Metric and air sampling

Soil assessment

The metric for assessing contamination of soil is the number of bacteria or genes per gram of soil or per millilitre (in a dilution made of 1 gram of soil).

Soil is full of bacteria and can therefore be a complex compartment to measure. When sampling soil, it is imperative to pool soil subsamples to obtain a representative sample. Furthermore, because soil is often analysed to assess the role of fertilisation with animal manure, information on the state of fertilisation (such as the last fertilisation event or the type of manure) is needed.

Standardised soil sampling is described in ISO 18400-104:2018. Detection of AMR is often carried out using molecular techniques identifying ARGs.

Air assessment

The metric for assessing contamination of air is bacteria or genes per cubic metre (m³) air or per time collected.

Sampling air often takes considerable time to collect enough DNA or bacteria to be able to analyse in the lab. Air can be sampled actively with pumps or passively with dust collectors or agar settle plates that, when left uncovered, will be colonised by bacteria falling from the air.

During the hours of sampling, many litres of air or fresh settled dust will be sampled: this amount is needed to have enough bacteria or genes to measure.