Pathogens (Fritz Lecture 1 - C. albicans)

Question 1

What is Candida albicans?

Answer 1

Dimorphic yeast that grows both in a filament form and as yeast

Question 2

Does C. albicans have either a) just a cell wall b) a cell wall and a cell membrane c) just a cell membrane

Answer 2

B) A cell wall and a cell membrane

Question 3

What are the 2 most important roles of the C. albicans cell wall?

Answer 3

1) Maintaining cellular shape

2) Providing resistance to environmental stress

Question 4

What is the structure of the C. albicans cell wall?

Answer 4

Majority are complex carbohydrates:
- Innermost layer is chitin
(polymer of N-acetylglucosamine subunits)
- Then layer of beta glucans e.g B-1-3 glucans
- Then outer layer of Mannan

Question 5

What is a common method to observe C. albicans?

Answer 5

Calcofluor white

• Binds to chitin
• Fluoresces when exposed to UV
• Very sensitive method

Question 6

Is C.albicans commensal or non-commensal?

Answer 6

Commensal

Question 7

Where does C.albicans usually reside?

Answer 7

In mucous membranes e.g mouth, vagina, gut and sometimes skin

Question 8

C. albicans infections can be subdivided into what 2 types?

Answer 8

Superficial and ‘deep-seated’

Question 9

Give an example of a superficial C. albicans infection

Answer 9

• Mild skin infections
• Infection of the tongue and oral cavity (thrush)
• Infection of the vagina (yeast infection)

Question 10

How can C. albicans cause more serious infection?

Answer 10

Dissemination within the blood to other internal organs

Question 11

Give an example of a ‘deep-seated’ C. albicans infection

Answer 11

Liver or kidney infections

Question 12

Who is most at risk of Candida infections?

Answer 12

Neonates (<1 year old) and immunocompromised individuals (elderly etc)

Question 13

What % of reported candida cases are in neonates and the elderly combined?

Answer 13

45%

Question 14

What are the 4 common classes of anti fungal drugs and give an example of each

Answer 14

1) Polyenes (e.g amphotericin B)
2) Azoles (e.g. fluconazole)
3) Pyrimidines (e.g flucytosine)
4) Echinocandins (e.g. caspofungin)

Question 15

What is empiric therapy?

Answer 15

Treatment applied before the confirmation of a definite diagnosis

e.g. giving antibiotics before diagnosing the specific bacterium causing the infection

Often used in fungal treatment before confirmation of infectious organism

Question 16

What are some problems with current fungal treatment?

Answer 16

Main problem is toxicity issues!
- amphotericin B nephrotoxicity

> Drug resistant non C.albicans species

Question 17

What is ergosterol and how is it synthesised in candida?

Answer 17

Ergosterol is a sterol found in cell membranes of fungi

Synthesised via the mevalonate pathway in candida

Question 18

Name the drug type used to inhibit ergosterol synthesis (one of the 4 main types)

Answer 18

Azoles
Mainly used: Fluconazole
Newer compounds:
- Itraconazole
- Voriconazole
- Posaconazole

Question 19

What is the main advantage of using azole drugs to treat candida infection?

Answer 19

Some give very few side effects

Question 20

How do the polyene drugs work?

Answer 20

Binds to ergosterol

Causes intercalation of the cell memebrane

Question 21

What are the advantages and disadvantages of using amphotericin B (a polyene drug)

Answer 21

Advantages:

• Very broad range
• Development of resistance is rare

Disadvantages:

• Many side effects
• Nephrotoxicity

Question 22

How do the echinocandins work?

Answer 22

Bind to and inhibit beta1-3 glucan synthase

Depletion of beta1-3 glycans in cell wall

Question 23

How does flucytosine work to combat Candida infections? (Pyrimidine drug)

Answer 23

Converted into 2 products:

1) Deaminated to 5-fluorouracil which interferes with RNA biosynthesis
2) Other product inhibits fungal DNA synthesis

Question 24

Why can flucytosine only be used in combination with another drug?

Answer 24

Because it gives rise to rapid development of resistance

Question 25

What is the current practice for diagnosing candida infection?

Answer 25

• bioMerieux API Microbial Identification (colourific analysis based on substrate metabolism)
• Minimum 24-48+ hour analysis

Question 26

Name some C. albicans virulence factors

Answer 26

C. albicans is an opportunistic pathogen
Virulence factors:
- Change its cell shape (polymorphism)
- Secretion of hydrolytic enzymes in particular aspartyl proteases
- Ability to adhere to epithelial cells

Question 27

How is C. ablicans polymorphism regulated?

Answer 27

By pH:
pH 4.0 - yeast (circular)
pH 7.4 - filamentous

By CO2:
Approx 5.5% CO2 will induce filamentation

Question 28

How is CO2 sensed by C. albicans

Answer 28

• Increased cellular CO2 reacts with water to produce bicarbonate (HCO3-)
• Bicarbonate activates soluble adenylyl cyclases (sACs)
• sACs generate cAMP
• cAMP signalling causes changed in gene expression resulting in filamentation

Question 29

What is Laryngeal cancer?

Answer 29

Cancer of the head and neck squamous cells

Question 30

How many people in the UK are diagnosed with laryngeal cancer each year?

Answer 30

2,300

Question 31

How is Laryngeal cancer treated?

Answer 31

Most cases:

• present early
• Therefore treated with radiotherapy or laser excision

Advanced or recurrent cases require laryngectomy (removal of the larynx)

Question 32

How do patients have their speech restored after a laryngectomy?

Answer 32

Surgical voice restoration (SVR) is the gold standard

- a speaking valve is inserted

Question 33

What is the main cause of early voice prothesis failure?

Answer 33

Yeast infections

Question 34

What is the % CO2 in air we inhale vs air we exhale?

Answer 34

Inhale = 0.039%
Exhale = 4-5.3%

Question 35

Why do speaking valves become infected with filamentous C. albicans?

Answer 35

Because the CO2 in exhaled air stimulates filamentation

Question 36

How many aspartyl proteinases does C.albicans secrete?

Answer 36

10!

Question 37

What protein is important for C. albicans adhesion to epithelial cells?

Answer 37

the Hwp1 protein