10. MRAS and VRE as hospital and community pathogens Flashcards
What is staphylococcus aureus?
- A very common pathogen
- Named for its gold colonies when cultured
- Grows in aerobic conditions
- It is very virulent
How is MRSA specifically tested for in the lab?
- Culturing in stress conditions
- This is salt agar at 30oC.
- This stress forces the expression of the MecA gene.
What environment is S. aureus found in?
- on mammals and there are specific strains for different mammals.
- It likes hot places like skin, nose and groin. Also mucous membranes
How is S. aureus spread?
- Direct contact
- Airborne
What are the risk groups for S. aureus infections?
- Diabetes mellitus patients
- Haemodialysis patients and other groups that have a lot of contact with healthcare settings.
- People who inject drugs.
- People living with HIV.
- People living in close quarters like prisons or homeless shelters
S. aureus pathogenesis: Cell Wall
- Polysaccharide capsule that inhibits phagocytosis
- Peptidoglycan and Teichoic acids activate the complement and cytokines like IL-1. This causes systemic immune activation
S. aureus pathogenesis: Enzymes
- Some are used for identification and diagnosis.
- ß-lactamases
- Coagulase to promote blood clotting
- Proteases
- Lipases
- DNases
- Phospholipase
S. aureus pathogenesis: Toxins
- Exotoxins like haemolysins that lyse WBC.
- Entertoxins that are heat stable so can cause food poisoning.
- Toxic shock syndrome toxin which is a superantigen causing hyper stimulation of T cells.
- Panton-Valentine Leukocidin (PVL) toxin is relevant transmission and disease presentation
What are the different clinical presentations of S. aureus infections?
- Soft tissue infections that normally come from an open wound and cause boils, abscesses and cellulitis.
- Bone and joint infections, which are from more invasive or long infections. A common cause of septic arthritis.
- Pneumonia but only in very sick patients
- Endocarditis - heart inflammation
- Bacteraemia
What are the 2 types of clinical S. aureus infections?
- Pyogenic infections
- Toxigenic infections
Pyogenic S. aureus infections
- Most common
- Start local and enter through breaks in the skin like surgical wound.
- Spread through the blood and invade the joints, kidneys and muscles. Also cause endocarditis.
Toxigenic S. aureus infections
- Commonly food poisoning with severe and rapid onset. Caused by a heat resistant toxin.
- Scalded skin syndrome
- Toxic shock syndrome which has severe manifestations like multi-organ failure and often end up need ITU care.
What is MRSA?
Methicillin-resistant Staphylococcus aureus
What is the cinical presentation of MRSA?
The same as S. aureus infections. the only difference is how you treat it
What is the mechanism of methicillin resistance in MRSA?
- Caused by the MecA gene.
- MecA produces an alternative PBP called PBP2a.
- This means penicillins cannot bind to it and it causes resistance.
- It is encoded on a mobile genetic element called SCCmec and can be acquired in horizontal gene transfer.
What is the origin of SCCmec genetic element?
- It has lots of different cross over points with S. Aureus.
- It may have originated in coagulase negative staphylococci and then transferred over. This could be a natural reservoir of resistance.
What was the pattern of emerging resistance to S. aureus and MRSA?
- S. aureus became resistant to penicillin quickly after it was introduced.
- Methicillin was introduced to treat these penicillin resistant infections in the 1960s.
- S. aureus very quickly also gained resistance to this as well.
- MRSA wasn’t a big problem until the late 90s.
- This was due to the emergence of MRSA-4, which carries other drug resistance mechanisms.
What is used to treat MRSA?
Vancomycin
What is the evolution of strains of MRSA?
- There are lots of different strains and they emerged in different places.
- It started with 1 sequence type that gained methicillin resistance.
- It then evolved over time depending on the location it spread to.
- This led to different strains being dominant in different locations.
- The different strains carry different genetic elements with different resistance mechanisms working with MecA.
What are the SCCmec subtypes?
They are different genetic elements with different resistance mechanisms and toxins. They all include MecA.
What are SCCmec subtypes 2 and 3?
- They are more common in hospitals
- They contain other resistance mechanisms to make it MDR.
- This MDR plasmid is big and comes with a fitness cost
What makes a S.aureus MRSA?
The presence of MecA
What are SCCmec subtypes 1,4 and 5?
- These remain susceptible to other antibiotic agents just not penicillins.
- Can contain PVL toxins
Community vs Hospital MRSA
Community:
1. SCCmec types 4 and 5
2. Increased virulence
3. often PVL+
4. Sensitive to fusidic acid, quinolones, tetracyclines and macrolides
Hospital:
1. SCCmex types 2 and 3
2. Carries a fitness cost.
3. Often PVL-
4. Often resistant to fusidic acid and tetracyclines
5. Maintains sensitivity to quinolones and macrolides
What is community onset MRSA?
MRSA when symptoms present in the community but the true origin is the hospital and it is likely to be a hospital strain.
What is community MRSA?
1, Originated in the community and no link to healthcare
2. Normally, a community strain.
What are the risk factors for community MRSA?
- Young people usually living in close quarters like student, professional athletes and military service members.
- exposure to prisons and prisoners
- Activities with regular skin to skin contact
- Exposure to antibiotic
- IV drug abuse
- Recurrent skin infections