We present a new platform for visual and spectroscopic detection of bacteria. The detection scheme is based on the interaction of membrane-active compounds secreted by bacteria with agar-embedded nanoparticles comprising phospholipids and the chromatic polymer polydiacetylene (PDA). We demonstrate that PDA undergoes dramatic visible blue-to-red transformations together with an intense fluorescence emission that are induced by molecules released by multiplying bacteria. The chromatic transitions are easily identified by the naked eye and can also be recorded by conventional high-throughput screening instruments. Furthermore, the color and fluorescence changes generally occur in shorter times than the visual appearance of bacterial colonies on the agar. The chromatic technology is generic and simple, does not require identification a priori of specific bacterial recognition elements, and can be applied for detection of both gram-negative and grampositive bacteria. We demonstrate applications of the new platform for reporting on bacterial contaminations in foods and for screening for bacterial antibiotic resistance.The emerging global risks of bioterrorism, the recurring incidents of bacterial food contaminations, and the need to monitor sterile environments in health care applications and other industries have pushed the development of methods for detection of pathogens to the forefront of technological and scientific research. Numerous technologies for reporting on bacterial presence have been developed (3,5,9,25,26). There are, however, limitations to existing bacterial detection techniques as rapid and generic approaches. Specifically, many bioanalytical techniques employed for pathogen detection (such as culture-based methods) provide results after relatively long time spans (several hours to days) (10). Other currently employed technologies often involve complex detection mechanisms that require specialized instrumentation, application by trained personnel, and the need for active operation (addition of reagents, initiation of chemical reactions, etc.), which overall do not lend their use in settings other than laboratory environments (3, 6). Furthermore, a prerequisite for many detection methods is the detailed understanding of the biochemical and structural properties of the bacterial species sought, limiting applications in the case of unknown pathogens or variants (9, 31).Various membrane-active compounds are released by bacteria to their environments (2, 30), a process that often has an essential functional role as a means for overcoming host defense mechanisms, allowing colony proliferation, and facilitating bacterial communication (17). Membrane-active peptides and toxins, in particular, are produced by bacteria, for example, pneumolysins secreted by streptococci (27) and ␣-toxin, which is the major cytolysin emitted by Staphylococcus aureus (1). Secretion of pore-forming exotoxins by bacteria is abundant, and endotoxins, such as lipopolysaccharides, which are often released by gram-negative bacteria, ...