Validation of Gentamicin and Streptomycin Stability for On-Orbit Microbial Monitoring

The Genomic Enumeration of Antibiotic Resistance in Space (GEARS) flight project seeks to characterize the frequency and genomic identity of antibiotic-resistant organisms on the ISS and expand in-space sequencing-based diagnostic capabilities. This project selects for antibiotic-resistant microbes on space station surfaces using agar plates containing antibiotics, which can require storage of these plates for several months. However, little published data is available on the longevity of antibiotics in agar plates. Here, we describe the process of antibiotic selection and validation of antibiotic stability for GEARS. A literature review was conducted on several antibiotics for their intrinsic and acquired resistance in Enterococcus and Staphylococcus and stability in solution and in agar. Gentamicin and streptomycin were selected for a long-term study tracking changes in bactericidal ability of the antibiotics in agar plates. After 6.5 months of storage at 4°C, we found that both antibiotic types remained stable throughout the test period, although they both exhibited evidence of slight degradation. These findings indicated that either antibiotic would be sufficient for the GEARS study based on stability under cold stowage conditions. Furthermore, this study generated information on long-term stability of antibiotics in agar, which can be useful for microbiology applications beyond spaceflight studies.