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Staphylococcus aureus:
is this new?
Gary Preston, PhD
10/24/2007
Humans & Their
Organisms
Many human body sites are normally inhabited by microorganisms…bowel,
skin, mouth, nose, etc. (Colonization). These same organisms
can cause disease if they reach body sites they do not normally
inhabit. For example, organisms in the nose and mouth can cause
pneumonia if they reach the lungs or brain; bowel organisms may
cause peritonitis if they are find their way into the abdominal
cavity such as via a burst appendix or the urinary tract infection
if they reach the bladder or kidneys; or skin organisms if they
find their way into a wound or break in the skin. Fortunately,
our immune response (antibodies and blood cells that fight microorganisms)
are on duty to assist with disposing of microorganisms that stray
from their usual locations.
Staphylococcus
aureus
Staphylococcus aureus (Sa) is one of the many species of staphylococcus
that can inhabit the skin and other body sites. It inhabits
the bodies of approximately 30% (1/3) of normal people. (Colonization).
This makes it available to enter wounds or the lower respiratory
tract and then possibly the blood where it can cause infection
(disease). It is also available to be transmitted by contact
from one person to another directly or indirectly on surfaces
and objects. The more of the organism available, such as during
an infection, the more available for transmission. Some people
are not colonized, some are intermittently colonized and some
are persistently colonized. Who has what today?
Antibiotic
Resistance & Virulence
Most antibiotics originated from substances produced by microorganisms
to kill other competing microorganisms. When microorganisms
(or any group of organisms) are exposed to stressful conditions,
those individuals which are genetically most capable of surviving
become predominant. This happened when penicillin (PCN) was
introduced in the late 1940’s and ‘50’s.
At first, Sa infections could be treated with PCN. Now almost
all Sa are resistant to PCN. Semi-synthetic PCN’s (methicillin,
oxacillin, nafcillin, etc.) replaced PCN for treating Sa infections
and soon (~1968) genes coding for resistance to these began
to spread amongst Sa. Vancomycin replaced penicillins for treating
Sa infections and, as you might anticipate, given enough time
and Vancomycin use, resistance to Vancomycin emerged in some
Sa (VRSA) in ~2002. Newer antibiotics can be used if/when VRSA
(now rare) is present and it will be important to monitor Sa
resistance to these. It is obvious by now that preserving the
effectiveness of antibiotics requires that their use be limited
to circumstances where they are really needed if the selective
pressure for resistance emergence is to be kept low.
The ability
to cause disease (virulence) is also determined by the organism’s genetic
code—which can vary from one organism to another even among Sa.
Tracking
Infections in Health Care and Public Health
Our hospital’s infection control program has for years
routinely monitored patterns of infection due to all organisms
in all patients…including but not limited to MRSA. While
our analysis guides intervention, since prevention depends
on appropriate patient care practices, we also monitor the
consistency of our prevention procedures: equipment cleaning,
hand hygiene, preoperative care, pneumonia prevention, IV line
care, etc. The more intensive the effort to improve practices,
the lower the risk of infection.
Public health tracking of infections is particularly important
when transmission of the infectious organism is not by predictable
contact from one person to the next but by the difficult to
control airborne route (e.g., measles, tuberculosis), when
many people may be infected by a single source such as food
or water (e.g., Salmonella, E. coli O157:H7, Shigella) or when
each infection comes with a high risk of serious outcome (e.g.,
death).
Staphylococcus aureus infections are very common and the organism
is transmitted by contact.
Infection
Prevention
Infections may be caused by organisms already present in or
on the body (endogenous) or organisms newly acquired (exogenous).
Prevention of infection requires attention to both potential
sources of organisms.
In healthcare, elevating the head of the bed of the intubated
patient helps prevent the patient’s own upper respiratory
organisms from reaching the lower respiratory tract. Not shaving
and thereby preventing
micro-nicks of the skin in which bacteria can grow and administering
antibiotics just before incision helps prevent surgical site
infection due to normal skin organisms. In and out of healthcare,
transmission of Sa and other organisms transmitted by contact
and not the airborne route is prevented by sanitation: cleaning
of objects, surfaces and hands after each person contact. Transmission
prevention may be accomplished by insuring that prevention
steps are reliably and consistently used all the time after
every contact.
Transmission
Prevention
The debate regarding the most effective transmission prevention
strategy is on.
One strategy assumes:
1. infected and colonized patients who have no signs of infection
are equally potential sources of organisms for transmission
to other patients;
2. routine cleaning and hygiene are insufficiently practiced
to reliably prevent transmission;
3. culture screening of patients can reliably identify who
has Sa at any given time and that culture results are necessary
to insure that hygiene and cleaning around culture positive
patients is adequate to prevent transmission;
but that
4. culture of healthcare workers need not be done because,
unless signs of infection and/or evidence of transmission
are present, healthcare workers are not likely to be significant
sources for transmission.
This is similar to the HIV transmission strategy that called
for testing of all persons so that appropriate disinfection
and sterilization steps could be followed selectively following
use with HIV positive patients.
A second strategy calls for increased and sustained attention
to reliable hand hygiene and cleaning routinely so as to prevent
transmission of not just MRSA but all Sa (from the 30% who
have it to the 60% who do not) but also other potential pathogens
such as Enterococcus (which can be resistant to Vancomycin
(VRE)), Clostridium difficile, E. coli, Pseudomonas, etc.,
etc. Reliable performance of these steps will minimize transmission
of microorganisms from patient to patient, patient to healthcare
worker and healthcare worker to patient no matter who has what.
This is similar to the strategy adopted to prevent HIV transmission
that does not depend on HIV testing of potential sources but,
rather, insures appropriate disinfection and sterilization
steps to insure that not just HIV but any blood borne pathogen
(hepatitis B and C, human T-lymphotrophic viruses, hepatitis
G, etc.) is not transmitted whether the individual potential
source patient is known have the organism or not.
Conclusion
MRSA is just one sub-type of just one of the organisms that
is commonly around us that can cause disease. Antibiotic
resistance has increased dramatically in all of potentially
disease causing microorganisms over the years that antibiotics
have been used in medicine and agriculture. Resistance will
continue to be a problem that can be minimized by appropriate
antibiotic use. Infections and transmission of microorganisms
that can cause disease will be prevented when we accept the
challenge to reliably and appropriately use the steps we
know can reduce infection due to microorganisms already present
and prevent transmission of microorganisms from where we
expect they might be to where we do not want them.
©2007 Healthcare Management Alternatives,
Inc. |
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