Fluorescence in situ Hybridization (FISH) in the Microbiological Diagnostic of Deep Sternal Wound Infection (DSWI).

Purpose: Postoperative mediastinitis after cardiac surgery is still a devastating complication. Insufficient microbiological specimens obtained by superficial swabbing may only detect bacteria on the surface, but pathogens that are localized in the deep tissue may be missed. The aim of this study was to analyze deep sternal wound infection (DSWI) samples by conventional microbiological procedures and fluorescence in situ hybridization (FISH) in order to discuss a diagnostic benefit of the culture-independent methods and to map spatial organization of pathogens and microbial biofilms in the wounds.

Methods: Samples from 12 patients were collected and analyzed using classic microbiological culture and FISH in combination with molecular nucleic acid amplification techniques (FISHseq). Frequency of and the time to occurrence of a DSWI was recorded, previous operative interventions, complications, as well as individual risk factors and the microbiologic results were documented.

Results: Tissue samples were taken from 12 patients suffering from DSWI. Classical microbiological culture resulted in the growth of microorganisms in the specimens of five patients (42%), including bacteria and in one case Candida. FISHseq gave additional diagnostic information in five cases (41%) and confirmed culture results in seven cases (59%).

Conclusion: Microbial biofilms are not always present in DSWI wounds, but microorganisms are distributed in a "patchy" pattern in the tissue. Therefore, a deep excision of the wound has to be performed to control the infection. We recommend to analyze at least two wound samples from different locations by culture and in difficult to interpret cases, additional molecular biological analysis by FISHseq.