Pseudomonas fluorescens

Pseudomonas fluorescens is not generally considered a bacterial pathogen in humans; however, multiple culture-based and culture-independent studies have identified it at low levels in the indigenous microbiota of various body sites. With recent advances in comparative genomics, many isolates originally identified as the “species” P. fluorescens are now being reclassified as novel Pseudomonas species within the P. fluorescens “species complex.” Although most widely studied for its role in the soil and the rhizosphere, P. fluorescens possesses a number of functional traits that provide it with the capability to grow and thrive in mammalian hosts. While significantly less virulent than P. aeruginosa, P. fluorescens can cause bacteremia in humans, with most reported cases being attributable either to transfusion of contaminated blood products or to use of contaminated equipment associated with intravenous infusions. Although not suspected of being an etiologic agent of pulmonary disease, there are a number of reports identifying it in respiratory samples. There is also an intriguing association between P. fluorescens and human disease, in that approximately 50% of Crohn's disease patients develop serum antibodies to P. fluorescens. Altogether, these reports are beginning to highlight a far more common, intriguing, and potentially complex association between humans and P. fluorescens during health and disease.

Over the past 15 years, the application of culture-independent methods for microbial identification has revealed a previously unappreciated complexity within human-microbe interactions. One interesting feature is that a number of these studies have identified the bacterium Pseudomonas fluorescens as a low-abundance member of the indigenous microbiota of various body sites, including the mouth, stomach, and lungs (1,–5). P. fluorescens has generally been considered nonpathogenic for humans, an assessment dating back to its earliest descriptions, by A. Baader and C. Garre, in Über Antagonisten unter den Bacterien (1887)

The bacillus [P. fluorescens] itself is not pathogenic. A culture applied to animals subcutaneously or injected into the peritoneum does not elicit a reaction. Even when introduced many times into fresh wounds it does not irritate healing by primary intention. Also, ingestion of cultures caused no harm to my stomach or intestines.

However, while far less virulent than P. aeruginosa, P. fluorescens can cause acute infections (opportunistic) in humans and has been reported in clinical samples from the mouth, stomach, and lungs. The most common site of P. fluorescens infection is the bloodstream. Most reported cases have been iatrogenic, with bacteremia attributable either to transfusion of contaminated blood products or to use of contaminated equipment associated with intravenous infusions. While not suspected of being an etiologic agent of pulmonary disease, we recently reported that P. fluorescens is routinely cultured at a low frequency from clinically indicated respiratory samples. Perhaps the most intriguing “association” between P. fluorescens and human disease is that approximately 50% of Crohn's disease patients develop serum antibodies to the I2 antigen encoded by P. fluorescens, and in some studies, this seroreactivity has correlated with the success of therapies aimed at the microbiome rather than the immune system. Altogether, these reports and others are beginning to highlight a far more common, and potentially complex, interaction between humans and P. fluorescens during health and disease.