May 16th, 2011 by Warren S. Joseph DPM FIDSA

For a number of years I have referred in my lectures to the word I have termed “Pseudomonaphobia”.  Basically, this is an irrational fear of the organism Pseudomonas aeruginosa (PA) when isolated from a culture of a wound in the foot.  I really believe that this arises in most Podiatrists during the residency interview process when, without fail they are asked by the interviewer “How would you treat a Pseudomonas infection in the foot?”  I remember being prepped for that question way back in 1981 when we were instructed that the correct response was “You use a combination of gentamicin and carbenicillin”.  Unfortunately, that was incorrect even back then since the more “current” schools were teaching their students to respond with a combination of tobramycin & ticarcillin, the so-called T&T therapy. (This may explain why I did not get my first choice of residency program!)  Fortunately, today the well prepared student should be able to recite about a dozen different, non-aminoglycoside options.  But, that is beside the point.  It really is a moot issue.  You see, despite the ability to readily culture the organism from lower extremity wounds, PA is RARELY a pathogen in lower extremity infections.   In fact, I would go as far as saying that about the only time PA should be empirically considered pathogenic is in a case of osteomyelitis following a puncture wound. 

Pseudomonas colonizing a heel wound

A number of randomized, controlled clinical trials have actually substantiated this viewpoint.  Most have looked at the clinical outcome of patients who grew PA from a wound/infection when treated empirically with an antibiotic that was ineffective against PA vs. one which had anti-pseudomonal activity. In this entry I will present 3 such trials.  

In 2002 Graham, et.al. published the results of the pivotal phase III trials comparing ertapenem to piperacillin/tazobactam for complicated skin and skin structure infections (PubMed link: http://www.ncbi.nlm.nih.gov/pubmed/12015692).  Of these two antibiotics ertapenem has no inherent anti-PA activity while pip/tazo does.  Despite this difference in spectrum, the positive clinical response in patients growing PA was 70% for ertapenem and 60% for pip/tazo.  In a very similar trial comparing the same two antibiotics specifically for diabetic foot infection, Lipsky et. al. had similar outcomes with a positive clinical response in 76.9% of patients on ertapenem vs. 70% for pip/tazo. (The SIDESTEP trial http://www.ncbi.nlm.nih.gov/pubmed/16291062).  More recently, in September 2010, Corey, et. al.  published the results of the clinical trials on the new anti-MRSA cephalosporin ceftaroline .  This drug has no inherent anti-PA activity yet there was a positive clinical response in 80% of the patients from which the organism was isolated.   

None of these trials are meant to show that the study drug had some previously undiscovered anti-PA activity.  They just point to the role of PA as a notorious colonizer of lower extremity wounds/infections. Nannini stated it quite succinctly in his review paper of ceftaroline (http://www.ncbi.nlm.nih.gov/pubmed?term=nannini%20ceftaroline):

“The demonstration of efficacy in patients with P. aeruginosa receiving ceftaroline, a pathogen against which ceftaroline has little activity, most probably reflects the presumptive role of P. aeruginosa as a colonizer rather than a true pathogen in many of these infections.”

Please do not misinterpret me.  I am not saying that PA is always a colonizer and is never pathogenic.  If this bug is found from a reliable, deep surgical culture, consideration should be given to covering it. However, this is a ubiquitous organism that has been found in tap water and on plants and vegetables.  Just culturing it from a superficial wound swab will lead the clinician to treat an organism that can most likely be handled with thorough debridement and topical therapy, with an anti-PA antibiotic that may be unnecessary.  This may increase resistance of the organism against these antibiotics so that they are not available for the next time…when we may really need them.

Posted in Antibiotics, Diabetic Foot, Infections, Osteomyelitis

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