MRSA (Boruchoff) - 5/2/16

Question 1

Describe the evolution of drug resistance in S. aureus.

Answer 1

S. aureus (1950’s) –> Pencillin-resistant S. aureus (1970s) –> Methicillin-resistant S. aureus (MRSA) (1990s) –> Vancomycin-resistant enterocci (VRE) (1997) –> Vancomycin intermediate-resistant S. aureus (VISA) –> VRSA (2002)

Question 2

Outbreaks of MRSA in the community

Answer 2

2002:

Passed from person to person in the community:

• Sports participants (sharing of towels, etc…)
• Inmates in correctional facilities
• Military recruits

A lot of it has to do with crowding and frequent contact

Question 3

Major clinical manifestations of Community-Acquired MRSA

Answer 3

Most common identifiable cause of skin and soft-tissue infections [presents with PUS]

Necrotizing pneumonia

Genetically different from hospital acquired-MSRA

Carriage less common in nares, so nasal screening may fail to detect

Question 4

Colonization vs. Infection

Answer 4

Colonization:

• Multiplication of an organism at a body site or sites without evidence of infection
• May or may not be a precursor of infection

Infection:

• Multiplication of organisms in the body of the host
• Usually involves invasion of tissue
• Usually accompanied by a measurable host response (inflammation, antibody production, cell-mediated immunity)
• May be clinical or sub-clinical

Question 5

What is the Iceberg Effect?

Answer 5

The number of people infected is less than everyone who is colonized

If an infection develops, it is usually from bacteria that colonizes patients (bacteria that colonize patients can be transmitted from one patient to another by hands of healthcare workers)

BACTERIA CAN BE TRANSMITTED FROM COLONIZED PATIENTS, EVEN IF PATIENT IS NOT CLINICALLY INFECTED

Question 6

Nasal Carriage of Staph

Answer 6

Staph niche = external nares

~33% of population in US are carriers of S. aureus

Question 7

Approaches to decolonization

Answer 7

Agents for nose and for skin

If you decolonize patients, they will be less likely to have staph infection

Question 8

Mupirocin

Answer 8

Topical antibiotic

“Pseudomonic acid” - derived from culture supernatants of Pseudomonas fluorescens

Bacteriostatic, but activity dependent on pH and inoculum size (may be bactericidal at low pH - skin pH disrupts protein synthesis

Question 9

Development of Mupirocin Resistance Among MRSA after widespread use of nasal mupirocin ointment

Mupirocin Resistance: Mechanisms

Answer 9

Intrinsic resistance (gram negatives) from inability to bind to tRNA synthetase (not from altered drug transport or increased drug destruction)

Two different mechanisms of resistance:

• “Low-level” resistance (MIC 8-25): point mutation in chromosomal coded miles
• “High-level” resistance (MIC>256): plasmid containing a novel miles gene - “mupA”

Question 10

MRSA Screening: What tests are available?

Answer 10

1. Traditional Culture and Susceptibility - Time to final result: 48-72 hrs

CHROMagar MRSA - 24-48 hrs

Expert MRSA/GeneXpert -