You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Xenobiotics and the human gut microbiome: metatranscriptomics reveal the active players. The human gut microbiome plays an important role in the metabolism of xenobiotics. In a recent issue of Cell, Maurice et al. (2013) identify the active members of the gut microbiome and show how gene-expression profiles change within the gut microbial community in response to antibiotics and host-targeted xenobiotics.
{"Chemical": []}