Bacteria were among the first life forms to appear on the face of the Earth and have the uncanny ability to overcome even the harshest of environments, from acidic hot springs to the ice-fields of the poles and deep portions of the Earth’s crust. They are so abundant, that they make up the majority of the total biomass on earth. Astonishingly, nearly 90% of the cells that move around with humans are actually bacteria , though since they are so much smaller than human cells, they make up only about 10 percent of our body-weight.
Sometimes bacteria are useful, sometimes not. For instance, we use them to make cheese, sauerkraut, and vinegar, to clear our polluted water and oil spills, as biological pesticides, or through genetic engineering to make insulin, growth factors, or antibodies. Critically, they also form an integral part of our immune and digestive systems. However, some bacteria infest plants and cause harvest disasters, grow in foodstuffs and cause fouling, infect production units for pharmaceuticals or agrichemicals, and most importantly cause diseases, such as diphtheria (Corynebacterium diphtheriae) tetanus (Clostridium tetani), cholera (Vibrio cholerae), tuberculosis (Mycobacterium tuberculosis) and even the “Black Death” (Yersinia pestis).
It is therefore not surprising that in many fields of medicine, biotechnology, and the food industry, the rapid detection of harmful bacteria is necessary to initiate appropriate control, disinfection, safety or treatment procedures. Normally, the presence and type of bacteria is tested for via standard microbiological and biochemical methods, or by detecting bacterial “fingerprints” based on their genetic or metabolic profiles. What all these methods have in common is that they are moderately elaborate and time-consuming. Recently, “artificial noses” that sniff out particular volatile chemicals released by bacteria have emerged in an attempt to overcome some of the limitations of more traditional methods. However, with these novel techniques, the specific and semi-quantitative detection of bacterial volatile chemicals is still a challenge, as are the costs involved.
In Discriminating Bacteria with Optical Sensors Based on Functionalized Nanoporous Xerogels, Crunaire and co-workers  describe a novel sensor based on a Xerogel matrix, which detects indole released from bacteria (over 85 species, including many pathogens). Indole is a metabolite that is produced by enzymatic splitting of the amino acid tryptophan into pyruvic acid, ammonia and indole. The indole can be detected colorimetrically after formation of the strongly colored, green-blue, azafulvenium chloride salt by reaction with Ehrlich’s reagent (p-dimethylaminocinnamaldehyde). The authors immobilized Ehrlich’s reagent in the hybrid organic-inorganic nanoporous matrix of the Xerogel.
A major advantage of Xerogels is the fact that the nanopore diameter can be controlled in such a way that indole is trapped in the matrix, but tryptophan is rejected, thereby avoiding interference by tryptophan during analysis. Furthermore, the sponge-like nature of the Xerogel concentrates indole and enhances the detection reaction, because the many cavities act as small isolated reactors.
Overall, this novel bacterial sensor offers several advantages over currently available indole detection methods, including low cost, non-interference by tryptophan, high stability, high sensitivity, and the ability to detect trace amounts over long periods of time. Particular application areas might include the monitoring of infections in industrial plants, for counter-terrorism purposes, hospital environments, clinical labs, and as rapid tests in community medical practices.
- Dykhuizen, D., Species Numbers in Bacteria. Proc Calif Acad Sci 2005, 56, (6 Suppl 1), 62-71
- Crunaire, S.; Marcoux, P.; Ngo, K.-Q.; Moy, J.-P.; Mallard, F.; Tran-Thi, T.-H., Discriminating Bacteria with Optical Sensors Based on Functionalized Nanoporous Xerogels. Chemosensors 2014, 2, (2), 171-181