Community-acquired MRSA soars among people with HIV in San Francisco

For people with HIV living in San Francisco, MRSA infections contracted outside hospital have become a significant problem, according to research presented at last week’s Forty-Sixth Interscience Conference on Antimicrobial Agents and Chemotherapy in San Francisco.

Methicillin-resistant Staphylococcus aureus (MRSA) infections usually occur in people with weakened immune systems in hospitals or other healthcare facilities such as nursing homes.

But community-associated MRSA (CA-MRSA) infections are those acquired by people who have not been recently hospitalised or had any sort of medical procedure. They can occur in otherwise healthy people and are on the rise in many countries.

Outbreaks of CA-MRSA have occurred across the United States over the past five years both in people with no risk factors as well as those with HIV disease.

This study tracked all Staphylococcus aureus infections in patients admitted to a HIV ward in San Francisco from 1996 to 2005.

Information on 544 Staph. aureus isolates from 544 individual patients were studied; 299 of the isolates were resistant to methicillin while 245 isolates were susceptible to the antibiotic, termed methicillin resistant Staph. aureus (MRSA).

Over those ten years the percentage of bacteria which were resistant to methicillin soared from 7.2% to 76.3%.

Ninety per cent of the MRSA isolated came from skin and soft tissue infections including abscesses, lesions, ulcers and wounds.

Researchers also looked for a specific gene in the MRSA bacteria which is associated with a strain seen in the community - not in hospital acquired cases. The gene - known as SCCmec IV - was already seen in 90% of isolates from 1996.

One particular strain of CA-MRSA - called USA1100 - caused a serious outbreak of infections in the HIV ward in the late 1990s.

In 2000 most of the CA-MRSA infections were made up of two strains - USA500 and USA1100.

But those strains have since been replaced by another - USA300 - which has already been identified in community outbreaks of MRSA wound infections in California, Georgia, Texas, Pennsylvania, Mississippi, and Minnesota

USA300 was first identified in this clinic in 2002 when it made up 33% of the CA-MRSA isolates. But by 2005 93% of isolates were of that strain.

That rise has been associated with an increase in multidrug resistance say the researchers.

USA300 is a very stable strain of CA-MRSA and is resistant to treatment with penicillin, oxacillin and erythromycin and is variably resistant to another antibiotic, tetracycline.

Some of the USA300 strains isolated as part of this study were resistant to every drug in four major antibiotic classes, making it a significant problem.

The researchers conclude that over the last decade community-acquired MRSA infections have replaced hospital-acquired MRSA infections as the cause of infections in the HIV population in San Francisco.

Resistance to multiple antibiotics is mainly associated with USA300 which is responsible to the vast majority of Staph. aureus infections over the past three years.