Effective photodynamic therapy against microbial populations in human deep tissue abscess aspirates

Abstract

Background and objective: The primary therapy for deep tissue abscesses is drainage accompanied by systemic antimicrobial treatment. However, the long antibiotic course required increases the probability of acquired resistance, and the high incidence of polymicrobial infections in abscesses complicates treatment choices. Photodynamic therapy (PDT) is effective against multiple classes of organisms, including those displaying drug resistance, and may serve as a useful adjunct to the standard of care by reduction of abscess microbial burden following drainage.

Study design/materials and methods: Aspirates were obtained from 32 patients who underwent image-guided percutaneous drainage of the abscess cavity. The majority of the specimens (24/32) were abdominal, with the remainder from liver and lung. Conventional microbiological techniques and nucleotide sequence analysis of rRNA gene fragments were used to characterize microbial populations from abscess aspirates. We evaluated the sensitivity of microorganisms to methylene blue-sensitized PDT in vitro both within the context of an abscess aspirate and as individual isolates.

Results: Most isolates were bacterial, with the fungus Candida tropicalis also isolated from two specimens. We examined the sensitivity of these microorganisms to methylene blue-PDT. Complete elimination of culturable microorganisms was achieved in three different aspirates, and significant killing (P < 0.0001) was observed in all individual microbial isolates tested compared to controls.

Conclusions: These results and the technical feasibility of advancing optical fibers through catheters at the time of drainage motivate further work on including PDT as a therapeutic option during abscess treatment.

Keywords: antimicrobial; bacteria; fungi; image-guided; methylene blue.

Figure 1. Methylene blue-PDT of bacterial clinical isolates

For each organism, three experimental groups (MB-Irradiated, MB-Shielded and Untreated-Irradiated) were assayed in duplicate in three separate experiments. Results are expressed as the mean ± S.D. of log₁₀ reduction in colony forming units (cfu) compared to the Untreated-Irradiated group. The absolute number of organisms in untreated groups = 1–3 ×10⁷ cfu/ml.

Figure 2. PDT of Candida tropicalis

Phenathiazine photosensitizer MB was compared to porphyrin photosensitizer TMP-1363. For each organism, three experimental groups (MB-Irradiated, MB-Shielded and Untreated-Irradiated) were assayed in duplicate in three separate experiments. Results are expressed as the mean ± S.D. of log₁₀ reduction in colony forming units (cfu) compared to the Untreated-Irradiated group. The absolute number of organisms in untreated groups = 1–3 ×10⁷ cfu/ml.

Figure 3. Methylene blue-PDT of polymicrobial aspirates in vitro

Each of the three experimental groups (MB-Irradiated, MB-Shielded and Untreated-Irradiated) was assayed in duplicate. Results are expressed as log₁₀ reduction in colony forming units (cfu) compared to the Untreated-Irradiated group. In each case, there was complete eradication of culturable organisms in irradiated samples treated with MB.