“What antibiotic are you giving?” asked the Microbiologist in a surly tone, thinking this was going to be a simple case of wrong antibiotic.
“I’ve been using Co-amoxiclav, but it doesn’t seem to be getting better” was the reply.
“Have you sent any samples to the lab?”
“I swabbed the wound and the result has come back showing a Coagulase-positive Staphylococcus; I wonder if this might be Staphylococcus pseudintermedius or possibly Staphylococcus aureus” said the Surgeon.
“Staphylococcus aureus is the most common bacterium we see in this situation” answered the Microbiologist, “why do you think it might be Staphylococcus pseudintermedius?”
“In our patients Staphylococcus pseudintermedius is often grown from these types of samples…oh, I forgot to mention, I’m a Veterinary Surgeon and the 3 year old patient is a Husky” was the reply…. “I should say too that the bacterium is resistant to beta-lactams… according to the report…
Now, we’re very lucky. Not only do we have five really lovely cats but we have an excellent Veterinary Surgeon who looks after them and cares for them as though they were her own. At the end of every visit we always get in to discussions about microbiology and the latest infectious conundrum in other animals she is treating. It is really clear to all of us that we have so much to learn from each other, although my wife was once warned by a vet to never let a doctor “treat” her cats!
So what is Staphylococcus pseudintermedius?
Back in 1976 a chap called Hajek described a bacterium he had isolated from various animals including horses, dogs and pigeons which was similar to both Staphylococcus aureus and Staphylococcus epidermidis; as it fell somewhere between the two of them he called it Staphylococcus intermedius. When others started looking at dog bacteria in more detail they realised that what for many years had been identified as S. aureus was in fact S. intermedius, and that this new bacterium was the more common pathogen in dogs. So that should have been the end of the matter…. only it wasn’t.
In 2005 as molecular genetics became more widely available it became apparent that there were in fact three closely related bacteria all being called S. intermedius so they were named individually, S. intermedius, S. delphini (originally found in dolphins) and finally S. pseudintermedius (the bacterium pretending to be S. intermedius). These three bacteria are now known as the “S. intermedius group”. Don’t you just love all the changes in names of bacteria as a result of molecular genetics… I know, I don’t either!
So why before the use of molecular genetics did we get in such a pickle?
Identification of Staphylococcus spp.
To understand where all of the confusion has come from you first have to know some old fashioned laboratory microbiology. Back in the days before MaldiTOF and molecular genetics the identification of Staphylococcus spp. was based on five main tests:
- Gram film
- Slide coagulase
- Tube coagulase
If a bacterium was identified as a Gram-positive coccus by Gram film then it was likely to be either a Staphylococcus spp. or a Streptococcus spp. Staphylococci tend to produce clusters and streptococci produce chains in Gram films, but if it’s still not clear which you are looking at, the next test is a catalase test. In the catalase test the bacterium is mixed with hydrogen peroxide (H2O2). If the bacterium is able to produce the enzyme catalase it breaks the H2O2 into oxygen and water producing bubbles which is a positive test; staphylococci are catalase positive, streptococci are catalase negative.
So now we have our catalase positive Gram-positive coccus Staphylococcus spp. we have to try and work out if it is S. aureus or not. We do this because S. aureus is the main pathogen in the staphylococcus family causing skin and soft tissue infections, bone and joint infections, endocarditis and intravenous device associated infections. It can be a nasty wee-beastie if it gets into the wrong place. S. aureus produces two types of coagulase: slide coagulase (clumping factor) and tube coagulase (free coagulase). In both of these tests the bacterium is producing a compound that makes plasma coagulate.
In the final test, DNase, S. aureus produces an enzyme that breaks down DNA (a DNase) and so would be described as DNase positive. The bacterium was cultured overnight on an agar plate containing DNA and the following day the agar plate was flooded with hydrochloric acid. The acid caused the remaining DNA to precipitate out making the agar go cloudy, except around the S. aureus where the DNA had been broken down and the agar remained clear.
Back in the day we reported laboratory results with a Staphylococcus spp. as “coagulase-positive staphylococcus” (meaning S. aureus) or “coagulase-negative staphylococcus” (meaning NOT S. aureus).
Yawn!!!! Stick with this …
In fact there are coagulase-positive staphylococci that aren’t S. aureus but they used to be referred to as “coagulase-negative” to distinguish them from S. aureus even though this wasn’t strictly speaking correct. I’ve mentioned one of these before, Staphylococcus lugdenensis, but there are others and these include the S. intermedius group. I suspect they were reported in this way to basically say that they were not S. aureus which is the most important staphylococcus causing human infections; essentially this “simplified the message to Doctors”. Nowadays we give all of the staphylococci their individual names… I’m not sure this is really any better as the names probably don’t really mean much to the doctors on the wards either… oh well.
Below is a table of the different staphylococci identification tests for the S. intermedius group compared to S. aureus.
Are the Staphylococcus intermedius group really emerging pathogens?
Staphylococcus pseudintermedius is said to be part of the normal flora of up to 90% of healthy dogs. In dogs it is said to be the most common cause of skin and soft tissue infections including infections of prosthetic material (e.g. orthopaedic plates, screws and joints). Essentially S. pseudintermedius can do to dogs what S. aureus can do to their owners.
The first case of human S. pseudintermedius infection was described in 2006 in a patient with infective endocarditis. This would suggest that up until this time it had never caused an infection in a human BUT… remember the name was only changed in 2005. Up until 2005/6 all S. pseudintermedius was called S. intermedius and this WAS a recognised cause of infection in humans before this time.
To be fair S. intermedius was always consider a less virulent bacterium than S. aureus, and was mainly described causing the kinds of infections the true coagulase-negative staphylococci caused, mainly prosthetic material infection and endocarditis. But some of these could easily have been S. pseudintermedius.
More recent studies of human patients with S. pseudintermedius infections have shown an associated contact with dogs in about 95%, and it is now believed that most S. pseudintermedius infections are zoonotic in nature; that is they are related to contact with animals. There is probably something about dogs that favours colonisation with S. pseudintermedius over S. aureus (although dogs can get infections with S. aureus too, quite possibly from their colonised owners!). One theory is that dogs have a more acidic skin than humans and this may favour S. pseudintermedius.
I suspect that these bacteria have recently genetically diverged (within a few hundreds to thousands of years… which is recent in terms of evolution… honest!) as each has become more adapted to its specific host; S. pseudintermedius to dogs and S. aureus to humans, S. delphini to dolphins! They are just too similar to my mind for there to be any other reason for these similarities.
Nowadays we use techniques like MaldiTOF and whole genome sequencing to identify bacteria and these methods are not tricked into calling S. intermedius a S. aureus but it is possible that as these new technologies “emerged” into everyday laboratory microbiology we became aware that what we have previously called one thing is actually another instead. It may not be the bacterium that is emerging as a pathogen but the technology emerging is now able to identify a pathogen that may have been there all along.
At the end of the day the treatment of Staphylococcus spp. is dependent on sensitivity testing. Whatever the staphylococcus is, in humans, if it is sensitive to Flucloxacillin then it should be treated first line with Flucloxacillin; if it is resistant then something like Teicoplanin or Vancomycin might be a suitable alternative.
In dogs the situation is somewhat different! (Note: Doctors should not treat dogs or cats). The first line beta-lactams are usually cephalosporins or Co-amoxiclav, but with the rise in Meticillin-resistant Staphylococcus pseudintermedius, alternatives such as Clindamycin and fluoroquinolones or tetracyclines are often required.
The isolates from our “puppy” patient was susceptible to Erythromycin and Clindamycin and therefore the dog was treated with Clindamycin orally. The wound healed nicely and the pooch made a full recovery.
P.S. Cute puppy huh?! :-)