MRSA screening

Question 1

What are the current testing techniques for MRSA?
- methicillin-resistant Staphylococcus aureus

Answer 1

swab cultures
polymerase chain reaction (PCR)
chromogenic agar plates

Question 2

How does swab culture testing work? What are the disadvantages?

Answer 2

swabs are taken from the nose, throat, or other suspected infection sites and are cultured on selective media to identify MRSA colonies

is time consuming which delayed diagnosis and treatment initiation
false negatives can occur if bacterial load is low or due to limitation of the culturing technique
is labour intensive so requires trained personnel and laboratory facilitates

Question 3

How does polymerase chain reaction testing work? What are the disadvantages?

Answer 3

these tests detect specific DNA sequences unique to MRSA providing quicker results than swab cultures

can be more expensive than culture based methods
requires specialised equipment and trained personnel
potential for false positive if there is contamination during the processs

Question 4

How does chromogenic agar plates testing work? What are the disadvantages?

Answer 4

these are selective media plates that change colour in the presence of MRSA

they have limited specificity - other bacteria may produce similar colour changes leading to potential misidentification

Question 5

Why is rapid testing for MRSA needed?

Answer 5

to
- reduce the spread of the organism
- reduce length of hospital stays
- reduce associated hospital costs

Question 6

What is LGX?

Answer 6

a hand held device used to detect MRSA
- takes nose swabs
- generates results in a short time

Question 7

How does LGX work?

Answer 7

LGX acts as a masked chromophore
- is fluorescent due to the presence of rhodamine fluorophore
- but is rendered non-fluorescent upon attachment to a substrate mimic

the substrate mimic consists of a tripeptide sequence
- valine, proline, arginine
- the substrate mimic is a substrate for enzyme staphylocoagulase which is diagnostic for SA presence (MRSA)

staphylocoagulase cleaves at the peptide linkage and frees the masked chromophore

rhodamine fluorophore becomes fluorescent due to the opening of the lactone ring which is no longer held by an amine group (changed to amide group due to peptide linkage cleavage)
- proves the presence of SA due to the enzyme staphylocoagulase activity

Question 8

How does LGX become fluorescent?

Answer 8

upon cleavage of the substrate mimic from LGX via enzyme staphylocoagulase, an amide group is left free

the amide typically pulls electrons from the unstable lactone ring to stop it from opening
- upon release of the tripeptide sequence/substrate mimic, the lactone ring opens which allows fluorescence

Question 9

What forms does rhodamine exist as?

Answer 9

lactone form which is colourless

zwitterionic form which is coloured
- the absence of electron withdrawing amide groups, rhodamine undergoes electron movement to form a zwitterionic compound which is highly coloured

Question 10

What are the advantages of LGX testing?

Answer 10

frees up lab resources
- only positive samples need further testing

rapid
- can be undertaken at bed side

potential downstream applications for prevention and management of surgical site infections

helps hospital infection control teams be more proactive

is cheaper than bacterial culture and PCR