Cath Lab Digest

Although it’s been almost 10 years, I’ll never forget the tone and inflection from my attending when she impressed upon me just how serious she considered ‘staph’ to be as a pathogen. At the time I was a new Infectious Diseases Fellow and I remember thinking that surely there must be some other bugs that would rate the same serious consideration. However, what I didn’t appreciate then—I do now.

Understanding Staph
In today’s ecology of microorganisms, the word ‘staph’ automatically brings to mind Methicillin Resistant Staphylococcus aureus (MRSA). However, over the last several years I have developed a much greater sense of appreciation for something that my attending knew back then—this germ should never be taken lightly. MRSA can cause a broad range of illnesses from simple recurrent furunculosis to severe invasive necrotizing pneumonia. For reasons—some that still elude me—MRSA can be cultured from wounds that seem to be on track toward healing; causing no obvious problem. Likewise, I have also seen MRSA cause severe, tissue/limb/life threatening, fulminant disease in patients who do not have the traditional risk factors of being colonized with or infected by staph aureus. It’s the great masquerader.

Lessons Learned In Time
In preparation for this article, I reviewed numerous medical publications including full peer reviewed articles and case reports; information available through the CDC; and Emerging Infectious Diseases. All of my resources were recent and published within the last 6 years. In order to fully appreciate MRSA as a pathogen, I think it is worthwhile to take a look at the historical arrival of MRSA on the clinical stage. MRSA was first identified in 1958. By 1961, it was documented as the cause of an infection in the United Kingdom. In 1968, reports of MRSA isolation were noted in the United States. By 1990, MRSA was found worldwide and was recognized as a cause of infection in ‘nontraditional’ patient groups. In 2003, continent - specific clones (ie, genetically distinct germs) were identified. We now have region specific clones across the U.S.

MRSA or Not MRSA
Whenever a provider writes an order for a ‘culture’, not only is extra cost added to the bottom line but a series of multiple events is set into motion—much like tipping over the first of many closely standing dominos. Hopefully the culture report will provide meaningful information to the provider thus enabling him to make an informed decision regarding possible changes in the patient’s treatment plan. This is why it is so imperative that, when a wound is cultured it is not collected from a haphazardly prepared wound bed or tissue space.
For those unfamiliar with what is involved in a microbiology lab in order to identify an organism, the first step is that a technician (or first year Infectious Diseases Fellow) - using the swab that was received in the lab - streaks a small glass slide and then inoculates an agar plate (plastic dish filled with a nutrient rich jelly-like substance that enhances bacterial growth). The agar plate is set aside to incubate and, the slide is streaked and allowed to dry. Once the streaked slide is sufficiently dried, various chemicals are applied sequentially, which will strain bacteria that were hopefully deposited there when it was streaked. This series of steps is called a ‘Gram Stain’; developed in 1882 by and named after Dr. Hans Christian Gram. Bacteria that stain darker violet to bluish purple are called ‘Gram positive’. Those that don’t are called ‘Gram Negative’. By microscopic examination, the now visible stained bacteria can be further described and identified based on their morphologic appearance. If the stained bacteria appear in clusters or in tetrads of round balls (cocci), they are called gram positive cocci (GPC’s), which are Staphylococci. Not know at the point is if the germ is Staph aureus or one of many other Staph species. GPC’s appearing in chains or pairs usually
are Streptococci.
Unfortunately, the next step in this determination cannot be completed until there is enough growth of the organism on the agar plate to allow the next chemical test to be performed. This usually takes a minimum of 24 hours for a colony of bacteria to appear on an agar plate. Only then can a “coagulase” test be done. This test capitalizes on the fact that Staphylococcus aureus is—of all the GPC’s—the only one with a cell wall protein called “coagulase”. It is one of many cell wall proteins found in Staphylococcus aureus that confers the potentially toxic effects to tissues. Through a one step bench-side test it can be determined whether or not the protein coagulase is present. If the test is positive, the organism is a “coagulase positive GPC”, which means that it is a Staphylococcus aureus. However, what is still unknown is whether the Staphylococcus aureus is MRSA or not. This determination requires testing against a panel of antibiotics and takes an additional 24 to 48 hours. If the coagulase test is negative, the organism is called a “coagulase negative GPC”. By convention, all coagulase negative staphylococci are collectively referred as ‘coag-negative staph’ and sometimes all non-aureus staph are collectively called ‘staph epidermidis’.