Gastrointestinal microbiota

What is the composition of normal flora in the mouth?

A complex community of bacteria.

What types of bacteria are found in the stomach and most of the small intestine?

Few bacteria, primarily lactobacilli and streptococci.

What types of bacteria are predominant in the ileum and colon?

Coliforms, which are facultative anaerobes, and obligate anaerobes like Bacteroides and Clostridium.

What is enteritis?

Intestinal inflammation caused by bacteria invading and damaging the mucosa and deeper structures.

What is dysentery?

Severe diarrhea characterized by the presence of mucus, blood, or pus.

What is enteric fever?

A systemic infection that spreads from the intestinal mucosa.

What is the significance of Helicobacter pylori in relation to gastritis and ulcers?

It is associated with peptic ulcers and gastric cancer, despite the stomach being traditionally considered sterile.

Who isolated Helicobacter pylori and when?

Barry Marshall and Robin Warren in 1983.

What role does urease play in H. pylori's pathogenicity?

It produces ammonia, raises the pH of the stomach, and has non-enzymatic pro-inflammatory functions.

What are the histological signs of H. pylori infection?

Infiltration of mucosa with neutrophils and mononuclear cells, often asymptomatic.

What are the diagnostic methods for H. pylori infection?

Serological tests, stool tests, and endoscopy with biopsy.

What is the treatment regimen for H. pylori eradication?

Quadruple therapy including two antibiotics (clarithromycin and amoxicillin), a proton pump inhibitor, and bismuth subsalicylate.

What defines diarrheal disease?

The passage of more than three loose or liquid stools per day.

What are enterotoxigenic pathogens?

Pathogens that generate enterotoxins and cause diarrheal disease by colonizing the mucosa.

What is cholera toxin and which bacteria produce it?

A toxin produced by Vibrio cholerae and enterotoxigenic E. coli that causes severe diarrhea.

What is the mechanism of action for invasive pathogens like Salmonella typhi?

They invade epithelial cells, survive in macrophages, and can cause systemic infections.

What is the role of C. difficile in pseudomembranous enterocolitis?

It can cause severe colitis, especially in individuals with predisposing factors like antibiotic use.

What are the symptoms of colitis without pseudomembrane formation?

Malaise, abdominal pain, nausea, watery diarrhea, and low-grade fever.

What is the impact of diarrheal diseases globally?

Approximately 2 billion cases annually, primarily affecting children under 2, leading to malnutrition and death.

What are the characteristics of enteroadherent pathogens?

They attach to the intestinal mucosa without intracellular replication, causing diarrheal diseases.

What is the significance of Rho proteins in cell signaling?

They regulate actin cytoskeleton, enzyme activity, and immune cell functions.

How do C. difficile toxins affect Rho proteins?

They glycosylate Rho proteins, inhibiting their function and blocking signal transduction pathways.

What is the prevalence of H. pylori colonization in developing vs. industrialized countries?

Less than 80% in developing countries and more than 40% in industrial nations.

What are the lifetime risks associated with H. pylori infection?

10-20% risk of ulcer and 1-2% risk of gastric cancer.

What are the common predisposing factors for C. difficile infections?

Use of antibiotics, long hospital stays, old age, and underlying illnesses.

What is the role of faecal bacteriotherapy in treating C. difficile infections?

It can be up to 95% effective in treating fulminant colitis.

EPEC (Enteropathogenic E. coli)

Creates 'A/E lesions' (Attaching & Effacing), sticks to intestinal cells using bundle-forming pili (BFP), destroys microvilli, rearranges the cell's cytoskeleton to create pedestals, and has a Shiga-like toxin.

EIEC (Enteroinvasive E. coli)

Actually invades cells, ruptures the phagosome, moves between cells laterally, and spreads infection to neighboring cells.

Cholera Toxin (CT)

Has 2 parts: A subunit (active part that does damage) and B subunits (5 of them that bind to GM1 ganglioside on cell surface).

A subunit of Cholera Toxin

Enters the cell and ADP-ribosylates the Gs protein, locking adenylate cyclase 'ON', leading to massive cAMP production.

B subunits of Cholera Toxin

Attach to GM1 receptor on cell surface.

CFTR

Opens in crypt cells, allowing Cl⁻ to flood out, causing water to follow ions, resulting in watery diarrhea.

Barrier Disruption

During infection, tight junctions break down, aquaporins are disrupted, leading to increased water in the lumen.

Bristol Stool Chart Type 3

Sausage with cracks.

Bristol Stool Chart Type 4

Smooth, soft sausage.

Bristol Stool Chart Type 7

Entirely liquid, indicative of cholera/ETEC.

H. pylori (Helicobacter pylori)

Survives stomach acid by producing urease, which converts urea to ammonia, neutralizing acid and creating a protective cloud.

Urease

Converts urea to 2 NH₃ + CO₂, with ammonia neutralizing stomach acid.

Location strategy of H. pylori

Lives in mucus layer (pH ~7), protected from gastric juice (pH 2), and uses TlpB for chemotaxis.

Consequences of H. pylori infection

Chronic inflammation (gastritis), peptic ulcers, and potential gastric cancer.

Urea Breath Test (UBT)

Diagnosis method involving administration of ¹³C-urea and measuring ¹³CO₂ in breath.

Clarithromycin

Antibiotic that stops protein synthesis (translation).

Amoxicillin

Antibiotic that stops cell wall building (peptidoglycan).

Bismuth subsalicylate

Coats mucosa, has mild antimicrobial and anti-inflammatory properties.

EPEC summary

Attaches and destroys gut surface.

EIEC summary

Invades and spreads between cells.

ETEC/Cholera summary

Toxin causes massive water secretion.

H. pylori summary

Neutralizes stomach acid and causes ulcers.

C. difficile

A bacterium that produces toxins leading to severe gastrointestinal disease.

Toxins A and B

Toxins produced by C. difficile that cause damage to the intestinal lining.

Hydrolytic enzymes

Enzymes produced by C. difficile that help in the breakdown of substances.

TNF-α

Tumor Necrosis Factor-alpha, a cytokine involved in systemic inflammation.

IL-8

Interleukin 8, a chemokine that attracts neutrophils to sites of infection.

Neutrophil recruitment

The process by which neutrophils are attracted to sites of infection or inflammation.

Epithelial tight junctions

Structures that help maintain the barrier function of epithelial cells.

Epithelial cell apoptosis

Programmed cell death of epithelial cells, often triggered by toxins.

Pseudomembrane formation

A layer of fibrin, mucin, and inflammatory cells that forms in severe colitis.

Watery diarrhea

A common symptom of C. difficile infection characterized by loose, watery stools.

Rho proteins

Small GTPases that act as molecular switches to control various cellular functions.

Endocytosis

The process by which cells internalize substances from their surface.

InsP6

Inositol hexaphosphate, a molecule that helps activate toxins in the acidic endosome.

Glucosylation

The process of adding glucose to a molecule, which inactivates Rho proteins in this context.

Cytoskeletal disruption

The breakdown of the cytoskeleton, leading to loss of cell shape and function.

Apoptosis

A form of programmed cell death that can be induced by C. difficile toxins.

Inflammation

A biological response to harmful stimuli, characterized by redness, swelling, and pain.

Cytotoxin assay

A test that detects Toxin B produced by C. difficile, considered the gold standard.

Enzyme immunoassay

A rapid test that detects Toxin A or B, known for its high specificity.

Latex agglutination

A fast, inexpensive test that detects glutamate dehydrogenase enzyme but has poor sensitivity.

Culture

A method to grow C. difficile in the lab, allowing for strain typing but is slow.

Commensal E. coli

Most E. coli strains that are harmless and reside in the intestines.

PAI

Pathogenicity Islands - foreign DNA with high %G+C

Transposons (Tn)

Jumping genes

Plasmids

Can transfer between bacteria

Phages

Viruses that insert DNA

EPEC

Enteropathogenic (diarrhea in infants)

EHEC

Enterohemorrhagic (bloody diarrhea, HUS)

EIEC

Enteroinvasive (dysentery-like)

ETEC

Enterotoxigenic (traveler's diarrhea)

EAEC

Enteroaggregative (persistent diarrhea)

DAEC

Diffusely adherent (diarrhea in children)

UPEC

Uropathogenic (UTIs)

Type III Secretion System

Directly injects effector proteins into host cells

Effects on host cells

Disrupts tight junctions (occludin, claudin) and aquaporins (AQP2, AQP3); causes water secretion

Bacteria that use T3SS

A/E bacteria (EPEC, EHEC), Salmonella, V. cholerae (via ZOT - zonula occludens toxin)

Key effector proteins

EspF, EspG, Map - multiple functions targeting occludin, claudin, actin filaments

EPEC - Localized adherence

Forms tight clusters and microcolonies in specific spots

EAEC - Aggregative adherence

Stacked brick pattern; bacteria stick to cells AND each other, forming thick biofilm

DAEC - Diffuse adherence

Evenly distributed across entire cell surface with no clumping

DAEC - Diffusely Adherent E. coli

Affects children 1-5 years old; causes diarrhea; produces SAT cytotoxin

EAEC - Enteroaggregative E. coli

Food-associated outbreaks; produces cytotoxin; forms mucus biofilm

Comparison Table: E. coli Pathovars

EPEC: Localized adherence, Shiga-like toxin; EHEC: A/E lesions, Shiga toxin; EIEC: Invasive, none; ETEC: Pili, LT/ST; EAEC: Aggregative, cytotoxin; DAEC: Diffuse, SAT

E. coli diversity

Mobile genetic elements create different pathogenic strains; Type III secretion system = molecular syringe