Lecture 3: Gram Negative Bacteria

Question 1

What is the largest group of human bacterial pathogens?

Answer 1

Gram-Negative bacteria.

Question 2

What are gram-negative bacteria?

Answer 2

Bacteria that have a lipopolysaccharide layer on their cell wall, and a thinner peptidoglycan layer than gram-positive.

They can be cocci, bacilli, and coccobacilli.

They are mostly motile. (All G-N bacilli are).

Question 3

What are some examples of gram-negative bacteria?

Answer 3

sp = species

Gram negative cocci:
- Neisseria meningitidis
- Neisseria gonorrhoeae

Question 4

What type of gram-negative bacteria account for almost half of all of them?

Answer 4

The facultative anaerobic bacteria:

• Enterobacteriaceae
• Pasteurellaceae

Question 5

Why are gram negative bacteria more pathogenic?

Answer 5

Because they are harder to treat with antibiotics. With gram +ve bacteria, antibiotics easily absorb into the peptidoglycan cell wall, but gram -ve have a lipopolysaccharide layer to their cell wall that is impermeable.

As

Question 6

What are enterobacteriaceae

Answer 6

A type of gram-negative bacteria that is usually coccobacilli or bacilli (1um x 1.2-3um).

If motile, these bacteria have peritrichous flagella.

They can have prominent capsules or loose slime.

All of them reduce nitrate to nitrite.

They can ferment glucose anaerobically, but grow better aerobically (2 mol of ATP anaerobic to 38 mol ATP aerobic).

They are all oxidase negative.

Question 7

What infections are individuals with cystic fibrosis susceptible to?

Answer 7

Pseudomonas infections.

Question 8

What causes the pathogenicity of enterobacteriaceae?

Question 9

Describe the general structure of a lipopolysaccharide.

Answer 9

It consists of a strain-specific O-polysaccharide that stretches outwards from the cell. In the middle is a core polysaccharide. And closest to the cell is a lipid A component.

Question 10

Describe the structure of Lipid A.

Answer 10

The structure consists of two

Question 11

Describe the structure of the core polysaccharide in LPS.

Answer 11

It is a chain of polysaccharides with slight branching, the structure of this polysaccharide, beginning from the O-polysaccharide, the structure goes:

Glc – NAG
I
Gal
I
Glc – Gal
I
Hep
I
Hep
I
KDO
I
KDO – KDO –(P)– Ethanolamine
I
(LIPID A)

Question 12

Describe the structure of the O-polysaccharide component in LPS.

Question 13

What are coliforms?

Answer 13

A group of enterobacteriaceae that colonise the gut, they can be opporunistic.

Question 14

Name some opportunistic coliforms.

Answer 14

E. coli
Klebsiella sp.
Serratia sp.
Enterobacter sp.
Hafnia sp.
Citrobacter sp.

Question 15

Name some opportunistic non-coliforms.

Answer 15

Proteus sp.
Morganella sp.
Providencia sp.
Edwardsiella sp.

Question 16

Name some truly pathogenic enterobacteriaceae.

Answer 16

Salmonella sp.
Shigella sp.
Yersinia sp.

Question 17

State the taxonomy of Escherichia coli.

Question 18

What is Escherichia coli?

Answer 18

A type of gram-negative enterobacteriaceae.

The most common and important coliform.

It has numerous O, H and K antigens used to identify particular strains, some of these strains are associated with virulence. e.g. O157.

They can transfer virulence plasmids amongst themselves.

Question 19

What diseases can be caused by Escherichia coli?

Answer 19

• UTIs
• Neonatal meningitis
• Gastroenteritis - an exotoxin called enterotoxin is produced and causes this.
• Severe to fatal hemorrhagic colitis

Question 20

What Escherichia coli strains are most associated with hemorrhagic colitis? What do they produce that causes this?

Answer 20

O157 and H7. They produce relatively large amounts of bacteriophage-mediated Shiga-like toxin, called Vero toxin (VT). Some strains can also produce Shiga-like toxin 2, similar but antigenically unique.