Comprehensive Study Notes on Salmonella Biology and Pathogenesis

Salmonella and E. coli belong to the same family of bacteria.
Salmonella is a Gram-negative bacterium, appearing pink under Gram stain.
It is classified as a facultative anaerobic bacterium, meaning it can survive in the presence or absence of oxygen.
The bacteria was discovered in 1885 and named after the veterinarian Daniel Salmon.

The genus is primarily divided into two main species: - Salmonella bongori: Primarily infects reptiles. - Salmonella enterica: Primarily infects all species of domestic animals.
There are more than $2,500$ subspecies (serotypes) categorized under these groups, though only certain types are commonly encountered in clinical veterinary practice.

Salmonella is a major cause of foodborne diseases globally, impacting public health and human safety.
It is a zoonotic disease, meaning it is transmitted from animals to humans.
Humans can contract the bacteria through contaminated milk, meat, or eggs.
In humans, specific strains like Salmonella typhi cause a systemic disease known as typhoid fever, characterized by high temperature and persistent fever.
Comparison of Persistent Fever Suspicions (Human Medicine): - If fever is continuous and accompanied by diarrhea: Suspect typhoid fever (Salmonella typhi infection). - If fever fluctuates, accompanied by sweating, back pain, and neck pain: Suspect Brucellosis.

It is a major causative agent of diarrhea, especially in young animals.
It can cause inflammation of the intestine (gastroenteritis) or progress to systemic disease (septicemia), particularly in birds.
Transmission Routes: - Fecal-oral route: Ingestion of contaminated food or water. - Food products: Meat, milk, or eggs. - Vertical transmission: Transmission from the mother to the offspring (e.g., from a hen to the egg; the egg becomes infected and the hatched chick possesses the bacteria).

Virulence increases as host adaptation moves from generalized to host-restricted ranges: - Generalized Infection: Infects any species. Disease is typically moderate and virulence is low. (e.g., Salmonella by types). - Host Adapted: Prefers specific species but occasionally infects others. (e.g., Salmonella choleraesuis in pigs). - Host Restricted: Reaches highest virulence and mortality. Only affects specific species. (e.g., Salmonella typhi in human; Salmonella gallinarum in chickens).

The preferred location for Salmonella colonization in the distal gastrointestinal area is the ileum.
Pathogenic steps: - Resistance to environmental changes: The bacteria survives the acidic pH of the stomach by removing proton or $H$ ions. - Adhesion: It uses fimbriae, flagella, and pili to adhere to the lymphocyte and move through the intestinal flow. - Invasion: Unlike E. coli, Salmonella enters the enterocyte cell (cell invasion). - Inflammatory response: Infection leads to an influx of neutrophils and macrophages. This "fight" between the immune system and the bacteria damages the enterocytes. - Diarrhea: Fluid leaks from damaged cells into the intestinal lumen, causing diarrhea.

A "carrier" is an animal that harbors the bacteria but shows no clinical signs of disease.
Carriers pose a significant biosecurity risk because they shed the bacteria into the environment, infecting healthy animals.
In farms, such as those in the United Arab Emirates, infectious diseases often spread when owners introduce new, clinically healthy-looking birds or animals to an existing flock without realizing they are carriers.
Statistical Example: In an experiment with Salmonella pullorum, $50\%$ of birds infected experimentally developed a carrier state while appearing clinically normal.

Salmonella utilizes Type III Secretion Systems (T3SS) governed by genes on the DNA known as Pathogenicity Islands.
Salmonella Pathogenicity Island 1 (SPI-1): - Functions as the primary needle system to fix the bacteria to the enterocyte. - Responsible for inducing the cell to swallow or engulf the bacteria (invasion).
Salmonella Pathogenicity Island 2 (SPI-2): - Responsible for survival inside the immune system Cells. - It deactivates the destroying/lysis mechanism of the macrophage. - It prevents the phagosome from fusing with antimicrobial vesicles (vacuoling system).
Contrast with E. coli: E. coli only possesses the attachment/toxin secretion system; it lacks the invasion and systemic survival mechanisms (SPI-2 equivalent) found in Salmonella.

The immune system recognizes specific structures on the pathogen called PAMPs (Pathogen Associated Molecular Patterns).
Examples of PAMPs in Salmonella: - Flagella. - Lipopolysaccharides (LPS). - Fimbriae.
Immune cells (white blood cells) have PRRs (Pattern Recognition Receptors), also called TPPR (Pattern Recognition Receptor), which identify these patterns.
Specific receptors: - TLR4: Identifies Gram-negative bacteria (LPS). - TLR5: Identifies bacterial flagella.
Hiding Mechanisms: - Salmonella typhi and Salmonella gallinarum can hide from the immune system. - They may hide the PAMPs/LPS under a capsule. - They may switch off the gene responsible for forming structural proteins (like fimbriae) so the immune system cannot identify them as they travel through the blood.

After the bacteria travels from the intestine via the macrophage to the lymphatic system, liver, and spleen, there are three possible fates for the animal: 1. Death: Caused by systemic septicemia or toxemia. 2. Recovery: The immune system produces antibodies and destroys the microbe. 3. Carrier State: The animal is clinically normal, but the bacteria persists (e.g., in the gallbladder of humans or the milk of cattle).
Mortality Statistics: - Human Typhoid: Responsible for $10\%$ mortality if left untreated. - Poultry Typhoid: Can cause up to $50\%$ mortality in bird flocks. - Systemic Strains: Salmonella dublin in cattle; Salmonella choleraesuis in pigs.
Observation: In Salmonella typhi, there is often less intestinal damage than other strains because the bacteria hides from the initial immune response to facilitate systemic spread.