BEACON Researchers at Work: Black in Science

This week’s BEACON Researchers at Work blog post is by Bradley Watson. Bradley spent two summers working as an undergraduate researcher at Kellogg Biological Station with BEACON faculty Kay Gross, and is now a master’s student at University of Nebraska Omaha. The original post appears on his blog here

I was writing this in a different mindset and with a different goal until Ferguson, MO happened. That ongoing tragedy popularized by racial tensions but equally influenced by economic inequality made me want to finish this little essay. I had forgotten how sheltered I was by my environment but I still think my experiences are relevant and representative of the struggles of other blacks who are infiltrating fields that were/are dominated by white people.

Bradley Watson and posterI was in a lab meeting this summer and offered to drive some samples of the prairie plants we study back to Omaha with me if we needed more time to process them. Our PI said that she could get the biomass processed here in Michigan and joked that I would be a huge target for the cops driving through Chicago in a car full of dried plants. All six of us at the lab meeting (me being the only black person) laughed our heads off. We extended the hypothetical situation into stories about me trying to explain that the dried grass in the black plastic bags was for an experiment leading the police to call our PI to ask if this story was true. Someone even said the word marijuana… IN A LAB MEETING! At the end of it all I was taken aback. My boss, a 60 something year old white Christian lady from the heartland of America had put herself in my shoes and recognized that things were a little different for me.

She used the phrase, “a black man.” Many people shy away from blatant acknowledgement of racial differences but she didn’t and I respect her for it. I have been one of a few blacks and often the only black person in my classes for a long time. I left the Bahamas where I was in the racial majority for school in Canada where I was often mistaken for being Nigerian or Jamaican but not really discriminated against because that country is so diverse. After a year there I moved to a school in Charleston, South Carolina. I knew racism was historically a part of southern culture but I always thought that being international, well spoken, and well-mannered (by my nature and to the point of being timid sometimes) meant that I wouldn’t be a victim of this racism… wrong! I remember the first girl to cross the street as I approached. It was on a well-lit side walk at around 10 pm. I was shocked when I realized that she was just avoiding walking past me but I rationalized it; she could just be avoiding the threat of a male instead of the threat of a black. Then there was a night when I was escorting a white female friend home from a party and stopped two fratastic guys from harassing her. They left her alone and laughed at me… slapping me on my ass as I walked by. I chalked that one up to their inebriation and the idiocy encouraged in some fraternities. One thing I could never justify was why blacks made up 10% of the population in the US but my school had nearly 100% black janitorial staff and more than 95% white professors. I remember just one black Biology professor and I’m only calling him black because it takes just a little yellowing of the skin or crimping of the hair to be perceived as such! The final wake up call for me was in a plant taxonomy class surrounded by white peers whom I got along extremely well with. I forgot how we got to this point in the conversation but the professor, another older white lady, looked at me and said something like “Do you know how long they tried to keep your people out of this school?” (The best use of “your people” I have ever heard, btw.) It was a shock for me; she recognized this bias and felt strongly about it while I was trying to ignore it. But what choice do I have? Am I supposed to think about my race all the time?

I think the answer to that is no, but I have to be aware of it. I just started a teaching assistantship and had some trouble with my paperwork… I admit that I was being a little slack but I had just gotten into town and was balancing departmental and international student paperwork and orientations simultaneously. I can’t help but imagine that my superiors consider the fact that I’m the only black TA when I screw up. So as a black man I feel pressure to avoid making mistakes at all costs whether it’s having too many friends riding in my car at one time or playing my music too loud in the lab. I just don’t feel like I’ll get as many second chances as others. Ironically, one of my good friends told me he thought people would take it easy on me since I’m a Black Bahamian. I don’t know which of the two adjectives would encourage this lax attitude the most in his mind.

LTER Tour Summer 2013This brings us to the issue of affirmative action. I was in my senior year of undergrad applying for a travel grant so we could show off the work our lab group had done that year. This time the professor I’m talking about was one of the sweetest ladies you’ll meet; she happened to be middle-aged and, surprise, white. She asked about my parent’s education and incomes and I had to admit that they both had master’s degrees and made OK money even though that wouldn’t help my chances of getting funded. In the end I didn’t get that funding so she took the time to call in and find out why. It turned out that I should have mentioned more of my research experiences than I had in my essay. Later I met a black professor and told him about this experience and he told me that there was other funding for black students that would have gotten me to that conference. This past summer I told a black administrator at one of my favorite schools that I couldn’t find funding there so I went to another institution. He also let me know that he knew where to find funding for me, a fellow black. Some people think these pockets of funding for minorities are unfair but I disagree. When the number of minorities in scientific leadership roles reflects the percentage of minorities in the general population I will concede that affirmative action is unnecessary.  I have been blessed with mentors and professors who invested in me regardless of (or possibly because of) my race but when I meet black professors there is something else there. The three ladies I mentioned earlier were the kind of allies M. L. K. was looking for when he wrote his letter from that jail in Birmingham but meeting black professors is like meeting Dr. King himself for me. Being in the presence of a successful black academic, or even their work for that matter, makes me feel like I now have to succeed because it really is possible since others have done it before me. I’m almost certain that female students experience the same emotions when they meet strong women in leadership roles.

So, yeah… black in science… black in this world. I just want to be treated like everyone else but I am no longer naive enough to expect that. This human experience is crazy, and what’s even crazier is that we can alter other’s experiences by sharing our own ;)

 For more information about Bradley’s work, check out his website.

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BEACON Researchers at Work: Mating behavior in treefrogs

This week’s BEACON Researchers at Work blog post is by NC A&T undergraduate Darian Mollock, who worked as an Undergraduate Research Apprentice (URA) at MSU’s Kellogg Biological Station in summer 2014.

getty labMy name is Darian Mollock, I am a junior double major in Lab Animal Science and Animal Science at North Carolina Agricultural and Technical State University. This summer I worked with Michael Kuczynski as an Undergraduate Research Apprentice at the Kellogg Biological Station studying sexual selection and communication in gray treefrogs. The project I worked on was investigating how male calling behavior may change based on an individual’s physical condition and age, and also how female mating preferences and choosiness are influenced by these factors. 

I arrived at the biological station shortly after the beginning of the gray treefrog breeding season. During this time of year, male frogs gather in large choruses where they call to attract females who then choose mates based on certain properties of the calls (length, rate, etc.). Part of our research involved examining female preferences and choosiness in mate selection. In order to assess female mating behavior we had to capture sexually receptive females in the field so we could bring them back to lab for behavioral testing. To ensure that females were sexually responsive we could only collect mated pairs that were already in amplexus (the mating position). Prior to coming to KBS I had never done late night field work in ponds before, so I was excited for my first night to begin. Strapping on my chest waders and head lamp, I headed into the pond to search for the mated pairs. After getting over the initial fear of being in a pond at night not knowing what was lurking in the abyss (I had heard stories of large snapping turtles…), I went in. It was a thrill collecting the gray treefrogs in their natural habitat, and the noise from all of the calling males was deafening. My first night we collected fifty mated pairs – one hundred frogs in total.

photo 3 (1)After we collect the mated pairs we bring the frogs back to the lab and store them in Tupperware containers until the females can be tested. To examine female mate choice we used a series of playback tests that we ran in a small sound chamber. The chamber consisted of a flat tabletop surrounded by sound-dampening blankets with a speaker placed at both ends of the chamber. We would place females in the center of this chamber and observe their response to different frog calls that we could broadcast from the speakers. Specifically, we broadcast attractive (long duration) calls and unattractive (short duration) calls from opposing sides of the chamber. Across multiple trials we would lower the playback volume of the attractive call relative to the unattractive call. A lower playback level would simulate the attractive male calling from farther away making it a more costly choice for the females. We examined whether females would switch their preference and move towards the unattractive call when the attractive call was played at lower volume. We then investigated whether a female’s age, size, or physical condition influenced whether she would switch her preference.

Now that the mating season has come to an end we are in the process of analyzing the data we have collected throughout the summer. With our data we will determine whether larger, better condition females are willing to travel greater perceived distances to reach a more attractive mate than smaller, poor condition females.

The overall ten week internship has been remarkable; I have grasped a vast amount of knowledge about field research hands-on. When you are around someone that is passionate about their research you cannot help but get inspired. I am glad I was able to assist as an undergraduate in Michael’s research project. I have picked up valuable skills during my internship that I will carry with me in my future endeavors.

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BEACON Researchers at Work: Directed and Real Evolution

This week’s BEACON Researchers at Work blog post is by University of Texas at Austin faculty Andy Ellington.

Ellington PortraitEvolution in Action. That’s the BEACON motto. It always struck me as a bit wishful. Because evolution is mostly glacial. Sure, it can bury you over time. But it’s like that scene in “A Fish Called Wanda” where Ken is running over Otto with a steamroller. You really have to be stuck to see it.

Anyway, my lab works on directed evolution. Which is the much faster, more engineered younger brother of natural selection. We can make molecules and pathways and organisms jump on our own timescale, which is typically shorter than the timescale of the typical graduate student thesis, although some of those can be glacial, too. We introduce many mutations at the outset or else carry out procedures that are inherently hypermutagenic, direct populations through bottlenecks with great speed, and amplify the survivors with efficiency. This works whether we’re talking about molecules in vitro or viruses or microbes that have decent replication cycles. It does not work if we’re talking about rabbits. We’ll have to work on that.

But this brings up the interesting and useful point I was not making. BEACON is as much about education as it is about science. And the general populace can think about something like a rabbit evolving much more easily than they can something like beta-lactamase (although the evolution of drug resistance is one of those canards that the Creationists generally steer clear of, for good reason). And since we can’t run fast cycles of rabbit evolution, turning fluffy bunnies into the sort of killer rabbits that stymied John Cleese in a different Python movie (Holy Grail with its Holy Hand Grenade of Antioch), it is not as easy to viscerally demonstrate evolution that resonates.

But of course we would all argue that it’s the same thing: generating and culling alleles is evolution. This is the fundamental definition. Diversity. Selection. Replication. At any timescale, in any system. I sometimes have trouble making this point when talking with colleagues about origins, because many biologists are closet Vitalists who sadly want to separate discussions of origins from discussions of evolution. Nonetheless, the fundamentals are sound, just like with the economy.

Now, how do we better represent these fundamentals to the public? My goal in BEACON has in part been to show Evolution in Action by developing kits that show ribozyme evoluton in real-time. Ribozyme populations placed into a very good environment of enzymes and substrates can lead to their own reproduction and winnow to those variants that are the most active, with a parallel fluorescence readout.

I’d now like to better encourage the spread of directed evolution throughout the BEACON community. I think we can begin to discuss how to encourage experiments that demonstrate evolution and that lead to products that are of commercial interest and value. And that these will feedback and reinforce our mission statement.

Since many BEACON-ites are interested in computer evolution, this will require a fundamental restructuring of how we meld computer and biological processes. We could have a wonderfully fun discussion of whether electrons replicate or not, and if they don’t whether or not computer algorithms are in fact evolving. But it would be a somewhat fruitless discussion, because in reality all we’re doing is moving the goalposts: just as I claim my enzyme bath allows ribozymes to replicate, computer scientists can claim that their cozy electronic interiors allow programs to replicate. Of course, there’s also the modeling of evolution that is not evolution itself.

Both types of research may find their way into biology. Have we ever really thought about how to make automatons and viruses more alike? If I wanted to make a virus that had at its core rule sets that were not unlike the rule sets in an algorithm, could I do that? That would be super hard and interesting. In contrast, can computer scientists make automatons that have at their core rule sets that resemble those of viruses. This, this is a much more worthy goal.

Modeling is a more obvious crossover, especially with Avida and all. Still, the use of Avida to retrodict and the use of Avida to predict could be refined. In particular, can viruses or other synthetic systems be set up to run down the same pathways that Avida has already plumbed, and will they get th same answers? This is particularly important because the time is coming, and coming quickly, when we will use computation to plumb fitness landscapes in advance of evolution. Tools already exist to begin to predict host:pathogen interactions, and the further ability to predict evolutionary dynamics of the random “viral chatter” that goes on between human populations and the many things assaulting human populations is of particularly import, given the recent news of how Koyaanisqatsi seems to be at our doorstep, at least for viruses.

I am reminded by a venerable senior colleague that the last reference is only for the olds, like me. For those of you of a certain age, you may remember the iconic crying Indian, saddened over the destruction of our environment, that would show up on public service announcements. Around that time, there was also a movie, Koyaanisqatsi, nominally translated from Hopi as “World Out of Balance,” that was anextended word picture that showed how we were moving away from nature. Yeah, we were all hippies back then. Anyway, when Ebola shows up on your doorstep and says ‘hi,’ it seems to me that the world is out of balance, and that it’s time to do something more than cry about it.

For more information about Andy’s work, you can contact him at ellingtonlab at gmail dot com.

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BEACON Researchers at Work: The genetic basis of biofilm formation

This week’s BEACON Researchers at Work blog post is by University of Washington graduate student Elyse Hope.

Elyse Hope“Remember to finish your full course of antibiotics” is a phrase we have probably all heard from a doctor at least once. Intuition tells us that a long course of antibiotics is designed to completely eliminate any pathogens from our bodies, making sure we don’t have any left over that might become drug-resistant. The information that is left out, however, is how a pathogen might evolve drug resistance. One way in which microbes (bacteria, yeast, and others) might survive an antimicrobial or antibiotic treatment is to form a biofilm, which is the trait I study in yeast. The premise of a biofilm is this: microbes can create proteins on their cell surfaces and even outside the cell, allowing the microbes to stick to surfaces and to each other in a mass (Verstrepen et al, 2004). When exposed to a stressor like an antimicrobial or other chemical treatment, cells on the outside of the biofilm may die, but cells on the inside of the biofilm might survive (Smukalla et al, 2008) and – once the stress has passed – repopulate. Drug-resistant microbes pose an increasing healthcare problem, and part of combating this problem is understanding the genetics behind antimicrobial resistance and how those genetics contribute to the types of biofilms we see. The primary goal of my research is to better understand the genetic basis of yeast biofilm formation and how the ability to form a biofilm evolves in yeast.

In Maitreya Dunham’s lab at the University of Washington, I am using many different strains of budding yeast Saccharomyces cerevisiae (the same yeast that makes bread, wine, and beer) to broadly investigate the genetics underlying biofilm formation. Most of what we know about yeast biofilms comes from working with a few well-studied laboratory strains, but we haven’t known until now whether these lab strains are representative of what we would see in yeast in the wild, including strains of yeast involved in infection. Recent work from a group in France (Liti et al, 2009) generated a collection of wild yeast from different sources all over the world, from Ethiopia to Malaysia to Pennsylvania, and from wine to cactus to palm tree nectar. I wanted to know: if we look at biofilms formed by these wild strains, do we see the same characteristics predicted by laboratory strains?

There are many different ways to look at yeast biofilms established in the literature, from how well the yeast stick to each other and to surfaces, to how complex they look when they grow together in a colony (Granek and Magwene, 2010; Stovicek et al, 2010). We studied five different visible traits related to biofilms in yeast and showed that most of these traits are uncorrelated. This means that whether a yeast strain can stick to other cells (trait 3) has little to do with how well it can form a “lacy” colony (trait 1) or stick to a surface (trait 5).


We also found that each of these wild strains has an entirely unique set of traits, both in strength and complexity, and that the traits are different depending on whether a cell is haploid or diploid (“ploidy”, how many copies of its chromosomes it has), which is a more complex picture of the relationship between ploidy and biofilm traits than was previously known from laboratory strains (Galitski et al, 1999; Reynolds and Fink, 2001).

Our next step is to look at changes in single genes and how they specifically contribute to each of these traits. My final goal is to make it possible for us to engineer strains of yeast with very specific biofilm characteristics, and to know what gene variants to look for as potential targets for small molecules. If we could find targeted ways to disrupt biofilms, then the microbes inside could be much more susceptible to drug treatments.

For more, see our paper here.


I have come to feel passionate about the work I am doing, and believing in the larger goal helps me meet the smaller goals of day-to-day effort. When I started graduate school, however, I had never even seen a yeast cell before. I started college with a computational background and planned to be a physics major, but genetics drew me in with its blend of logical problem-solving and real-world applications, and I declared a biology major instead. My senior year I joined a lab that had just started a collaboration with a genomics group. My PI sent me a paper about genome sequencing to see if I would be interested in going that route with my research. I had never read anything more interesting in my life; I was astounded by what had been accomplished with sequencing so far, as well as the implications for what could be accomplished in the future. I later entered a Genome Sciences graduate program so I could work on realizing those possibilities. I had every intent of focusing on sequencing and staying computational, but I had an amazing conversation with a PI who worked on yeast and envisioned a very real-world project on biofilms that would integrate genetics, sequencing, and bench work. She took a chance on me that I would love yeast as much as she did, without any experience whatsoever, and she was right. 



Galitski, T., A. J. Saldanha, C. A. Styles, E. S. Lander and G. R. Fink, 1999 Ploidy Regulation of Gene Expression. Science 285: 251-254.

Granek, J. A., and P. M. Magwene, 2010 Environmental and genetic determinants of colony morphology in yeast. PLoS Genet 6: 1-12.

Liti, G., D. M. Carter, A. M. Moses, J. Warringer, L. Parts et al., 2009 Population genomics of domestic and wild yeasts. Nature 458: 337-341.

Reynolds, T. B., and G. R. Fink, 2001 Bakers’ yeast, a model for fungal biofilm formation. Science 291: 878-881.

Smukalla, S., M. Caldara, N. Pochet, A. Beauvais, S. Guadagnini et al., 2008 FLO1 is a variable green beard gene that drives biofilm-like cooperation in budding yeast. Cell 135: 726-737.

Stovicek, V., L. Vachova, M. Kuthan and Z. Palkova, 2010 General factors important for the formation of structured biofilm-like yeast colonies. Fungal Genet Biol 47: 1012-1022.

Verstrepen, K. J., T. B. Reynolds and G. R. Fink, 2004 Origins of variation in the fungal cell surface. Nat Rev Microbiol 2: 533-540.

For more information about Elyse’s work, you can contact her at ehope at u dot washington dot edu.

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BEACON Researchers at Work: Evolution and the nano-scale

Today’s BEACON Researchers at Work blog post is by NC A&T faculty Dr. Joseph L. Graves Jr.

Dr. Joseph L. Graves Jr. Associate Dean for Research & Professor of Biological Sciences Fellow, American Association for the Advancement of Science Section G: Biological Sciences Joint School of Nanoscience & Nanoenginneering North Carolina A&T State University & UNC Greensboro

Dr. Joseph L. Graves Jr.
Associate Dean for Research & Professor of Biological Sciences;
Fellow, American Association for the Advancement of Science
Section G: Biological Sciences;
Joint School of Nanoscience & Nanoenginneering,
North Carolina A&T State University & UNC Greensboro

One nanometer is defined as 1 x 10-9 meter. This is about the size of one glucose molecule. The nucleus of a human cell is about 600 nm’s across and the average bacterium is about 1000 nm’s in size. Clearly many important biological phenomena occur at the nanoscale, including molecular self-assembly. The rate of the development of nanotechnology over the last twenty years has been astounding. Its applications are all around us. From sunscreens and dental adhesives, to the components of high technology devices. Nanotechnology is being proposed to build bioactive and biodegradable scaffolds for tissue engineering and for controlled drug delivery to treat chronic diseases.

The global production of nanomaterials (NMs) is expected to grow exponentially. It is estimated that global production of NMs will reach 104–105 tons annually for structural applications by 2020. Other projections include: 103 tons in skin care products, information communications technology industries >103 tons, biotechnology 10 tons, and environmental industry 103–104 (source Royal Society and Royal Academy of Engineering Report, 2004). Yet little has been done to study the ecotoxicity of NMs, particularly issues such as bioaccumulation in food chains; or impacts of NMs on bacteria and other microorganisms. Indeed, metallic and metallic oxide nanoparticles are already being touted as the miracle “cure” for multi-drug resistant bacteria (Rai et al. 2012). This means that bacteria will be increasingly exposed to metallic/metallic oxide NPs and other NMs both intentionally (as antimicrobial applications) and unintentionally (run-off from industrial processes.)

Reading these glowingly optimistic reports from those working with engineered nanoparticles (eNPs) was disconcerting. There were two major flaws in this thinking. First was the idea that bacteria would be widely susceptible to noble metals (copper, silver, gold, etc.). Second was even if this were currently true, that bacteria would not rapidly evolve resistance to them. It turns out that neither assumption is true. Bacteria have an array of resistance mechanisms to heavy metals in general and silver in particular (Silver and Phung 2006; Mijnendonckx et al. 2013). It turns out that essentially all bacteria have genes for toxic metal ion resistances. Amongst those best studied include those for Ag+, AsO2-, AsO4(3-), Cd2+ Co2+, CrO4(2-), Cu2+, Hg2+, Ni2+,Pb2+, TeO3(2-), Tl+ and Zn2+. The largest group of resistance systems functions by energy-dependent efflux of toxic ions. Fewer involve enzymatic transformations (oxidation, reduction, methylation, and demethylation) or metal-binding proteins (for example, metallothionein SmtA, chaperone CopZ and periplasmic silver binding protein SilE). Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers (Silver and Phung 2006).

Evolution of Nanoparticle Resistance

To test the second assumption, I decided to utilize a relatively “naïve” bacterium Escherichia coli K12MG1655 to determine how rapidly this strain could evolve increased silver nanoparticle resistance. This strain of E. coli did not have any of the sil (silver resistance) genetic elements in its genome. However, like most bacteria its genome normally contains heavy metal sensing genes such as the cus system. The Cus system aids in protection of cells from high concentrations of silver and copper. The histidine kinase CusS of the CusRS two-component system functions as a silver/copper responsive sensor kinase and is essential for the induction of the genes encoding the CusCFBA efflux pump. The efflux pump works by removing the toxic concentrations of the metal from the interior of the bacterial cell.

Scanning electron microscope picture of E. coli bacterium with AgNPs associated with cell wall (picture by M. Tajkarimi, JSNN).

Scanning electron microscope picture of E. coli bacterium with AgNPs associated with cell wall (picture by M. Tajkarimi, JSNN).

Our experiment was simple. We conducted an experimental evolution protocol using “off the shelf” E. coli K12 MG1655 and exposed it to increasing concentrations of 10nm citrate-coated spherical nanoparticles. We cultured the cells using Davis Minimal Broth with dextrose 10% as a sole carbon source, enriched with thiamine hydrochloride 0.1% in 10 ml of total culture volume maintained in 50 ml Erlenmeyer flasks. The flasks were placed in a shaking incubator at 37o C for 24 hours. This is generally considered a non-stressful growing media for E. coli. The cultures were propagated daily by transfers of 0.1 ml into 9.9 ml of DMB. The control populations were maintained in this medium without the addition of silver nanoparticles (AgNPs). The treatment populations were exposed to increasing concentrations of spherical 10nm citrate-coated AgNPs. Both the control and treatment groups were replicated five-fold. In this way we could determine if any of the mutations that arose in the control or treatment groups were shared by more than one of its populations.

After determining the minimum inhibitory concentration (MIC) for this strain of E. coli, we exposed the treatment group to a concentration less than MIC so that some bacteria could survive. We wanted to allow enough survivorship such that a sufficient number of bacteria were left in the culture with potential silver-resistant mutations. In the non-exposed bacteria we would normally observe a 2-log increase in the culture over 24 hours (this is about 6.5 generations resulting in 106 founders growing to about 108 per ml of culture). At the beginning of the day the cultures would look “clear” but by the end of the day they would be “turbid” to the eye. Our goal was to observe the same sort of growth in the silver nanoparticle treated populations. This happened rapidly.

The table below shows the number of generations in this experiment that were kept at a given AgNP concentration:

Generations Exposure Concentration
1 – 50 50 mg/l
51 – 140 100 mg/l
141 – 265 125 mg/l

Thus by generation 50 (~ 9 days later!) we were observing turbid cultures in the AgNP treated group exposed to 50 mg/L of 10nm AgNPs. After an additional 90 generations we were observing turbidity in 24 hours for the treated group at 100 mg/L of 10nm AgNPs and so on.

Undergraduates Quincy Cunningham and Herve Nonga explain MIC experiment results to Dr. Chandra Jack at Annual BEACON Congress, 2014.

Undergraduates Quincy Cunningham and Herve Nonga explain MIC experiment results to Dr. Chandra Jack at Annual BEACON Congress, 2014.

We tested the control and treatment groups at generation 250 for population growth over 24 hours in a range of concentrations of 10nm citrate-coated AgNPs, 10nm PVP-coated AgNPs, 40 nm citrate-coated AgNPs, 40 nm PVP-coated AgNPs, and bulk silver nitrate (AgNO3). Both controls and treatments were able to grow at 50 and 100 mg/L, but not surprisingly, the treatment populations showed superior growth compared to the controls at 250 mg/L, 500 mg/L, and 750 mg/L. In other words, the treatment group was now AgNP resistant, relative to the control bacteria.

Genomics of Resistance

The experimental evolution protocol used in this study indirectly demonstrated that genomic changes must have occurred between the control and treatment bacteria. Next generation sequencing was used to investigate the genomic changes more directly. This process is facilitated by the fact that E. coli K12MG1655 has already been fully sequenced, allowing us to compare the genomic features of both our control and treatment populations against a reference genome for this bacterium. Thus we sequenced the “off-the-shelf” MG1655, our controls, and our treatments and compared their genomes to the reference using the breseq bioinformatic pipeline (developed by the Dr. Jeffrey Barrick, U. Texas – he is another BEACON scientist.) We sequenced the ancestral bacteria (off-the-shelf), generation 100 controls and treatment; generation 150 controls and treatment; and generation 200 controls and treatment via Illumina sequencing technology. Their sequences are then trimmed of Illumina adaptors, aligned to the reference genome, and genetic variants called against the reference genome. The E. coli genome contains ~4.7 million base pairs – so this is not an unsubstantial computational task.

Going into the sequencing, I had expected to see a large number of genetic differences between the controls and the treatment populations. In fact, the results showed that not only did AgNP resistance evolve quickly, but that it didn’t take a great deal of genomic changes to achieve the result! The breseq pipeline allows one to investigate point mutations (SNPs), and deletions, insertions, insertion elements (indels). The genomic story was told mainly by three point mutations! As we are still finishing this study, I will not hang my hat on these results as of yet. However, stay tuned…the nature of science requires that we check our results. What we can say at this point is that it seems relatively easy for bacteria to evolve resistance to metallic nanoparticles (as they did to traditional antibiotics.) Care should be utilized before we intentionally and accidentally introduce these nanomaterials into our ecosystems. That is because evolution is always in action, even where you don’t suspect it.


Gudipaty, S.A. and McEvoy, M.M., The histidine kinase CusS senses silver ions through direct binding by its sensor domain, Biochim Biophys Acta 1844(9): 1656-61, 2014 doi: 10.1016/j.bbapap.2014.06.001.

Mijnendonckx, K., Leys, N., Mahillon, J., Silver, S., Van Houdt, R., Antimicrobial silver: uses, toxicity and potential for resistance, Biometals 26(4): 609-21, 2013. doi: 10.1007/s10534-013-9645-z.

Rai, M.K., Deshmukh, S.D., Ingle, A.P., and Gade, A. K., Silver nanoparticles: the powerful nanoweapon against multidrug-resistant bacteria, J. Appl. Microbiol. 112(5): 841-52, 2012. doi: 10.1111/j.1365-2672.2012.05253.x.

Silver, S. and Phung, le T. A bacterial view of the periodic table: genes and proteins for toxic inorganic ions, J. Ind. Microbiol. Biotechnol. 32(11-12): 587-605, 2005.


This research project would not have been possible without the work of the following individuals; Mehrdad Tajkarimi, graduate student, Nanoscience Department, UNC Greensboro; Quincy Cunningham, undergraduate student, NCATSU; Herve Nonga, undergraduate student, Michigan State University; Adero Campbell, undergraduate student, Bennett College; and Dr. Scott Harrison, Biology Department, NCATSU.

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