Hello everyone, I am Dr. A. Pajma, a Stain Professor, Department of Biochemistry, Bowen-Svegan and the College, Saini Kpuri, Second Abed. In this video, we will be looking at phase variation in Salmanilla. The contents covered in this video are understanding the term phase variation, structural features of Salmanilla, flasalum proteins, mechanism of phase variation, and importance of phase variation. Coming to the term phase variation, as we all know that bacteria have thousands of proteins encoded in their genes and these genes. They call for variety of proteins which includes the enzymes needed for metabolism and the proteins which are involved in sticking to their host and also the proteins to make up the flasilla for their motility and many more proteins which are essential for their survival in the host. And expression of some of these proteins can be interchangeably switched on and off. This process is known as phase variation and phase variation is reversible process which helps the bacteria to quickly generate them diversity among the population and to adapt to rapidly changing environments and to where immune response of the host. As like many other bacteria, even the Salmanilla entry curve also use this technique to switch between different types of the flasolin protein and as a result, flasilla with different structures are assembled. And coming to the structural features of this Salmanilla flasilla and all these Salmanilla species are non-spoir forming, predominantly motel-entero bacteria with cell diameters ranging from 0.7 to 1.5 micrometers and lengths from 2 to 5 micrometers and they all have the peritrecus flasilla that means all the body will be surrounded by the flasilla. And among is all the Salmanilla species in a specific example that is Salmanilla entry curve, 0- Typhymurium which has approximately 6 to 10 flasilla that are arranged around the cell peritrecusly. And coming to the structure of the individual flasilla, each individual flasilla is composed of three distinct substructures. And if you observe this diagram, this consists of three different substructures. The first one is called as a basal body and a transmembrane motor. I hope that links the motor and the filament and the last one is the filament. So, this is the basal body and this is the transmembrane motor and this is the hook and this is the filament. And this diagram is the electron micro scope diagram of the Salmanilla typhymurium where it is harboring the human cells where we can see the flasilla around the rod shaped bacterium and all the bacteria that are harboring the human cells. And each filament is approximately 10 micrometers in length and composed of so many subunits of the flasolin protein, nail it 20,000 subunits of the flasolin protein either of the type flasolin B. Any one of the flasolin protein only will make up the flasilla either C or B in a particular bacterium. The flasolin C and the flasolin B proteins are identical in their first 71 amino acids and also in the last 46 amino acids. So, where the difference is coming is in the middle of the amino acid sequence of the total protein of either flasolin C or the flasolin B where the divergence is present and which results in the distant antigensities. And most of the Salmanilla species, the alternate between the expression of these two different flasolin genes, they are either flasolin C or the flasolin B at a rate of 10 to the power of minus 3 to 10 to the power of minus 5 per cell generation. So, it is not so frequently happening only at the rate of 10 to the power of minus 3 to 10 to the power of minus 5 per cell generation. And coming to the mechanism of the phase variation, this happens by a specific phenomenon called as site specific DNA inversion. And if we look at this diagram, here we can see the genes which codes for the flasolin B along with another gene which is called as the flasolin A. These two genes are controlled by single promoter. So, this is like an operand regulated by the promoter with the sequence which is oriented in this way. And this promoter is been flanked by two recombination sites called as the Hix L. This is the Hix L and the left side and Hix R on the right side. When the promoter is in this particular orientation, it is able to transcript the genes of the flasolin B and A. And the respective proteins, flasolin B protein and the flasolin A proteins will be synthesized from the respective mRNA. And this flasolin A is not any flasillar protein, but it is in a repressor for another flasillar protein gene that is the flasillar flasolin C gene. So, if we observe here, this flasolin A protein goes and binds to them promoter operator region of the flasolin C gene and putting this gene to off. So, at a particular point of time, we can see only flasolin B is expressed, not the flasolin C is expressed. And coming to the phase variation by process of the DNA inversion at the site specific of these two recombination sites with the help of a specific enzyme called as the hind recombination, which helps in the inversion of this particular DNA segment that is a promoter region in this orientation, this promoter is no more able to transcript the operand, which consists of the flasolin B and flasolin A genes. So, neither flasolin B protein will be synthesized, neither flasolin A. So, once there is no presence of the flasolin A to repress the flasolin C gene, this flasolin C gene will be expressed and ultimately flasolin C protein will be synthesized. So, if we observe here again flasolin B protein is not synthesized, but at the same time from the other segment of the DNA, we can see the flasolin C is been expressed. So, that is the mechanism by site specific DNA inversion. And what is the use of this kind of the phase variation in the bacteria is basically to evade the immune response from the host, this is called as the immune evasion. There are many studies recently also showing that the flasolin proteins, either of the B at the C, they will see the immune response especially by the stimulation of the toe-like receptor and which in turn leads to the mobilization of the nuclear factor NFK and stimulation of the tumor necrosis factor alpha. And this tuber necrosis factor alpha is involved in the regulation of immune cells. So, what we are understanding from this is the flasolin proteins are able to stimulate the immune response in the host system. And most telymphocytes, especially of the CD4 plus telymphocytes which are generated again is the infection with the salmonella, they are directed towards the salmonella flasolin epitopes. So, flasolin epitopes are capable of illustrating the immune response even with respect of the telymphocytes. So, if the host is able to generate the immune response, the bacteria looks to evade by changing the flasolin proteins from one type of the flasolin proteins to the other type of the flasolin proteins. So, the host takes time to again elicit the immune response by the time the bacteria will be able to multiply and survive in the host system. That is the advantage for the bacteria by having this phase variation by evasion of the immune response from the host. Though there are evidences that the immune deviation by the phase variation is being operated by the flasolin proteins, its role in the pathogenesis is need to be well understood further because there is there are no studies where this flasolin proteins themselves are the causative factors for the pathogenesis of the salmonella. There are other antigens which are more responsible for the pathogenesis and the resources are taken from these mention. Thank you.