Systems for Detection of Pathogens I

Question 1

Taxonomy

Answer 1

naming micro organisms

Question 2

Commensal Non pathogen (in host)

Answer 2

PRESENT but NOT CAPABLE of causing disease in the host

Question 3

Zoonotic Non pathogen (in carrier)

Answer 3

PRESENT but only CAPABLE of causing disease in ANOTHER host

Question 4

Commensal Opportunist (in host)

Answer 4

PRESENT and CAPABLE of causing disease in the host but only in certain circumstances

Question 5

if you are infected with a pathogen, will it always be active?

Answer 5

no, can be latent, like TB

Latent TB can sit around for years until you become old or you have an operation or any situation where your immune system is not able to cope with the organism

30% of humans are affected, 18% of which have latent disease

Question 6

pathogen

Answer 6

A microbeCAPABLE of causing a specific degree of host damage

Question 7

what is meant by a “good sample”

Answer 7

sample must have from sterile sites that is free from contamination - non sterile sites require decontamination of normal flora

if the sample has a high volume or relatively low infected pathogen load it requires concentration (centrifugation, filtering)

Question 8

Do we really need to culture?

Answer 8

we don’t need to culture as long as we know what organism is there

Question 9

Microscopy - adv and disadv?

Answer 9

• easy to perform
• rapid screening
• specific immunoflourescence staining possible
• used for large organisms

DISADV

-Not Sensitive
-screening sputum smears requires
at least 10,000 orgs per ml to be visualised
-General stains are not specific
-Labour intensive (expensive)
=Requires specialist interpretive expertise

Question 10

staining usefulness?

Answer 10

useful when looking at shape and structures, whether bacteria are gram negative or positive

Question 11

what method is good for looking at viruses down a light microscope?

Answer 11

Immunofluorescent staining with pathogen specific conjugated antibody

Question 12

culture - different types of media?

Answer 12

Non Selective Media
eg. Blood Agar

Semi Selective Media
eg. MacConkey Agar, DCA, CLED

Selective growth temperatures
eg. Campylobacter species

Aerobic/Anaerobic Culture

Microaerophilic Culture

Question 13

role of selective media?

Answer 13

gets rid of everything else and make sure you only grow the pathogens

Question 14

in what situations could anaerobic bacteria affect humans?

Answer 14

anaerobic bacteria could affect humans who have frostbite, as there will be dead tissue with no oxygen (e,g at the tips of fingers, where tissue might degrade), produces a toxin and you get gangrene

Question 15

Systematic Bacteriology examples

Answer 15

Metabolic function and sugar utilisation tests for identification

Question 16

Antibiotic sensitivity plate testing

Answer 16

looking at resistance

Resistant = grow all the way up to the discs

Question 17

key difference between bacteria and viruses?

Answer 17

Viruses are intracellular – bacteria have a cell wall on the outside they can live outside cells, but viruses cannot – they NEED a host to survive.

Question 18

what will you need in order to grow and detect viruses?

Answer 18

a cell line
-vero cells are a type of immortal cell line

direct antigen detection
-ELISA

Question 19

Classical culture and identification - adv and disadvantages?

Answer 19

Advantages

Cheap simple, reliable reagents
Sensitive
eg. Single organisms can be grown and identified
Validated specificity
eg. ‘Gold Standards’ with multiple parameters
Direct in vivo measurement of effectiveness of therapy
eg Antibiotic sensitivity

Disadvantages

Some pathogens cannot be grown
Some pathogens cannot be well differentiated by biochemistry alone
Slow: culture requires at least overnight incubation:
Viral = 3-10 days Mycobacterial = 6-12 weeks
Some pathogens grow too slowly to aid rapid diagnosis (TB)
Labour intensive (expensive)
Requires specialist interpretive expertise (more expensive)