MIC 4124 8a - Salmonella

Salmonella enterica is considered a __ pathogen

foodborne

Salmonella enterica

Gram-negative opportunistic pathogen; Gamma-proteobacteria

Salmonella enterica structure

- Peritrichous Flagella

- motile

- Rod shaped

Where can Salmonella inhabit?

both inside a host (intestinal tract) and in the environment (water, soil, plants, human/animal excretion)

Salmonella enterica host range

Broad host range (usually; Can infect a wide range of hosts):

- humans

- food animals

- pets & wild animals (most often chickens, ducks, reptiles)

A salmonella enterica infection is the leading cause of...?

foodborne gastroenteritis; frequently food-poisoning related

- also contracted from pets

Causative agent of Typhoid fever

Salmonella Typhi

Typhoid fever

- An enteric fever caused by a specific type of salmonella

- Plague of Athens (400 B.C.): almost 100,000 deaths

- Globally: ~270,000 deaths/year

- Illness develops between 6 hours and 1 week after infection

Serovar

A variation within a species; also called serotype

Types of salmonella enterica serovars

- Typhoid serovars

- Non-typhoid serovars

Typhoid serovars

Human-restricted host (only infect humans)

- Typhi

- Parathyphi

- Sendai

Symptoms of typhoid serovar infection

Enteric fever; abdominal pain; transient diarrhea or constipation; and a salmon-coloured maculopapular rash on the trunk

Non-typhoid serovars

- Thyphimurium

- Enteriditis

includes all other non-typhoidal disease-causing serovars

Symptoms of non-typhoid serovar infection

Gastroenteritis; abdominal pain; vomiting; and inflammatory diarrhea

- Some non-typhoidal Salmonella (NTS) strains can also cause bloodstream infections in some parts of the world; 20-25% lethal

Long term challenges of a salmonella infection

- Chronic carriage

- Reactive arthritis

Chronic carriage

Occurs when S. enterica persist in the body long after initial infection, through infections with or without obvious symptoms

- persists even with antibiotic treatment

- can occur with typhoidal and non-typhoidal serovars

Which organ is a common reservoir for Salmonella allowing for chronic infection?

Gall bladder

Challenges associated with chronic carriage

- Bacteria can be shed from host and spread to others

- Antibiotic treatment cannot clear all bacteria from the body (ex, gallbladder) can cause recurrent infections

Reactive arthritis

Inflammatory response that occurs in other part(s) of the body; usually occurs after infection is cleared

Treatment of salmonella

Antibiotics: quinolone, macrolide, or cephalosporin classes

- antibiotic resistance growing problem

- MDR & XDR strains becoming problematic

Drug resistant Salmonella in agriculture

In S. enterica derived from chicken, resistance to at least one drug class is common and multi-drug resistance also observed

- Antibiotic resistant Salmonella are widespread in the environment

Salmonella phylogeny

Two species in the Salmonella genus

- enterica (>2600 serovars, 7 subspecies)

- bongori

Which subspecies of Salmonella enterica is responsible for the majority of salmonella related human/warm-blooded animal disease?

Salmonella enterica subsp. enterica

- Most often the other subspecies infect cold-blooded animals

Model pathogen in Salmonella infection

S. typhimurium

S. typhimurium vs. S. typhi

S. typhimurium has a broad host specificity while S. typhi only infects humans

Pseudogene

genetic region that resembles a protein-encoding region, but is likely non-functional (ex, frameshift, premature stop codon, etc)

S. enterica serovar Typhimurium LT2 genome

4.86 Mb circular, 4489 ORFs (38 pseudogenes)

S. enterica serovar Typhi CT18 genome

4.81 Mb circular, 4599 genes (204 pseudogenes) (many pseudogenes)

- Includes effector genes (not normal). Were unneeded functions lost during the evolution to a human only host?

- May explain why it infects humans only

Along with pseudogenes, Salmonella's genome contains...?

Salmonella pathogenicity islands (SPIs), which include genes important for survival in host

- suggests frequent gene acquisition through horizontal gene transfer

What is encoded on SPI-1 and SPI-2?

Two type III secretion systems (T3SS)

- these T3SSs secrete effector proteins to mediate infection

SPI-1 T3SS is important for...?

early infection

- turned on before infection; predominantly involved in inducing pathogen uptake into SCV

SPI-2 T3SS is important for...?

later stages of infection

- turned on after uptake into SCV; predominantly involved in promoting bacterial survival, replication and spread

Are SPI-1 and SPI-2 functional at the same time during infection?

No, SPI-1 T3SS is turned off once the bacteria enters the cell and SP2-T3SS is then turned on

How does Salmonella survive within a host cell?

Salmonella is an intracellular vacuolar pathogen

- Resides in compartments termed Salmonella-containing vacuoles (SCV)

How many effector proteins does Salmonella make?

>30 effectors produced to mediate infection

Why is Salmonella an important pathogen in research?

It is a commonly used model organism to study bacterial pathogenesis and host response to infection

S. enterica route of entry

Salmonella spp. is a food-borne pathogen so it enters in the mouth

Survives low pH of stomach

In the intestinal epithelium: enters M cells & other epithelial cells

Gastroenteritis serotypes induce...?

local inflammatory response

Systemic illness serotypes enter...?

macrophages and spread

How many membranes must salmonella T3SS pass through to reach the host?

3 membranes (2 bacterial and 1 host) to secrete effector proteins into host

- (2 bacterial because salmonella is gram-negative)

Salmonella effector protein secretion timing

- Effector protein is intricately timed; proteins are only made when needed

- Effectors are often multifunctional; have evolved multiple domains to target different host processes

- Some effectors play important roles in some cell types, but not all

- Some effectors can function in multiple cell types, but can have different consequences (ex, due to specialized pathways)

When a Salmonella pathogen infects a non-phagocytic host cell, what happens?

the bacteria must induce itself into cell

Steps of Salmonella infection in epithelial cells (Non-phagocytic) (7)

1. SPI-1 T3SS effectors induce invasion ruffle → bacterial uptake into SCV

2. Endocytic pathway begins to target SCVs

3. SPI-2 T3SS effectors secreted to promote bacterial survival & replication

4. Salmonella inhibits lysosomal pathway

5. SCVs migrate from cell periphery to the perinuclear region and multiply near Golgi

6. Effectors induce the formation of tubules (Salmonella-induced tubules (SITs)) that extend to the cell periphery

7. SCVs migrate along microtubules to cell periphery (for cell-to-cell spread)

Invasion ruffle function

facilitates bacterial uptake into host cell

Where do SCVs multiply in the host cell

At the Golgi body

How do we visualize a salmonella infection?

Fluorescence microscopy: stain for both bacteria & host (SCV); often use antibodies to detect

LAMP proteins

Markers for SCV membrane

- Must stain for host (SCV) as well because we have to show that salmonella is in an intracellular vacuole as it should be