Historically, b-lactam antibiotics are the more widely used antibiotics (class="squire-citation ltx_cite" class="ltx_cite"  data-bib-text="@article{Garau_2005, doi = {10.1128/aac.49.7.2778-2784.2005},  url = {http://dx.doi.org/10.1128/aac.49.7.2778-2784.2005},  year = 2005, 
...
author = {G. Garau and A. M. Di Guilmi and B. G. Hall},  title = {Structure-Based Phylogeny of the Metallo-~-Lactamases},  journal = {Antimicrobial Agents and Chemotherapy}  }" data-bib-key="Garau_2005" contenteditable="false">Garau 2005,   contenteditable="false" style="cursor: pointer">href="#Garau_2005">Garau 2005)

antibiotic href="#Van_Boeckel_2014">Boeckel 2014 ESAC 2009, ECDC 2012)

antibiotic  resistance is usually studied in clinical environments

The target of antibiotic resistance genes in natural environments should be considered, specially if we consider the evidence that these genes have a long evolutionary history in natural environments (class="squire-citation ltx_cite" class="ltx_cite"  data-bib-text="@article{Aminov_2009, doi = {10.1111/j.1462-2920.2009.01972.x},  url = {http://dx.doi.org/10.1111/j.1462-2920.2009.01972.x},  year = 2009, 
...
author = {Rustam I. Aminov},  title = {The role of antibiotics and antibiotic resistance in nature},  journal = {Environmental Microbiology}  }" data-bib-key="Aminov_2009" contenteditable="false" style="cursor: pointer"> contenteditable="false">  href="#Aminov_2009">Aminov 2009, class="squire-citation ltx_cite" class="ltx_cite"  data-bib-text="@article{Hall_2004, doi = {10.1016/j.drup.2004.02.003},  url = {http://dx.doi.org/10.1016/j.drup.2004.02.003},  year = 2004, 
...
title = {Evolution of the serine $\upbeta$-lactamases: past,  present and future},  journal = {Drug Resistance Updates}  }" data-bib-key="Hall_2004" contenteditable="false" style="cursor: pointer"> contenteditable="false">  href="#Hall_2004">Hall 2004, class="squire-citation ltx_cite" class="ltx_cite"  data-bib-text="@article{Garau_2005, doi = {10.1128/aac.49.7.2778-2784.2005},  url = {http://dx.doi.org/10.1128/aac.49.7.2778-2784.2005},  year = 2005, 
...
author = {G. Garau and A. M. Di Guilmi and B. G. Hall},  title = {Structure-Based Phylogeny of the Metallo-~-Lactamases},  journal = {Antimicrobial Agents and Chemotherapy}  }" data-bib-key="Garau_2005" contenteditable="false" style="cursor: pointer"> contenteditable="false">  href="#Garau_2005">Garau 2005). Thus, natural environments apear appear  such as a reservoir of potential ARGs to pathogenic bacteria (class="squire-citation ltx_cite" class="ltx_cite"  data-bib-text="@article{Berglund_2015, doi = {10.3402/iee.v5.28564},  url = {http://dx.doi.org/10.3402/iee.v5.28564},  year = 2015, 
...
author = {Björn Berglund},  title = {Environmental dissemination of antibiotic resistance genes and correlation to anthropogenic contamination with antibiotics},  journal = {Infection Ecology {\&} Epidemiology}  }" data-bib-key="Berglund_2015" contenteditable="false">Berglund 2015,   contenteditable="false" style="cursor: pointer">href="#Berglund_2015">Berglund href="#Versluis_2015">Versluis  2015) that cannot be ignored.

metageno mes ignored.

Metagenomic studies have been revel the presence of ARGs in different environments, including non anthropogenic impacted environments (Donato 2010, Yang 2013, Bhullar 2012, Segawa 2012, Forsberg 2014), but metagenoms  offer the opportunity to assess antibiotic resistance in non clinical environments

The presence and diversity of b-lacatamases in different environments could shed light about both b-lactamase gene transfer between environments and the anthropogenic impact in natural environments.

Data set

Shotgun metagenomic sequences obtained by Illumina sequencing process were used in this study and downloaded from two repositories, MG-RAST (http://metagenomics.anl.gov/) and EBI METAGENOMICS (https://www.ebi.ac.uk/metagenomics/;jsessionid=5B7CC5F7C4C8CD7C9FB5F7E8A4D1C621). A total of 232 metagenomes related to X different projects (Table S1) embedding 4.7 billion sequences, were retrieved after quality control steps performed by each repositorie. Each metagenome was associated with different sampling habitats including soil (undisturbed and agricultural soils), fresh water, ocean, glaciers, human gut, animal gut (rumen and feces), and effluents from wastewater treatment plants.

Construction of a comprehensive β-lactamases database 

Characterization of metagenome sequences associated to β-lactamase genes was preceded by construction of an  extensive β-lactamase database (EX-B) that integrated four clinically-important  publically-available databases: The Lahey β-lactamase database,