Survival Of Microbial Bacteria Essay

1506 words - 7 pages

Microbial world frequently face abrupt changes in the surrounding environment. For the survival in such a dynamic environment, it is advantageous to take decision of future action at the population level. Bacteria do so by a phenomenon called quorum sensing (QS), which is sensing the presence of other bacteria in surrounding environment, and taking the decision at certain threshold cell density. In QS active state, bacteria induces certain set of target genes at population level. QS among bacteria is mediated by autoinducers (AIs), whose concentration is a function of cell density, and above a certain concentration of cells AI mediate QS. There are three classes of AIs, (a) AI-1: Chemically ...view middle of the document...

AI molecules are frequently detected using QS biosensor strains. During last decade several biosensors have been developed to detect and quantify AI molecules. Most of the biosensors are designed to sense either AI-1 or AI-2. These biosensors are efficient in detecting presence of AI(s) in a bacteria, as well as in screening of QS inhibitors(Tello et al., 2013, Anbazhagan et al., 2012). A bacterial biosensor is a genetically recombinant bacterium, which harbors a QS genecircuit of interest coupled with an easily detectable and quantifiable reporter gene circuit. Reporter gene generates different classes of readable output including fluorescence, color pigments, and luminescence. QS molecules can also be screened using several analytical methods including HPLC and Mass Spectrometry(Rutherford and Bassler, 2012). But, in comparison to chemical based methods, biosensors are easy to use and inexpensive to construct and provide real time data in physiologically relevant environmental conditions. Most of quorum sensing biosensors express a reporter gene expressed from a QS responsive promoter. This promoter gets activated when a complex of AI and QS transcriptional activator bind with the promoter. Some biosensors respond to a very specific to a particular type of AI, and some respond to a group of AIs. Regularly used biosensors are mentioned in table 5.1.In following sections, we will first discuss several methods for the detection of AHLs, and then biosensors available to detect different classes of AIs.
5.2 Methods to screen AI producers
Several qualitative and quantitative methods have been developed during last decade to detect and characterizeAIs. Selection of suitable screening method depends on the type of study being carried out. Choice of biosensors also plays a role in screening of AHL producers. Some biosensors are very specific to a particular type of AI, and some responds to a range of AIs. So, it is recommended to use different classes of biosensors, if the physical and chemical properties of AI produced by the tester strain is unknown. In following sections, we will discuss commonly used detection techniques, along with their merits and demerits.
5.2.1 T-Streak plate method
T-streak plate is easy and rapid method to qualitatively screen AI producers. Here, biosensor and tester strains are streaked on agar plate forming a T-shape. If biosensor strain responds to AI produced by tester strain, it produces a particular type of visible output depending on reporter gene present in biosensor. Biosensors like Chromobacterium violaceum strain CV026, and biosensors with reporter lacZare compatible with this method(McClean et al., 1997). C. violaceum CV026 produces a visible violet pigment violacein.lacZ encodes enzymeβ-galactosidase, which breaks X-gal into a blue color product, which can be visually detected. As we discussed above, T-streak is rapid, easy to perform and does not require any instrument, but it reports only presence or absence...

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