Systems for Detection of Pathogens I Flashcards Preview

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Flashcards in Systems for Detection of Pathogens I Deck (25)
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1
Q

What’s in a NAME?

A

Names provide us with the opportunities to define boundaries
It is up to the test system to define these boundaries
and provide a measure that informs us
“To what extent can any one microbe be able to cause disease”

2
Q

Mycobacterial genus - how many species and obligate human pathogens

A
147 current species
Only three are obligate human pathogens
M .tu berc u los is
M .leprae
M .u lc erans
3
Q

How can we define a pathogen?

A

A microbe
CAPABLE of causing
a specific degree of host damage

4
Q

Commensal Non pathogen (in host) - define w/examples

A

PRESENT but NOT CAPABLE of causing disease in the host
eg. E.c oli
B ac teroid es thetaiotaomic ron ‘good bacteria’

5
Q

Zoonotic Non pathogen (in carrier)

- define w/examples

A

PRESENT but only CAPABLE of causing disease in ANOTHER host

eg. E.c oliO157:H7 is subclinical in cattle

6
Q

Commensal Opportunist (in host) - define w/examples

A

PRESENT and CAPABLE of causing disease in the host
but only in certain circumstances
eg. B ac teroid es fragilis
Coagulase Negative Staphylococcus (CNS)

7
Q

Positive samples in relation to diagnosis - always 100%?

A

Not all positive samples are diagnostic of active disease

8
Q

Sterile sites vs non sterile sites in tests - give examples

A

Sterile sites must be free from contamination
eg. Skin flora in blood cultures

Non sterile sites require decontamination of normal flora
eg Faeces, Mouth, Skin

9
Q

Which samples require concentration for testing - give examples

A

Samples with high volume or relatively low infected pathogen load
require concentration (centrifugation, filtering)
eg CSF, Ascites, 24 hr Urine

10
Q

Desribe sequence of events from preparation to identification phase (compare culture vs direct)

A

PP for culture = Enrichment
Purification
Amplification

PP for direct = Concentration
Sample treatment

ID same for both =

Molecular DNA/RNA
Gross morphology (Microscopy)
Chemical composition (HPLC MassSpec)
11
Q

Microscopy - advantages

A
Advantages
Easy to perform
Rapid screening
Some parasites have SPECIFIC morphology
eg. S c his tos oma mans onii
Specific Immunoflourescence staining possible
12
Q

Microscopy - disadvantages

A
Disadvantages
Not Sensitive
eg. M yc obac teriu m tu berc u los is
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 (more expensive)
13
Q

Bacteriology - what does it rely on

A

Bacteriology
This relies on the ability of the test system
to be able to grow the pathogen

14
Q

Bacteriology - types and examples of media

A

Media:

Non Selective Media
eg. Blood Agar

Semi Selective Media
eg. MacConkey Agar, DCA, CLED

Selective growth temperatures
eg. Campylobacter species

15
Q

Describe diagnosis for Streptococcus pneumoniae

A
Optochin sensitivity
with gram stain microscopy
and colony morphology
is DIAGNOSTIC for
S treptoc oc c u s pneu moniae
16
Q

Classical metabolic testing - results for catalase

A

E.coli = +ve

Clostridium perfringens –ve

17
Q

Classical metabolic testing - results for indole test

A

Can cleave indole from tryptophan
(Indole test)

E.coli = +ve
Clostridium perfringens –ve

18
Q

Describe the identification of Enterobacteriacae

A

Metabolic function and sugar utilisation tests for identification of Enterobacteriacae
Eg. Salmonella, Shigella, E.coli

19
Q

Food Poisoning - describe microscopy test

A

Stool → Direct microscopy for cysts/eggs of amoebae or parasites

20
Q

Food Poisoning - describe gram stain test

A
incubate
microaerophilically at 42ºC
(85% N2, 10% CO2
, 5% O2
)at 42ºC

Spread on Campy CVA
selective media plate

Positive growth
(silver grey colonies)

Gram stain

Gram negative curved rods

Oxidase positive

Campylobacter jejuni

21
Q

Food Poisoning - describe antibiotic sensitivity testing

A

incubate
aerobically at 37ºC

M acC onkeyagar &
D esoxycholate agarplates

Identify bacteria by biochemical profiling
(API 20E strip)

Salmonella
typhimurium + Shigella
flexneri

=

Antibiotic sensitivity testing
disc diffusion plates

22
Q

Virology culture requires what and list its effect

A

Culture
Requires permissive cell lines
eg.Vero cells (Kidney epithelial)
for H erpes s implex

Cytopathic Effect
Immunofluorescent staining of culture
Direct Antigen Detection
ELISA
eg. Influenza Virus
23
Q

ELISA titres to specific antigens can be measured by

A

Multiple samples single
antigen with signal positive
colour cut off

24
Q

Classical culture and identification - advantages

A

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
Easily archived
eg. Epidemiology

25
Q

Classical culture and identification - disadvantages

A

Disadvantages
Some pathogens cannot be grown eg. M yc obac teriu m leprae
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
eg. M yc obac teriu m tu berc u los is
Labour intensive (expensive)
Requires specialist interpretive expertise (more expensive)

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