- • A variety of eARGs were ubiquitous in the ARGs-rich environments.
- • Contributions of eARGs and iARGs to the spread of antibiotic resistance were assessed.
- • The transformation efficiency of adsorbed eARG was higher than that of free eARG.
The emergence and spread of antibiotic resistance has pose a huge threat to both human health and environmental ecosystem. However, little is known regarding the pool of ARGs in extracellular DNA (eDNA). In this study ten ARGs (sul1, sul2, tetW, tetX, ermA, ermB, blaTEM, ampC, cat and cmr) and class I integron (intI1) in the sludge from hospital, pharmaceutical industry, wastewater treatment plant (WWTP), and swine manure, and sediment in urban lake in the form of both eDNA and intracellular DNA (iDNA) were evaluated by quantitative polymerase chain reaction (qPCR). The results showed that every gram of sludge dry weight contained from 7.31 × 103 to 1.16 × 1010 copies of extracellular ARGs (eARGs) and from 1.04 × 105 to 2.74 × 1012 copies of intracellular ARGs (iARGs). The sludge from hospital with the highest ratio of eARGs to total ARGs (11.02–89.63%), followed by the sediment from urban lake, implying that most of the ARGs in these regions were contributed by eARGs. The relative abundance of eARGs were higher than iARGs in sludge from WWTP and pharmaceutical industry, moreover, 1/3 and 5/9 detected eARGs were higher than the ARGs in the iDNA extracted from sludge of hospital and sediment from urban lake, respectively. Furthermore, the transforming ability of eARGs suggesting that adsorbed eARG is more preferentially coupled to the competent cells than free eARG. These findings highlight the need to focus attention on the contribution of eARGs to the dissemination of antibiotic resistance into environment, and also future needs in mitigating the spread of eARGs in the environment.