All posts by Rashmi Madhavan

Genetic Variation of a Bacterial Pathogen within Individuals with Cystic Fibrosis Provides a Record of Selective Pressures

            In the paper from Lieberman et al, the bacterial pathogen B. dulosa was studied to find out whether mutations within the bacteria’s genome led to a genetic fix in the population or resulted in more genetic diversity. The two different models the hypothesis was based on were the dominant-lineage model, which suggested that that the presence of more beneficial mutations would eventually select for superior dominant lineages of genes, and the diverse-community model, which stated that adaptive lineages reached an intermediate frequency in the population, leading to general genetic diversity with many coexisting gene lineages. The study that the researchers conducted found that the B. dulosa that colonized in the patients with cystic fibrosis had been colonizing with multiple genetic lineages for at least 5 years, showing that they had been developing like that for quite some time. They additionally found that many of the selective pressures acting on the pathogens were the same across all of the patients, showing that the diverse mutations were fairly stable across the sample population. Overall, the study’s support of the diverse-community model allowed them to conclude that in bacterial populations, the beneficial mutations that emerge and compete with each other actually prevent any one dominant lineage from becoming fixed in the population, and rather evolve together to fight off common selective forces. 

The Drunken Monkey Hypothesis

The topic I chose for my oral presentation recently came back in the news, with the publication of the book The Drunken Monkey: Why We Drink and Abuse Alcohol by Dr. Robert Dudley, a professor of biology at the University of California Berkeley. The book goes into greater detail about the hypothesis I discussed in class about alcoholism stemming from the consumption of fermented fruit. The theory, which Dudley refers to as a the “Drunken monkey hypothesis,” proposes that our tendency to favor alcoholic drinks and become addicted to them stems from the fact that humans have developed a powerful sensory bias towards alcohol that relates it to nutritional reward. As detailed in an article in The Huffington Post by Dudley, titled “How Evolution Explains Why Humans Drink and Abuse Alcohol.” Primates would have eaten fruit living in more tropical climates, which would have exposed them to more rapidly ripening fruit that contains ethanol due to fermentation. The consumption of these fruits by primates and our subsequent evolution from them suggest that we may have developed a genetic adaptation that primes us to want to consume more of it. It makes sense that early primates would have been drawn to this as a food source in the first place – fruit flies were the first ones to figure out that the smell of alcohol indicated a good source of calories, and alcohol stimulates feeding in modern humans through the aperitif effect (apéritifs are alcoholic drinks that are served before meals to stimulate the appetite).

The drunken monkey hypothesis points toward an evolutionary mismatch in humans with our modern consumption of much higher levels and concentrations of alcohol, especially with the evolutionarily recent process of distillation. Animals in the wild do not get drunk, since fruit contain very low levels of alcohol. But it leaves humans with the evolutionary results of being drawn to alcoholic substances.

Sexual Relationships and Pathogen Evolution

The reading by Nahmias and Nahmias from earlier this month detailed the wide breadth of influencing factors that played a role in the spread of sexually transmitted infections (STIs) throughout human populations all over the world. Although the connection between sexual activity and disease was established a long time ago, the epidemiology behind STIs continues to be ever multi-faceted, making the development and execution of successful interventions very difficult. The paper also explained how the nature of the transmission allows for interesting trends in the evolutionary paths taken by the infectious agents. Many factors were cited as contributing to the spread of STIs, including travel, ecological change, economic inequality and public health issues, among many others, all of which created a complex web of interrelationships in the disease epidemiology.

The article “Love and Sex Influence Disease Evolution” talks about a study from August 2006 in The American Naturalist that examined in detail one of those particular influences on sexually transmitted pathogens. The scientists that authored the study, Eames and Keeling, concluded that the length of time sexual partners stay together has a significant influence on the evolution of multiple strains of a particular infection. It showed that some strains evolve as better suited for monogamous pairings with little chance of a rapid change in host environment (considered slow strains because they have the ability to persist in a host for a long time), while others are better adapted for short term relationship (considered to be fast strains because they cannot utilize resources in order to persist in one host). The study gives further proof for why different strains of a single infection can exist without selecting against each other.

The Controversy Surrounding Vaccination

Dr. Mina’s presentation from a couple weeks ago gave the class a look into the many controversies surrounding the administration of vaccines. With his studies revealing that live-attenuated vaccines correlating with a greater susceptibility to upper respiratory tract infections, the dilemma becomes more complicated in regards to whether or not to let this information influence people’s thinking on what is strongly accepted by the medical and scientific communities as recommended practice. Although there have been many controversies related to vaccines throughout history, the most recent and relevant trigger for the modern anti-vaccine movement is generally considered be a study by Andrew Wakefield in a 1998 issue of the UK’s The Lancet that the MMR shot (used to inoculate against measles, mumps and rubella) caused colitis and autism. This resulted in a widespread abandonment of vaccination in the UK and Ireland, as well as significant influence on public perception of vaccines. The study has since been very publicly declared fraudulent (due to manipulation of evidence and other forms of ethical misconduct), with heavy scientific backing from multiple subsequent studies that concluded that there was no significant evidence to support Wakefield’s conclusion.

A recent article in the Huffington Post by Jennifer Raff, a postdoctoral fellow at the University of Texas with a Ph.D. in genetics and anthropology, addresses those parents who still remain seduced by the arguments of the anti-vaccine community, primarily by utilizing a wealth of scientific evidence to back up every single one of her claims both against the anti-vaccine movement and in support of vaccination in general. By linking her words to many different scientific publications, Raff provides readers with concrete evidence to counter several claims made by the anti-vaccine movement, including papers that negate the idea that measles, chicken pox, influenza, and whooping cough are not dangerous or deadly, that natural infection is a better vaccination, that side effects are not well known, or that vaccines cannot be trusted for a whole host of reasons. On the flip side, she also provides evidence for how vaccines are tested with great scrutiny for effectiveness and safety, and most importantly, encourages people to do their research before subscribing to a mentality that is literally killing children.

Hygiene Hypothesis – New Developments

Dr. Garcia’s presentation last class on host-parasite interaction of gut microbes touched on what is known as the hygiene hypothesis. The hygiene hypothesis is a very relevant and interesting topic in connection to evolutionary medicine because it takes into account the history of human exposure to microbes and how our immunoregulatory circuits developed in relation to the presence of microorganisms, especially with gut and skin flora. There is even evidence of evolved dependence to some of these microorganisms due to coevolution. With the changes brought about by technological modernization, the decreasing presence of microbes in our immediate environment has exacerbated systems of inflammation to cause a growing set of chronic autoimmune diseases to emerge. This connection is described in great detail in the book The Hygiene Hypothesis and Darwinian Medicine, edited by Graham A. W. Rook, who is a prominent name in this field of study.

An article in Science Daily from March 2012 titled “Getting the Dirt on Immunity: Scientists Show Evidence for Hygiene Hypothesis” details new supporting evidence for the theory provided by Brigham and Women’s Hospital about this modern predicament. The hospital’s study specifically provided an underlying biological mechanism explaining the hypothesis for the first time in its history, using “germ-free mice” as models. These mice, which were completely lacking in bacteria or any other microbes, were compared to mice living in normal microbial environments and were found to have exaggerated displays of inflammation due to hyperactivity of a unique class of T cells previously linked to disorders such as asthma and colitis in the lungs and colon. Most important of the research’s findings was that exposure to microbes during the first years of life, even if they no longer were as adults, still led to normal immune function, showing the important of early immune conditioning to microbes.