Metagenomics can also help in understanding microbial competition and communication. This research may have far-reaching applications to all environments, whether they are within the digestive tract of humans or in the deep-sea vents in the oceans. Functional metagenomics can identify small molecules important to microbial survival, such as antibiotics. Metagenomic libraries can be assessed for antimicrobial activity using functional assays to identify new antibiotics. Additionally, sequence-based analysis of metagenomic libraries can identify synthases that make novel polyketides (antibiotics related to erythromycin and rifamycin). Other functional metagenomic screens have been used to identify quorum-sensing molecules. These are indicators of bacterial population density. Because many bacteria infect only eukaryotic cells or make toxins when they are present in sufficient numbers, interference with quorum sensing provides a new approach to antibacterial therapy. Thus, this area is of direct clinical importance. New quorum-sensing molecules have been identified by a detection system that used the reporter GFP. When clones expressed a quorum-sensing molecule, this activated the expression of GFP, making the bacteria fluorescent.