Salmonella and Sigella

Professor James M. Small MD PhD from the Department of Pathology and Microbiology presides over the discussion of Salmonella and Shigella.
Focus on gram-negative bacteria classified under the genera Salmonella and Shigella.

Identify the two species of Salmonella and methods for subtyping.
Describe clinical diseases and virulence factors associated with both Salmonella and Shigella.
Discuss laboratory tests to differentiate these bacteria from other coliforms.

Salmonella: A genus of bacteria encompassing multiple species.
Shigella: Another genus of bacteria known for causing gastrointestinal infections.
Typhoid Fever: A severe illness caused primarily by Salmonella typhi.
Gram-negative bacteria: Bacteria that do not retain the crystal violet stain used in the Gram staining method.
Peptidoglycan: A polymer that makes up the cell wall of many bacteria, especially gram-positive.
Flagella: Hair-like structures that aid in the motility of bacteria.
H antigen: Antigen found on flagella.
Pili: Hair-like appendages that allow bacteria to adhere to surfaces.
Fimbria: Similar to pili, these structures help with attachment.
LPS (Lipopolysaccharide): A major component of the outer membrane of gram-negative bacteria, serving as an endotoxin.
Lipid A: The toxic component of LPS.
Endotoxin: A toxin associated with the outer membrane of gram-negative bacteria.
Non-lactose fermenter: Refers to bacteria that do not ferment lactose, which is a sugar.
Hydrogen Sulfide (H2S): A gas produced by some bacteria during metabolism.
Capsule antigen Vi: A capsule antigen found in some Salmonella species that aids in evading the immune system.
Motile and Non-motile: Describes whether a bacterium is capable of movement.
Shiga toxin: A potent toxin produced by Shigella, which inhibits protein synthesis in host cells.

Global health implications; vaccination against Typhoid Fever is emphasized before travel.
Practical implications in food safety, demonstrated by the importance of hygiene after handling raw poultry.

Do not retain crystal violet stain; appear red or pink after counterstaining.
Thinner peptidoglycan layer sandwiched between inner and outer membranes.
Alcohol treatment degrades the outer membrane, making it fragile.
Outer membrane contains lipopolysaccharides (LPS), which can trigger significant immune responses.

Morphology: Gram-negative rods, non-lactose fermenting, do not produce H2S, and non-motile.
Environment: Exclusively found in humans and some primates.
Virulence Factors: - Produces Shiga toxin that affects the 60S ribosomal subunit, halting protein synthesis. - Invades intestinal epithelial cells, leading to dysentery.
Disease Transmission: Spread via fecal contamination; common in crowded, unsanitary conditions.
Symptoms: Sudden cramps, watery and bloody diarrhea, and fever usually 2-4 days post-infection.
Diagnosis: Bacterial culture from stool samples.
Treatment: Rehydration therapy; antibiotics may be used, albeit with growing resistance.

Morphology: Gram-negative bacilli, non-lactose fermenting, produce H2S, and are flagellated (motile).
Species: Significant species include S. bongori and S. enterica, the latter encompassing over 2500 serovars.
Antigens: - H: Flagellar antigen - O: Somatic antigen found in LPS - Vi: Capsular antigen (also referenced as K antigen).
Disease Mechanism: Infection is initiated through ingestion of contaminated food/water; the bacteria invade the intestinal mucosa and produce enterotoxins, activating adenylate cyclase which leads to diarrhea.
Common Serovars: - S. enterica enterica, S. enterica Typhi, S. enterica Choleraesuis.
Disease Presentations: - Gastroenteritis: Most common form, often self-limited. - Typhoid Fever: Systemic disease with severe infections; transmitted via asymptomatic human carriers. - Septicemia: Can be caused by various species, commonly associated with S. Choleraesuis.
Diagnosis: Culture, PCR techniques.
Treatment: Antibiotics (e.g., third-generation cephalosporins) not usually necessary for uncomplicated enteritis; increased resistance noted in strains like S. enterica Typhi.

Structure: Comprises two membranes with a thin peptidoglycan layer in between.
Endotoxins: LPS activates macrophages and initiates a strong immune response.
Exotoxins: Protein secreted by bacteria, includes neurotoxic examples (like tetanus and botulism).
Classification: Divided based on morphology (e.g., cocci, bacilli).

Available vaccines for Typhoid Fever provide immunity, with different types like polysaccharide injections and live attenuated vaccines.

Fecal-oral transmission; low infectious dose; primarily impacts children in day care settings.
Shiga toxin-producing strains can lead to severe dysentery, particularly Shigella dysenteriae.

Distinction between types of diarrhea: inflammatory vs non-inflammatory.
Bacterial infections can lead to differing symptoms, with bloody diarrhea suggesting a more invasive organism.

Questions posed throughout the lecture to encourage retrieval of material covered, focusing on specific typical traits of Salmonella and Shigella, including their diagnostic characteristics, virulence factors, and clinical presentations.

Series of clinical vignettes presented to apply knowledge about differentiating pathogens based on characteristics and laboratory findings.
Examples of correct answers with explanations given for clarity about why other choices are incorrect.