Thursday 6 September 2007

Staphylococcus aureus

Science and Staphylococcus aureus
(The little “BUG-ger” That Got Me)


“Big bugs have little bugs on their backs to bite them, Little bugs have lesser bugs and so forth ad infinitum…”


Microbiology. I first fell in love with the beauty and the science of microbiology while attending The University of Western Ontario (UWO) in London Ontario. My fascination with science began much earlier in a Hamilton public school. It was an epiphany when struck with the realization that the world around us does not behave randomly but rather is governed by strict rules and predictable properties which continue to be refined and discovered. From that moment on, my perception of the world changed. Science was all around us - everywhere, and all the time. My hobbies and interests and life itself were suddenly seen and approached in a different light. Science was the answer to all questions. (Behavior of politicians excepted!)

From my first Microbiology class, bacteria fascinated me. As if they were people, I was introduced to these microbial characters one by one and began to realized they too had quirks and personalities. They could be grouped and identified. Some were good, some not so much. Like people, they liked to live in different locations and each preferred to “eat” different foods. “Family Trees” could show how they were related to each other. Many were industrious workers which could be harnessed to produce products we need or destroy those we don’t. As in society, some were parasites which preferred to live off others. Microbes could themselves catch infections and become “sick”. Bacteria were “intelligent” in the sense that they could detect and respond to adverse environmental conditions, such as antibiotics, and adapt accordingly. They were survivors. Microbes were not just static “Germs” but rather were ever evolving complex life forms.

After obtaining my honours degree in Microbiology & Immunology and a certificate in Medical Laboratory Technology, I spent over thirty years employed by various medical institutions. At times I have been involved in teaching, research and the day to day diagnosis of medically important infectious diseases. I have found it rewarding passing on knowledge of this fascinating discipline to students, managed to co-author some medical papers based on my research, and had the tremendous satisfaction of diagnosing the day to day infectious diseases afflicting ailing patients. My last 20 years have been spent as a Microbiology Laboratory Technologist at a local acute care community hospital.

Whether your doctors suspects infection by bacteria, fungi, viruses or parasites, your specimens will be examined by a highly trained and dedicated Medical Laboratory Technologist, registered with the Collage of Medical and Laboratory Technologists of Ontario (CMLTO). Medical technologists occupy one corner of the ‘medical health triangle’ (1)

It is rather ironic that it was a bacterium that ultimately found revenge and caused an infection on my spine leaving me a paraplegic. Bacteria are all around us. We become colonized by bacteria from the environments we reside in. A hot soapy shower may reduce the numbers temporarily however in as little as twenty minutes they will start to emerge from where they were hiding and re-colonize your body. It is a fact of life! No doubt my day to day presence in a hospital setting during the last 30 years had me carrying bacterial strains which could be traced back to my workplace. Regardless, my infection was not directly work related but probably resulted from my run down state after a 10 day stretch at work. I developed a sprained back with familiar symptoms and thought that bed rest might help. Not finding anyone willing to take over his established medical practice, I found myself without a family physician shortly before my symptoms developed. I opted for a trip to a local walk in clinic rather than tolerate a painful six hour wait in my hospital’s crowded emergency department. Muscle relaxants and pain killers were prescribed as I crawled my way back to bed and sank into a hazy semi-conscious state. Too painful to shower, I survived in a sweaty incontinent state unwilling to eat while continuing to deteriorate. Perhaps it was through my pneumonia, a urinary tract infection, or I may have inoculated myself with resident bacteria through a scratch. Regardless, I emerged from my state of oblivion long enough to realize that my legs would not move. My wife called an ambulance and as my hospital’s emergency department continued to exist in an overcrowded state, I was diverted to another community’s hospital. After numerous tests, it was determined that I had a urinary tract infection (UTI) which was probably the source of my blood culture infection and state of septicaemia (2). An MRI (3) ultimately revealed that I had an abscess on my spinal column. The culprit was an old friend of mine - Staphylococcus aureus. As serious at this situation was, the bacterium was not the highly resistant MRSA (4) strain and so could be treated with less toxic antibiotics.

A course of antibiotic therapy was initiated but did not clear the abscess, therefore surgery was required to excise and drain the abscess. Post surgical antibiotic therapy was administered for 6 additional weeks as intravenous Oxacillin to ensure complete eradication of my newly acquired companion. Sadly neither surgery nor antibiotic therapy could resolve my paraplegic condition.

A veterinarian that has been bitten by a dog would hardly give up his practice nor develop a hate of canines. Likewise, I continue to love bacteria and hope to someday get back to my laboratory and my fascinating microscopic friends.

Staphylococcus aureus: (A Most Basic Primer)

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Staphylococcus aureus is a coccoid (5) bacterium which divides in two planes giving it a “grape-like” clustered appearance under the microscope. A biological stain known as the “Gram Stain” colours them purple.

Cell wall components determine if a particular bacterium is either Gram Positive (Purple) or Gram Negative (Red). This initial differentiation of bacteria provides information as to how to proceed further for identification and a rough idea of what antibiotics could be excluded from consideration.

Specimens are plated (6) on appropriate bacteriological media (think food) and allowed to grow up from microscopic cells to visible colonies comprised of millions of cells. Macroscopic appearance and growth patterns on various media provide further clues.

The Genus Staphylococcus contains numerous species however Staph aureus is probably the most medically significant. Staph aureus can appear a golden-yellow colour when grown on Sheep Blood Agar whereas the less invasive Staph epidermidis appears as white colonies.
Staph aureus can also produce enzymes called haemolysins which, as the name implies, lyses the blood causing a clear zone around each colony.

Picking the colonies with a wire or loop, they can be tested by various methods or inoculated into a variety of media to see what enzymes they possess and what products they use or produce. The patterns obtained help to differentiate a particular bacterium from other species as well as other genera.

Basic Biochemical Traits of Staph aureus:

Beta-haemolytic - enzymes present lyse the blood cells clearing the area around the colonies.
Catalase Positive - Staph species poses enzymes which can split 3% hydrogen peroxide into oxygen and water to give observable bubbling when colonies are submersed in it.
Coagulase - Staph aureus produces an extra cellular protein which causes plasma to produce visible clumps when mixed together with the bacterial cells. Staph epidermidis does not
DNAse - Staph aureus can produce and enzyme which degrades DNA. When incorporated into a growth medium with an indicator the degradation of the DNA can be detected if present. Other Staph do not produce DNAse but some other genera do.
Mannitol Fermentation - Staph aureus can ferment this particular sugar where Staph epidermidis cannot. Resulting acid production from the fermentation can be detected by colour indicators.
Salt Tolerance - Staph aureus has a particularly high tollerance to salt concentrations. In the lab they can be routinely tested against 6% sodium chloride (NaCl or salt) but can often tolerate up to 15% NaCl.
Facultative Anaerobe - Staphylococci species are “Facultative Anaerobes” meaning they like to grow in the presence of air or oxygen (aerobic) but can survive nicely in (anaerobic) environments or those deprived of oxygen.

In addition to these products, Staph aureus can produce a myriad of enzymes, proteins and factors which can aid the microbiologist in identification and aid the bacterium in defeating host defence mechanisms thereby assisting it in establishing infections. Some of these are Staphylokinase, Leukocidin and Exfoliatin toxin.

Staph aureus can exist as normal flora on healthy people. They are often found in the nasal passages and the rectal area. Staph aureus is considered to be opportunistic in that it can become a pathogen under certain conditions.

Illnesses Produced by Staphylococcus aureus:

Boils and Pimples (folliculitis)
Food Poisoning
Septicemia (Blood Poisoning)
Toxic Shock Syndrome
Osteomyelitis (Invasion of the bone)
Surgical Wound Infections
Scalded Skin Syndrome

Historically Staphylococci were sensitive to penicillin however over the years, antibiotic pressure selected for those rare bacterial cells that could produce an enzyme capable of degrading penicillin. Now virtually all Staph aureus strains are resistant to penicillin and should be considered as such. Modified penicillin derivatives were created which were resistant to these enzymes. Hospital acquired infections are often caused by the antibiotic resistant strain (MRSA) and can only be treated with vancomycin or a newer alternative. Vancomycin is considered to be the last best antibiotics in the arsenal against Staph aureus. VRE or vancomycin resistant strains are now showing up leaving few alternatives for treatment. Until recently, infections acquired outside hospitals have been treated with penicillinase-resistant ß-lactams. However, many of the community acquired (CA) Staphylococcal infections are now methicillin resistant. These organisms are uniformly resistant to penicillins and cephalosporins. The infections have been treated with combination therapy using sulfa drugs and minocycline or rifampin.

So now I, the practitioner who would on any other day diagnose this sort of infection, now became the patient and had to put my diagnosis in the hands of my colleagues. They did not disappoint.


(1) The Medical Health Triangle - Three components compose what is known as the Medical Health Triangle with Doctors at the pinnacle. The Doctors are supported by a base of Nurses and Technologists.
Many specialties and disciplines are found within each group. As Technologists usually would have the least patient contact, their contribution to a patient’s health is under appreciated.

(2) Septicaemia - A condition where bacteria have invaded and are actively growing in the blood stream.

(3) MRI - Magnetic Resonance Imaging: a non-invasive method used to create images of physiological and pathological features within the body. Principles of action are beyond the scope of this post.

(4) MRSA - Methacillin Resistant Staphylococcus aureus - an acronym for this particular strain of bacteria which can defeat the action of the antibiotic of choice, methacillin/oxacillin. Greatly reduces the choices of antibiotics which can be used to cure this infection.

(5) Coccoid: Bacteria can exhibit a variety of shapes and forms. The two most common forms are “coccoid” round spheres (coccus) or elongated rods (bacilli).

Gram Positive Cocccus vs. Gram Negative Bacillus

(6) Plating - Specimens suspected of being infected with bacteria are “plated” or inoculated onto media which can assist in their identification. Media can serve several purposes:
a) Growth Media - encourages the growth of any bacteria present.
b) Selective Media - which will encourage the growth of suspect bacteria while inhibiting bacteria which are unimportant to the disease process.
c) Differential Media - have components such as indicators which allow the identification or detection of certain bacteria by their metabolic properties.

Staphylococcus aureus = Staph aureus = S. aureus

Further reading on Staphylococcus aureus