Preliminary Study On The Source Tracking Of Antibiotic Resistance Genes Contamination In Lettuce

The soil-crop system is an important pathway for the spread of antibiotic resistance genes(ARGs)from environment to human,and the ARGs in edible parts of stems,leaves and fruits are an important source for human exposure.Exploring the ARG transfer pathway and the dominant host bacteria in soil-crop system is beneficial to the precise control of crop ARGs pollution.Here,greenhouse pot experiments were conducted to analyze the contributions of internal(endosphere-root endosphyte-leaf endosphyte-phyllosphere)and external(soil-air-phyllosphere)transfer pathways to the ARGs accumulation in the lettuce phyllosphere.Phyllosphere resistome and the location of ARGs in chromosome and mobile gene elements in host bacteria were also characterized.The main conclusions are as follows:(1)The soil-air-phyllosphere pathway contributed the most to phyllosphere ARGs.The total abundances of six typical ARGs(tet M?tet A?tet Q?tet W?sul1?sul2)were 3.06×10^-4and 2.74×10^-5copies/16S r RNA copies in the rhizosphere and phyllosphere,respectively.After blocking the soil-air-phyllosphere pathway by mulching and the background air-phyllosphere by air-filtering,total ARGs in the phyllosphere reduced by 80.7%?98.7%(89.5%on average).Film mulching is not only useful for soil heat and moisture preservation,but also has the potential in controlling phyllosphere ARGs pollution.(2)After blocking the external transfer pathway and the input of background air ARGs,there were 127 and 89 ARGs in the rhizosphere and phyllosphere,respectively.Multidrug and macrolide-lincosamide-streptogramin B(MLSB)resistance genes were relatively abundant.More than 80%of ARGs were located on the chromosomes of host bacteria,indicating relatively low horizontal gene transfer(HGT)potentials.Pseudomonas sp.7SR1 was identified as the dominant ARG host,which accounted for 1.0%and 47.1%of the total bacteria in the rhizosphere and phyllosphere,respectively.Identifying the dominant ARGs and host bacteria would further benefit the targeted control of ARGs pollution in phyllosphere and reduce the risk of human exposed to ARGs.