Methicillin-resistant Staphylococcus aureus among HIV Positive Individuals in San Francisco

Methicillin-resistant staphylococcus aureus (MRSA) infections acquired in hospitals and healthcare settings can be severe as they can only be treated with intravenous vancomycin - an antibiotic that has high toxicity.

Methicillin-resistant Staphylococcus aureus (MRSA) -- strains of the bacteria that have developed resistance to a once-standard antibiotic -- is a growing problem among people with HIV. Though previously seen mainly in hospitals, MRSA is now commonly transmitted in the outside community. S. aureus causes conditions ranging from boils and other skin and soft tissue infections to septicemia (bloodstream infection).

At the recent ICAAC, researchers from the University of California at San Francisco reported on a 10-year survey of MRSA among gay men and people with HIV receiving care at the San Francisco General Hospital outpatient HIV/AIDS clinic (a.k.a. "Ward 86"), looking at prevalence, genetic similarity, and resistance to antimicrobial agents.

Over 10 years (1996-2005), 543 S. aureus clinical isolates were collected. MRSA isolates were characterized by pulsed field gel electrophoresis, the methicillin-resistance determinant SCCmec gene element, and presence of Panton-Valentine-Leukocidin (PVL) genes.

Results

MRSA prevalence increased from 7.2% of all S. aureus isolates in 1996 to 76.3% in 2005.

The total number of patient with culture positive MRSA isolates increased 2.7-fold, from 42 to 114.

The SCCmec IV element was omnipresent (in more than 90% of isolates) from 1996 onward.

S. aureus genotypes carrying the SCCmec IV gene were distributed between 4 clones until 2001:

- ST8:USA500 (45%);
- ST59:USA1000 (20%);
- ST30:USA1100 (30%) (PVL+);
- ST1:USA400 (5%) (PVL+).

The ST8:USA300 (PVL+) genotype was first identified in 2002, and increased from 33% of isolates that year to more than 90% in 2004-2005.

The percentages of community-acquired MRSA with resistance to various other antibiotic agents was as follows:

- methicillin alone: less than 2%;
- erythromycin: 15%;
- erythromycin + ciprofloxacin: 26%;
- erythromycin + ciprofloxacin + clindamycin: 16%;
- erythromycin + ciprofloxacin + clindamycin + tetracycline: 10%.

Trimethoprim-sulfamethoxazole resistance was seen only in methicillin-susceptible isolates, and decreased from 31% to 15% during the study period.

Conclusion

Over the past decade, community-acquired MRSA has replaced methicillin-susceptible strains as the main cause of S. aureus infections in the HIV positive population, the researchers concluded. Resistance to multiple antibiotics is mainly associated with ST8:USA300, the clone responsible for the vast majority of S. aureus infections identified over the past 3 years.

In a second study, the same researchers reported on an analysis looking at specific ST8:USA300 isolates collected in San Francisco, Oakland, and Los Angeles. They found that subtype ST8:USA300-0114 represented 81% of the isolates of the 28 subtypes identified. Some isolates were also resistant to erythromycin (88%-100%), clindamycin (3%-38%), ciprofloxacin (46%-82%), tetracycline (15%-40%), and rifampin (0%-4%), varying based on location. Resistance to at least 3 non-beta-lactam antibiotics was most common in San Francisco.

"Multidrug resistance in ST8:USA300 is an alarming reality," they concluded

10/17/06

References

H Carleton, F Perdreau-Remington. A Ten Year Survey of Staphylococcus Aureus (SAUR) Isolates Causing Infections Among Gay Men and People with HIV in San Francisco. 46th ICAAC. San Francisco, CA. September 27-30, 2006. Abstract C2-1142.

F Perdreau-Remington, H Carleton, R Bush, and others. Regional Variation of Multidrug Resistance in Community-Associated Methicillin-Resistant Staphylococcus Aureus (CA-MRSA) ST8:USA300 in Northern and Southern California. 46th ICAAC. Abstract C2-1131.

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