Question

1. a) Describe the mutations that differentiate Y. pestis from Y. pseudotuberculosis, and what phenotypes these mutations confer.

b.) explain the basic types of evidence that shows vaccines work, and why the originally proposed connection between autism and vaccines has been debunked

Answer 1

1. a) Yersinia pestis and Y. pseudotuberculosis are closely related to each other, yet, show some genetic differences with each other. Y. pestis has evolved from the latter one almost 20,000 years back. The interesting fact though is, Y. pestis is more pathogenic in nature and causes plague (Mode of transmission-vector: fleas), compared to Y. pseudotuberculosis which causes gastrointestinal distress (Mode of transmission: contaminated water, food, aerosols, soil etc.)

Although 75% of the genetic material between aforementioned species show almost 97% similarity, still some features categorize them separately on the pathogenicity column.

The main type of mutation differentiating Y. pestis from Y. pseudotuberculosis is that the genome of the former one is constantly changing which includes loss and gain of genetic elements. One simple example could be transposable elements that are constantly changing their position, as well as being removed from the genome. Other type of mutation being genetic re-arrangements which include re-assortmens, recombinations etc. Another form of mutation is the transcriptional regulation which controls gene expression at a higher level, leading to pathogenesis or virulence of an organism.

The type of phenotypic characteristics imposed by these mutations is mainly the pathogenicity of the organism, host range (Y. pestis resides in an intermediate host, the fleas before it infects humans whereas Y. psudotuberculosis mainly predominates in contaminated food source, soil, water etc.), living conditions, mode of infection etc.

b.) Vaccines are engineered substances which contain part of or a whole material of a microorganism(e.g., bacteria, virus etc.) (either in dead or attenuated/live form) that helps in boosting or enhancing the both wings of an immune response- humoral and cell mediated which involve induction of B-cells (antibody producing cells) and T-cells (cell mediated) both.

The best and simple example can be given with vaccines against small pox. Edward Jenner, the father of immunology injected an 8 year old boy with cowpox lesion, which by the way is a dormant form of the disease, pox. Then after few days he injected the boy with actual small pox pustules. This as to his theory prevented the boy from dying. The reason behind this was to create a immune response in that boy's body which eventually produced immune cells to fight the foreign particle, the cowpox infectious agent. Then when the true immunogen (small pox pathogen) was injcted, body's immune cells created a heightened immediate immune response. This happened because of the production of memory cells from the B-cell lineage, also responsible for producing antibodies against specific antigens. As the two pathogens shared some level of similarity in their antigenic profile, the immune system recognized the small pox pathogen next time and eliminated from the body, providing full protection.

The reason for cancelling out the idea that vaccination causes autism, is because vaccination is given in childhood times after the child is born but research ha shown that autism is caused purely because of genetic disturbances which happens, of course, before birth, in the mother's womb. Reason for autism can be many, varied from- Parents' advanced age, pregnancy and birth complications, transfer of the changed genes from either of the parents to their child directly (also called genetic pre-disposition), spontaneous occurence of genetic changes in early embryo and/ or sperm or egg. Correlation of the two events of vaccination and acquiring autism both at an early age doesn't mean one is affected by the other.