Alkaline hydrothermal treatment of gentamycin mycelial residues: characteristics of disintegration, solid-state fermentation, and antibiotic resistance genes reduction

The aim of this study was to promote the decomposition of gentamicin mycelial residues (GMRs) and removal of antibiotics resistance genes (ARGs) by alkaline hydrothermal treatment (AHT). The treated GMRs were also inoculated with fungi and converted into protein feed by solid-state fermentation (SSF). The conditions of AHT were reaction temperature (140–200 °C), residence time (0–60 min), and NaOH concentration (0–0.6 M). The effects of AHT on GMR organic release and ARGs, as well as on SSF of Candida utilis and Geotrichum candidum, were explored. The results showed that AHT under different conditions significantly reduced the particle size of the GMRs and promoted the release and solubilization of organic matter. However, both C. utilis CFU and G. candidum CFU kept decreasing as the reaction temperature increased. This was attributed to the fact that higher treatment temperatures resulted in the deterioration of nutrients and production of many non-biodegradable substances, which inhibited SSF of GMRs. Extended residence time is more conducive to promoting the release and hydrolysis of polysaccharides from GMRs, which is more suitable for C. utilis SSF. Increasing the concentration of NaOH can promote the release of more proteins in the GMRs, which can better satisfy the demand for high protein concentration in the fermentation matrix of G. candidum. In addition, AHT was able to reduce the abundance of most ARGs by untargeted disruption of gene structure, but it was difficult to remove plasmid gene IncN. High temperature, long residence time, and high NaOH concentration (> 0.4 M) are more favorable for the removal of ARGs. Considering the efficiency of SSF and the removal of ARGs, GMRs’ optimal treatment temperature of AHT was 140 °C. In addition, long residence time is suitable for C. utilis SSF of GMRs and high NaOH concentration is suitable for G. candidum SSF.