Earthworm gut as an enabler for the dissemination of tetracycline resistance genes: evidence from controlled feeding experiments

Vermicomposting has been proven to be effective in combating tetracycline resistance genes (TRGs) in organic waste, such as animal manure and sewage sludge. However, the influences of tetracycline resistant bacteria (TRB) in organic waste during earthworm conversions on the fate of TRGs remain poorly understood. Hence, we prepared diets treated with either sensitive or insensitive tetracycline resistant bacteriome (STRB or ITRB) to earthworm midgut fluid and investigated the fate of bacterial communities and TRGs (including <i>tet</i>B, <i>tet</i>C, <i>tet</i>Z, <i>tet</i>L, and <i>tet</i>X) and their response to tetracycline during earthworm conversions in a controllable environment. Results showed that the bacterial composition of casts was highly complex, yet diet-derived bacteria were either minimal or undetectable. Notably, TRGs abundance in casts was dramatically higher than that in diets, indicating that the earthworm gut is a hotspot for TRGs dissemination. Furthermore, the increase in TRGs abundance was more pronounced in casts treated with ITRB compared to those treated with STRB, and this increase was suppressed by tetracycline exposure in casts treated with ITRB. This phenomenon may be due to the fact that diet-derived TRB and/or tetracycline alter the microbial community (e.g. relative abundance of <i>Aeromonas</i>). In conclusion, this study highlights the diet-derived TRB amplify the hotspot effect of earthworm gut on tetracycline-resistance gene dissemination, but regulated by tetracycline.