Microbiological analyses: antibiotic residues versus antibiotic elimination

Background: The aim of the study was to compare the direct inoculum method of corneal preservation media in the presence of antibiotics with the same method after elimination of antibiotics with the RESEP syringe.

Materials and methods: 32 samples of cold storage media (EUSOL-C) were obtained from the Eye Bank of Rome after donor cornea storage for up to 7 days. Aliquots of 3 ml of the media were either inoculated directly in trypton soy broth and fluid thioglycollate medium or treated with RESEP syringe to eliminate antibiotic residues and then inoculated in growth media. 10 samples of sterile broth, treated with RESEP syringe and tested by direct inoculum method, served as controls. Positive samples were identified genetically.

Results: 16 out of 32 tested samples (50%) resulted contaminated when assessed by the direct inoculum method without RESEP syringe treatment. The same samples treated with RESEP syringe showed 25/32 (78%) of positive samples. 28% of the positive samples, which were not detected without antibiotic elimination, were contaminated by Staphylococcus spp, Bacillus spp, Propionibacterium acnes, Pseudomonas spp and Candida spp. None of the control samples was contaminated.

Conclusions: The elimination of antibiotic residues from samples allowed to reveal an additional 28% of contaminated samples as compared to the direct inoculum method without elimination of antibiotics. Our study demonstrated that the antibiotics present in the corneal preservation media may interfere with the microbial growth during microbiological analyses by direct inoculum method and lead to false negatives.