Emerging Staph Strains Increasingly Dangerous
September 7, 2005
A study of how the immune system reacts to strains of antibiotic-resistant Staphylococcus
aureus bacteria — emerging strains that sicken otherwise healthy people,
or so-called community-acquired infections — has shown for the first time that
these strains are more deadly and better at evading human immune defenses than
more common S. aureus strains that originate in hospitals and other
In a paper released today online in The Journal of Immunology, scientists from
the National Institute of Allergy and Infectious Diseases (NIAID), part of the
National Institutes of Health, describe how community-acquired S. aureus strains
that survive treatment with the methicillin family of antibiotics can evade immune
defenses. Infections from community-acquired methicillin-resistant S. aureus,
or MRSA, are difficult to treat and are increasing nationally at an alarming
rate, say experts.
Scientists at NIAIDs Rocky Mountain Laboratories (RML) in Hamilton, MT, and
colleagues examined the ability of MRSA strains to cause disease in mice and
avoid destruction by human white blood cells called neutrophils. Neutrophils,
which typically ingest and then kill harmful bacteria, make up about 60 percent
of all white blood cells and are the first line of defense against bacteria.
Scientists know that community-acquired strains differ from hospital strains,
but they dont know why the community strains cause more serious infection in
otherwise healthy people.
S. aureus has become resistant to many commonly used antibiotics. Up to 20% of all Staphylococcus isolates are resistant to penicillin, which has led to the introduction of flucloxacillin and cloxacillin as first-line antistaphylococcal antibiotics.
An increasing problem since the 1950s has been resistance of S. aureus to flucloxacillin, oxacillin, and similar β-lactam antibiotics that are deactivated by β-lactamase. As methicillin is used in laboratories to assess for this type of resistance, the term Methicillin-resistant Staphylococcus aureus (MRSA) is in use to denote these strains. MRSA is generally sensitive to the glycopeptide antibiotics vancomycin and teicoplanin.
In 1997, physicians were alarmed to encounter staph strains that resist even vancomycin, to which it had previously always been sensitive. Due to this resistance, S. aureus is sometimes referred to as a superbug.
The work also identified specific S. aureus genes that potentially
control the bacteriums escape from neutrophils. Among thousands of S. aureus genes
analyzed in the five different strains used in the study, the scientists identified
a large group of genes whose role in helping spread infection is unknown. RMLs
Frank DeLeo, Ph.D., the investigator who directed the study, and colleagues plan
to determine if some of the unknown genes help promote disease. If they can learn
which genes control the ability of S. aureus to evade and destroy neutrophils,
their work could lead to new medical treatments.
Each day physicians around the world are stymied by the inability to effectively
treat patients suffering from severe S. aureus infections, says NIAID
Director Anthony S. Fauci, M.D. There is a critical need to develop new treatments
against late-stage disease caused by antibiotic-resistant strains, and this promising
work offers several new approaches.
According to the Centers for Disease Control and Prevention, recent reports
of community-acquired MRSA infections raise concern If MRSA becomes the most
common form of Staphylococcus aureus in a community, it will make treatment
of common infections much more difficult. The April 7, 2005, issue of The
New England Journal of Medicine refers in an editorial to an epidemic
of MRSA in the community.
S. aureus strains acquired in health-care settings can be challenging
to resolve because of antibiotic resistance, which limits the choices for treatment.
But the situation can become more serious with the newer community-acquired strains,
says Dr. DeLeo. We do not know why cases of community-acquired MRSA infections
are increasing, let alone how they flourish, he says. But scientists do know
the community strains can cause more severe forms of disease.
Mild S. aureus infections such as impetigo, which typically forms small
blisters on the faces of children, or cellulitis, an inflammation of skin or
muscle tissue, can easily be treated and usually resolve in a matter of days.
But S. aureus disease also can be much more severe and difficult to
treat, affecting vital organs and leading to toxins poisoning the blood and infection
overwhelming the heart. One of the most severe types of disease is necrotizing
pneumonia, where bacteria destroy lung tissue.
The reason that some mild infections become severe or fatal is not well understood,
but virulence is often associated with certain strains, says Jovanka Voyich,
Ph.D., of RML, the studys lead author. To cause human disease, bacterial pathogens
must avoid being killed by neutrophils. These results, says Dr. Voyich, suggest
that community-acquired MRSA causes disease in healthy people in part because
it has enhanced ability to circumvent killing by neutrophils.
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NIAID is a component of the National Institutes of Health, an agency of
the U.S. Department of Health and Human Services. NIAID supports basic and
applied research to prevent, diagnose and treat infectious diseases such as
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malaria and illness from potential agents of bioterrorism. NIAID also supports
research on transplantation and immune-related illnesses, including autoimmune
disorders, asthma and allergies.
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This is a NIH news release. The original version appears here