Is Pseudomonas A Legitimate Pathogen Or A Notorious Colonizer In Lower Extremity Infections?

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 when it is isolated from a culture of a wound in the foot. I really believe this arises in most podiatrists during the residency interview process when, without fail, the interviewer asks them, “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 and 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 our ability to readily culture the organism from lower extremity wounds, P. aeruginosa is rarely a pathogen in lower extremity infections. In fact, I would go so far as to say that about the only time P. aeruginosa should be empirically considered pathogenic is in a case of osteomyelitis following a puncture wound.

A Closer Look At The Literature Findings On Pseudomonas

A number of randomized, controlled clinical trials have actually substantiated this viewpoint. Most have looked at the clinical outcome of patients who grew P. aeruginosa from a wound/infection when they were treated empirically with an antibiotic that was ineffective against P. aeruginosa versus one that had anti-pseudomonal activity.

In 2002, Graham and colleagues published the results of the pivotal phase III trials comparing ertapenem (Invanz, Merck) to piperacillin/tazobactam (Zosyn, Pfizer) for complicated skin and skin structure infections.1 Of these two antibiotics, ertapenem has no inherent anti-P. aeruginosa activity while pip/tazo does. Despite this difference in spectrum, the positive clinical response in patients growing P. aeruginosa was 70 percent for ertapenem and 60 percent for pip/tazo.

In a very similar SIDESTEP trial comparing the same two antibiotics specifically for diabetic foot infection, Lipsky and co-workers had similar outcomes with a positive clinical response in 76.9 percent of patients on ertapenem versus 70 percent for pip/tazo.2

More recently, in September 2010, Corey and colleagues published the results of the clinical trials on the new anti-MRSA cephalosporin ceftaroline (Teflaro, Forest Laboratories).3 This drug has no inherent anti-Pseudomonas aeruginosa activity yet there was a positive clinical response in 80 percent of the patients from which the organism was isolated.

None of these trials is meant to show that the study drug had some previously undiscovered anti-Pseudomonas aeruginosa activity. They just point to the role of P. aeruginosa as a notorious colonizer of lower extremity wounds/infections. Nannini stated it quite succinctly in his review paper of ceftaroline: “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.”4

Please do not misinterpret what I am saying. I am not saying that P. aeruginosa is always a colonizer and is never pathogenic. If this bug is present in a reliable, deep surgical culture, one should consider 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 with an anti-Pseudomonas aeruginosa antibiotic, which may be unnecessary when the wound can most likely be addressed with thorough debridement and topical therapy.

This may increase resistance of the organism against these antibiotics so they are not available for the next time … when we may really need them.

References

1. Graham DR, Lucasti C, Malafaia O, et al. Ertapenem once daily versus piperacillin-tazobactam 4 times per day for treatment of complicated skin and skin-structure infections in adults: results of a prospective, randomized, double-blind multicenter study. Clin Infect Dis. 2002; 34(11):1460-8.

2. Lipsky BA, Armstrong DG, Citron DM, et al. Ertapenem versus piperacillin/tazobactam for diabetic foot infections (SIDESTEP): prospective, randomised, controlled, double-blinded, multicentre trial. Lancet. 2005; 366(9498):1695-703.

3. Corey GR, Wilcox M, Talbot GH, et al. Integrated analysis of CANVAS 1 and 2: phase 3, multicenter, randomized, double-blind studies to evaluate the safety and efficacy of ceftaroline versus vancomycin plus aztreonam in complicated skin and skin-structure infection. Clin Infect Dis. 2010; 51(6):641-50.

4. Nannini EC, Stryjewski ME, Corey GR. Ceftaroline for complicated skin and skin-structure infections. Expert Opin Pharmacother. 2010; 11(7):1197-206.

Editor’s note: This blog was originally published at https://www.leinfections.com/antibiotics/%E2%80%9Cpseudomona-phobia%E2%8… and has been adapted with permission from Warren Joseph, DPM, FIDSA, and Data Trace Publishing Company. For more information about the Handbook of Lower Extremity Infections, visit www.leinfections.com/ .