Gram Negative Bacilli

gram negative bacilli - the enterics

• rod shaped

• live in the intestinal tract of humans/animals

• thin outer layer

• stain pink with the gram stain method

found as part of

• normal intestinal flora BUT can also cause disease

4 major families of the gram negative bacilli

• Enterobacteriaceae (Salmonellae, Shigellae, E. coli )

• Vibrionaceae (Vibrio, Campylobacter)

• Pseudomonadaceae (Pseudomonas)

• Bacterioidaceae

how are the 4 major families of gram negative bacilli divided:

• into groups based on biochemical and antigenic properties

emb media:

• lactose fermenters —> dark purple/black

• inhibits gram positive bacteria

MacConkey Media

• lactose fermenters are pink/purple

• inhibits gram positive bacteria

what type of media are EMB and MacConkey, Selective or Differential?

• Selective: EMB agar and MacConkey agar inhibit gram positive bacteria, so they select for gram negative to grow

• Deifferential: distinguish bacteria based on metabolic differences (lactose fermentation)

biochemical classifications

• H2S production

• hydrolysis of urea

• liquefy gelatin

• decarboxylation of amino acids

What are surface antigens?

Molecules on the surface of bacteria that the immune system can recognize and that vary between bacterial strains.

What is the O-antigen?

• The outermost part of lipopolysaccharide (LPS) in Gram-negative bacteria.

• It is highly variable between enteric bacteria and helps distinguish different strains.

What is the K-antigen?

• A capsular antigen that forms a capsule around the bacteria.

• It covers the O-antigen and helps bacteria evade the immune system.

What is the H-antigen?

• The flagellar antigen, made from flagellar protein subunits.

• Motile bacteria only, because they have flagella.

Which antigen is associated with flagella?

H-antigen.

Which antigen is associated with the capsule?

K-antigen.

Which antigen is found in LPS?

O-antigen.

What type of enteric disease involves bacteria that attach to intestinal cells but do not invade them?

A diarrheal disease where bacteria bind to intestinal cells without entering them and cause symptoms through toxin (enterotoxin) release.

What causes diarrhea in enteric infections where bacteria attach to intestinal cells but do not invade them?

Exotoxin release, specifically enterotoxins, which cause fluid and electrolyte loss leading to watery diarrhea.

What is an enterotoxin?

A toxin that acts on the intestines, causing fluid and electrolyte secretion leading to diarrhea.

What type of diarrhea is typically produced by non-invasive enteric bacteria?

Watery diarrhea with no fever.

Why is there usually no fever in these infections?

Because the bacteria do not invade tissues or cause systemic infection.

Example of a bacterium that causes watery diarrhea without invading intestinal cells?

Vibrio cholerae

Which bacterium is a common cause of diarrhea involving intestinal cell invasion?

Shigella.

What symptoms are commonly associated with bacterial invasion of intestinal cells?

Bloody diarrhea and fever.

Why is fever often present in invasive diarrheal infections?

Cell invasion and tissue damage trigger inflammation, leading to a fever response.

What happens in diarrhea where bacteria invade intestinal cells?

Bacteria attach to intestinal cells and then invade them using virulence factors.

toxin release destroys cells —>

bloody stool

What happens when bacteria invade lymph nodes and the bloodstream during diarrheal infection?

The infection spreads beyond the intestine, causing systemic symptoms and inflammatory diarrhea.

What symptoms occur when intestinal bacteria invade tissues and spread systemically?

Abdominal pain, diarrhea with red and white blood cells, fever, headache, and increased white blood cell count.

Why are red blood cells found in the stool during invasive diarrheal infections?

Damage to the intestinal lining causes bleeding into the stool.

Why are white blood cells found in the stool during invasive diarrheal infections

The immune system sends white blood cells to fight bacteria invading intestinal tissues.

Why does fever occur in invasive diarrheal infections?

The infection spreads beyond the intestine, triggering a systemic immune response.

Which bacteria can cause diarrhea by invading lymph nodes and the bloodstream?

Salmonella Typhi, Yersinia enterocolitica, and Campylobacter jejuni.

What are the three classifications of diarrheal disease caused by enteric bacteria?

1. Diarrhea with / without systemic invasion

2. Diarrhea with intestinal cell invasion

3. Diarrhea with invasion of lymph nodes and bloodstream

What happens in diarrhea with / without systemic invasion?

Bacteria attach to intestinal cells but do not enter them; they release enterotoxins that cause fluid and electrolyte loss.

What type of diarrhea occurs when bacteria attach to intestinal cells but do not invade them?

Watery diarrhea, usually no fever.

What causes diarrhea in infections where bacteria do not invade intestinal cells?

Enterotoxin release that stimulates fluid secretion in the intestine

What happens in diarrhea with intestinal cell invasion?

Bacteria bind to and invade intestinal epithelial cells, causing cell damage and inflammation.

What symptoms are commonly seen in diarrhea caused by intestinal cell invasion?

Bloody diarrhea and fever due to inflammation and immune response

What occurs in diarrhea with invasion of lymph nodes and bloodstream?

Bacteria invade intestinal cells and spread to lymph nodes and the bloodstream, causing systemic infection.

What symptoms are associated with bacteria that invade lymph nodes and the bloodstream?

Abdominal pain, diarrhea with white and red blood cells, fever, headache, increased white blood cell count.

What is the key difference between intestinal cell invasion and invasion of lymph nodes/bloodstream?

• Intestinal invasion: infection remains in the intestine

• Lymph node/bloodstream invasion: bacteria spread systemically beyond the intestine.

What infections can enteric bacteria cause besides diarrhea?

Urinary tract infections (UTIs), pneumonia, bacteremia, and sepsis.

What are nosocomial infections?

Infections acquired in a hospital or healthcare setting.

Which bacteria commonly cause nosocomial infections?

E. coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter, Serratia, and Pseudomonas aeruginosa.

Pseudomonas aeruginosa:

opportunistic pathogen, often infects burn patients and can disseminate through body into CNS