Bacterial-sensing circuits may be triggered by molecules originating from the environment (e.g., nutrients and chemoattractants). Bacteria also actively probe the environment for information by releasing molecular probes to measure conditions beyond the cell surface: a process known as telesensing. Perceiving the environment beyond is achieved by sensing environmentally induced changes in those probes, as occurs when a siderophore chelates an iron atom or a quorum-sensing signal is inactivated by a specific enzyme or adsorbent. This information, captured by chemical and physical changes induced in specifically produced molecules transiting through the environment, enables bacteria to mount a contextually appropriate response.