

• Motile, Gram-negative rods
• Facultative anaerobes
• Resistant to bile salts, although poor resistance to acids ^[contrast with [[#Shigella General Characteristics]]]
• Non-lactose fermenting, but do ferment xylose
  
  • XLD agar can be used to differentiate Salmonella from Shigella
• Grow within a wide temperature range (≤54°C)
  
  • Grows optimally at 37°C
  • *Psychrotrophs (2-4°C)

Question 9

Describe salmonella taxonomy

Answer 9

• Taxonomy based on genomic relatedness show that there are two Salmonella species:
  
  • S. bongori
  • S. enterica
• S. enterica has a further 6 subspecies:
  
  • S. enterica subspp. arizonae
  • S. enterica subspp. diarizonae
  • S. enterica subspp. enterica
  • S. enterica subspp. houtenae
  • S. enterica subspp. indica
  • S. enterica subspp. salamae
• There are >2500 serovars/serotypes for S. enterica.
• Most of the pathogenic Salmonella serovars are found within S. enterica subspp. enterica
  
  • e.g., S. enterica subspp. enterica serovar Typhimurium generally shortened to S. enterica serovar Typhimurium, and Salmonella paratyphi ?.

Question 10

Describe classification of salmonella disease

Answer 10

• Non-typhoidal: Cause acute gastroenteritis
  
  • Most infections caused by S. enterica serovar Typhimurium and S. enterica serovar Enteritidis.
  • Large animal reservoir, transmitted to humans via food in developing and developed countries.
• Typhoidal: Cause enteric (typhoid/paratyphoid) fever
  
  • Most infections caused by S. enterica serovar Typhi and S. enterica serovar Paratyphi.
  • S. enterica serovar Typhi associated with asymptomatic carriage (gall bladder usually the reservoir).
  • Infect humans only, common in developing countries/areas of poor sanitation, faecal-oral route of transmission.

Question 11

Describe salmonellosis gastroenteritis

Answer 11

• Most common form of salmonellosis in developed countries, typically associated with low mortality.
• High infectious dose (~10^5 cells)
• Incubation: 2-7 days, Duration: 2-7 days
• Symptoms: vomiting, fever (more with Typhi), abdominal cramps, watery diarrhoea with PMNs
  
  • Inflammatory diarrhoea: due to invasion
  • Osmotic diarrhoea: due to loss of absorptive surface - similar mix to shigellosis diarrhoea
• Usually self-limiting, antibiotics not prescribed unless:
  
  • Extraintestinal disease develops (typically bloodstream infection)
  • Severe gastroenteritis * * * *
  • Infants and immunosuppressed individuals

Typical Sources of Non-Typhoidal Salmonella
- Commensals in birds (and other animals)
- Poultry meat and eggs are common sources of infection

Question 12

Describe salmonellosis enteric fever

Answer 12

• In Australia, most cases are acquired overseas i.e. returned travellers
• Low infectious dose (10–100 organisms)
• Incubation: 7–28 days (Typhi), 1–15 days (Paratyphi)
• Symptoms: Prolonged fever, vomiting, abdominal cramps, diarrhoea with PMNs, headache, myalgia, nausea, (+/-) rash of flat rose-coloured spots.
  
  • Inflammatory + Osmotic diarrhoea
• Humans are the only hosts for these species
• Transmission: Faecal-oral route and person-to-person spread by chronic carrier
• After infection, people can carry the organisms for months to years

Question 13

Describe treatment of typhoidal salmonellosis

Answer 13

• Antibiotics should be started as soon as enteric fever has been confirmed.
• Identify and treat carriers of S. enterica serovar Typhi and S. enterica serovar Paratyphi:
  
  • Antibiotics
  • Or cholecystectomy (if antibiotics fail); sometimes patients will elect to do this
• Ideally, Typhoid carriers should be excluded from jobs involving food handling and preparation.

Question 14

Describe salmonella virulence strategies

Answer 14

• Invasiveness:
  
  • Intracellular survival and multiplication in intestinal epithelium and macrophages.
• Type III secretion system and effector proteins (SPI-1) required for initial invasion into intestinal epithelium.
• Type III secretion system and effector proteins (SPI-2) required for replication in intestinal epithelium and macrophages.
• LT-like toxin and other enterotoxins.
• Capsule (S. enterica serovar Typhi only).

Question 15

Describe the lifecycle of typhoidal salmonella (ie. in systemic infectons)

Answer 15

• Organism is consumed.
• Enter lamina propria via M cells.
• Taken up by macrophages.
• Migrate in macrophages to mesenteric lymph nodes and multiply.
• Released into the bloodstream (1° bacteraemia, low level, typically asymptomatic).
• Removed from blood by macrophages in spleen, bone marrow, liver; multiply and re-enter bloodstream (2° bacteraemia, onset of symptoms i.e. headache, malaise, fever).
• Re-enter GIT via gall bladder, replicates asymptomically but enters GIT and sheds, or causes bouts of typhoid fever

Question 16

Describe campylobacter general featuers

Answer 16

• Motile, Gram-negative “comma-shaped” rods.
• Microaerophilic (5-7% O2).
• Most species grow well at 42°C (thermophilic).
• Taxonomically classified in the same order as Helicobacter ^[when first characterised H pylori was called ‘Campylobacter sthng’].
• Campylobacter jejuni and Campylobacter coli are the species most commonly implicated in human GIT infections.

Question 17

Describe campylobacteriosis

Answer 17

• Gastroenteritis, can lead to Guillain-Barré syndrome.
• Low-Medium infective dose (~500 organisms)
• Incubation period of 2 – 11 days.
• Duration: 3 days – 3 weeks, relapses take place in 15 – 25% cases.
• Symptoms include abdominal cramps and fever, vomiting rare, bloody diarrhoea + PMNs (inflammatory and osmotic diarrhoea).
• Rare complication- GBS (see also [[Immunology B4 - Lecture 3]])
• Organism is excreted for several weeks unless treated with antibiotics (erythromycin), but typically self-limiting

Question 18

List souces of campyliobacter

Answer 18

• Common GIT commensal of chickens and other animals (cattle, pigs, dogs, wild birds, etc.).
• Common cause of foodborne illness in developed countries (particularly contaminated chicken and unpasteurised milk).
• While invasive, mechanism not well understood.
• Infrequently causes invasive/extraintestinal disease esp. if host is immunocompromised
• Inflammatory process consistent with invasion of intestinal epithelium, M-cell uptake, and with the production of bloody diarrhoea + PMNs.

Question 19

Describe campylobacter virulence startegies

Answer 19

• Invasiveness:
  
  • Intracellular survival and multiplication.
• Surface structures:
  
  • Capsule important for virulence, epithelial cell adherence, and invasion.
  • Lipooligosaccharide (LOS) required for epithelial cell adherence and invasion.
  • LOS structures of C. jejuni can display molecular mimicry of neuronal gangliosides (linked to Guillain–Barré syndrome).
  • Flagellum required for colonization, virulence and epithelial cell invasion, and also acts as a secretion apparatus for invasion antigens.
• Cytolethal distending toxin:
  
  • Causes DNA damage promoting inflammation and cell death.

Question 20

Describe L mono general chars

Answer 20

• Motile, Gram-positive rod.
• Widespread among animals and the environment.
  
  • Found in plants, soil, and surface water samples in addition to being in sewage, food, and human/animal faeces.
• Outbreaks associated with high-risk foods:
  
  • Unpasteurised milk, soft-serve ice-creams, and soft cheeses (brie, camembert, ricotta), raw vegetables, cold deli meats, raw seafood.
• Infectious dose (<1000 organisms).
• **Grow optimally at 37°C, but Psychrotroph**: 2–4°C.
• Infections most common in immunocompromised individuals, pregnant women, neonates, and the elderly <65.

Question 21

DESCRIBE listeriosis

Answer 21

• Symptoms:
  
  • In healthy adults it is usually asymptomatic.
  • Flu-like illness and/or Gastroenteritis: nausea, vomiting, fever, diarrhoea + PMNs.
  • Inflammatory and osmotic diarrhoea.
  • In 5 - 10% of the population, L. monocytogenes is present in the intestinal tract.
• In at-risk populations: bloodstream infection, sepsis, chorioamnionitis, meningo-encephalitis
  
  • One of the few bacterium that can cross both the placental and blood-brain barrier.
• Treatment: IV benzylpenicillin ^[higher doses may be required i.e. for CNS infections]

Question 22

Describe L mono virulence strategies

Answer 22

• Adhesion:
  
  • Listeria adhesion protein (LAP)
  • Fibronectin binding protein A (FbpA)
• Invasion:
  
  • Invasion A (InlA)
  • Invasion B (InlB)
• Vacuolar Rupture:
  
  • Listeriolysin O (LLO)
  • Phospholipase A (PlcA)
  • Phospholipase B (PlcB)
• Intracellular Spread:
  
  • Actin-assembly-inducing protein (ActA)

Question 23

Describe infectious diarrheoa in Au and pathogens under surveillance

Answer 23

Infectious Diarrhoea In Australia
- inverse relationship between incidence and severity
- reported figures are always lower because most disease is mild/not investigated
- Camp most, followed by salmonella, EHEC, Listeria
—

Pathogens & Illness Under Surveillance
- State and Territory legislation requires laboratories and doctors to report (notify) infections of the following in a timely fashion:
- Campylobacter (Campylobacteriosis)
- Non-typhoidal and typhoidal Salmonella (Salmonellosis)
- Shigella (Shigellosis)
- EHEC (Haemolytic Uraemic Syndrome)
- Listeria (Listeriosis)
- (Hepatitis A)