Dr. Krassimira Hristova

Krassimira R. Hristova

Assistant Professor
Wehr Life Sciences, 208
(414) 288-5120

M.S., 1987, Sofia University "St. Kliment Ohridski" , Bulgaria
Ph.D., 1994, Sofia University "St. Kliment Ohridski", Bulgaria
Post-doctoral Training, University of Illinois at Urbana-Champaign, IL
Research Associate Microbiologist, University of California at Davis, CA


The research in my laboratory is based on key concepts and emerging trends in molecular and environmental microbiology to support research experiences in environmental toxicology, antibiotic resistance, and biodegradation of pollutants.


Ecology of Antibiotic Resistance


Currently, we are focused on studies of the antibiotic resistance determinants in environmental and clinical settings. There is growing understanding that antibiotic resistance is a natural property of all bacteria, and that environmental microbes are a likely reservoir of resistance encountered in  medical clinics. Our recent findings suggest that while emergence of resistance may be limited in the clinic, the urban waterways in which the treated waste and storm water is released increases the proliferation of resistance. Characterization of the E. coli isolated from sediment near waste water treatment plant effluent and urban waterways indicates that there is a greater abundance of multiple resistance to several different classes of antibiotics, including beta-lactam antibiotics, compared to clinical E. coli isolates from a Milwaukee hospital. In addition, E. coli plasmids from the effluent sediment correlated more with antibiotic resistance than influent, and plasmids containing resistance gene(s) to multiple antibiotics showed an ability to be transferred via conjugation. The goal of a new project, recently funded by the NOAA Wisconsin Sea Grant, is to investigate a novel role of the invasive Dreissenid mussels, zebra and quagga, on the dissemination of antibiotic resistance genes (ARGs) in the Great Lakes coastal ecosystem through horizontal gene transfer (HGT). We hypothesize that the primary source of ARG proliferation in Great Lakes coastal ecosystems is the gastrointestinal microbiome of humans and animals consuming antibiotics and invasive mussels that concentrate bacteria in their gut. The project goal is to characterize the surface water resistome in Lake Michigan and an inland Wisconsin lake by characterizing ARG diversity, HGT rates, and the impact of zebra and quagga mussels on the dissemination of ARGs. This work will provide novel information of how the interactions of many ecosystem factors, including treated wastewater, invasive species, and the microbiome of coastal waters relate to environmental reservoirs of antibiotic resistant bacteria.


Environmental Health and Safety of Nanomaterials


An additional project in the lab is aiming to understand the toxic effects of metalnanooxides on human health in the event of long term or accidental exposure, in order to engineer less toxic nanomaterials, and prevent injury to human cells. Nanomaterials are used in many commercial products and new applications in biomedicine, yet their fate, potential toxicity, and mechanisms of internalization in biological cells have not been well defined. The main objective is to identify the pathways and mechanisms underlying interactions between metal oxide NPs and eukaryotic cells. The potential mechanisms of toxicity of two types of engineered CuO nanoparticles (NPs) to Saccharomyces cerevisiae were investigated and related to distinct physicochemical properties such as size, shape, dissolution, aggregation, and agglomeration. Spherical, 8 nm CuO NPs showed significant inhibition of yeast cells’ metabolic activity, whereas 50 nm CuO NPs with irregular morphology showed much less effect. Aged NPs in the cells growth media were more toxic than fresh NPs dispersions implicating the importance of the NPs-media components’ (organic molecules and salts) interactions on their toxicity. The impact of Cu2+ (as CuSO4) and the two engineered CuO NPs on the cell respiration and mitochondrial function is different as studied with oxygen consumption rate, ETC mutants, and mitochondrial membrane depolarization measurements. The possible neurodegenerative effect of CuO NPs is currently under investigation with C. elegans as a genetic model. Future research will focus on studying how CuO NPs exposure impacts cellular copper homeostasis (transporters, Cu-binding methallothioneins and transcription factors) and what are the key factors of protection against Cu-induced oxidative stress under elevated intracellular Cu concentrations.


Biochemistry and genetics of an MTBE degradation pathway


A third project involves studies of the genetic regulation of bacterial MTBE degradation pathway and the expression and purification of the tert-butyl alcohol (TBA) hydroxylase proteins. Methyl tertiary butyl ether (MTBE) is often associated with gasoline spills and has formed long plumes in groundwater. MTBE is a primary groundwater contaminant in the US with low biodegradation rate under oxygen-limited conditions. Its downstream metabolite, TBA, is a potential carcinogen. In the bacterial strain Methylibium petroleiphilum PM1, two novel oxygenase enzymes are involved in the oxidation of MTBE (a flavin-dependent monooxygenase) and TBA (a two-component Rieske type oxygenase), but little is known about their regulation, structure and function. Ongoing research involves biochemical characterization of the functional role of this novel TBA hydroxylase through analysis of the enzymatic kinetics of the purified enzyme, substrate range and specificity, and efforts on solving the crystal structure. The long-term goal of this project is to understand the regulation of MTBE pathway upon biodegradation of organic contaminants and design bacterial strains with improved capabilities for bioremediation of gasoline spills.



Selected Publications

Kappell, A. D., Y. Wei, R. J. Newton, J. Van Nostrand, J. Zhou, S. M. McLellan, and K. R. Hristova. 2014. The polycyclic aromatic hydrocarbon degradation potential of Gulf of Mexico native coastal microbial communities after the Deepwater Horizon oil spill. Frontiers in Microbiol., online first May 2014 doi: 10.3389/fmicb.2014.00205


Schmidt, R.; Tantoyotai, P.; Fakra, S.; Marcus, M.; Yang, S. I.; Pickering, I.; Bañuelos, G.; Hristova, K. R., and Freeman, J.  2013. Selenium biotransformations in engineered aquatic ecosystem for bioremediation of agricultural waste water via brine shrimp production. Env. Sci. Technol., 47: 5057–5065. 


Schmidt, R., D. Klemme, K. M. Scow, and K. R. Hristova. 2012. Microbial Biosafety of pilot-scale bioreactor treating MTBE and TBA contaminated drinking water supply. J. Hazardous Materials. 209-10: 524-528.

Shi, J.Y., Abid, A.D.,  Kennedy, I.M., Hristova, K.R., and Silk, W.K. 2011. To duckweeds (Landoltia punctata), nanoparticulate CuO is more inhibitory than the soluble copper in the bulk solution. Environmental Pollution. 159: 1277-1282.

Son, A., Kennedy, I.M., Scow, K., and Hristova, K.R. 2010. Quantitative gene monitoring of microbial tetracycline resistance using magnetic luminescent nanoparticles. J. Environ. Monitoring, 12: 1362–1367.

Schmidt, R., Battaglia, V., Scow K., Kane, S., and Hristova, K.R. 2008.  A novel enzyme, MdpA,  is involved in MTBE degradation in Methylibium petroleiphilum PM1.  Appl. Env. Microbiol. 74: 6631-6638.


Hristova, K.R., Schmidt, R., Chakicherla, A., Legler, T., Wu, J., Chain, P., Scow, K.M., and Kane, S. 2007. Comparative transcriptome analysis of Methylibium petroleiphilum PM1 exposed to the fuel-oxygenates methyl-tert-butyl ether and ethanol. Appl. Env. Microbiol, 73: 7347-7357.



Meeting Presentations


A.D. Kappell, and K. R. Hristova. 2014. Antibiotic Resistance and Markers of Gene Transfer of Escherichia coli from Urban Waterways. At the American Society of Microbiology 114th General Meeting. May 17-20, 2014, Boston, MA.


M. Mashock, A.D. Kappell, and K. R. Hristova. 2013. Cytotoxicity of CuO Nanoparticles to Saccharomyces cerevisiae is Mediated by Cell Surface Attachment and Impact on Respiratory Metabolism. At the 34th SETAC North America Conference, Nashville, TN, November 16-21st, 2013.


A.D. Kappell, A. Skowronski, N. Ahuja and K.R. Hristova. 2013 The human impact on antibiotic resistance: Resistance in urban waterways. At the Microbial Ecology and Water Engineering Conference, Ann Arbor, MI, July 7-10, 2013.


N. Hallaj, M. Mashock, and K. R. Hristova. 2012. Mechanisms of CuO nanoparticle toxicity to Saccaromyces cerevisiae. At the 1st Sustainable Nanotechnology Conference, Hilton Hotel, Arlington, VA, November 3-6th, 2012.


M. Mashock, and K. R.Hristova. 2012. Effects of Nanoparticles on symbiotic relationship of Rhizobium etli and Phaseolus vulgaris. At the 1st Sustainable Nanotechnology Conference, Hilton Hotel, Arlington, VA, November 3-6th, 2012.


A.D. Kappell, M. DeNies, R. Newton and K. R. Hristova. 2012 The impact of wastewater treatment plants on antibiotic resistance. At the Midwest Microbial Pathogenesis Conference, MMPC 19, 7-9 September 2012. Milwaukee, WI, USA.


 A. Kappell, J. Sun, and K. R. Hristova. 2012. Characterization of tert-butyl-alcohol-hydroxylase, a Rieske oxygenase in MTBE degradation pathway of Methylibium petroleiphilum PM1. At the 14th International Symposium on Microbial Ecology, ISME 14, 19-24 August 2012, Copenhagen, Denmark.




Way Klingler Young Scholars Award (2013)

Current Lab Personal


Michael Mashock (Ph.D. student)

Anthony Kappell (Postdoctoral Researcher)

Nicholas Callard (undergraduate)

Tylor Zenon (undergraduate)

Kyle Antene (undergraduate)

Killy Paavola (undergraduate)

Jose Nieves (undergraduate)

Charlie Metcalf (Summer Research Program 2014)

Dr. Hristova is currently accepting new Ph.D. students     



Former Lab Members


Yin Wei (Postdoctoral Researcher)

Neha Ahuja  (BS 2014)

Nadia Hallaj (BS 2012)

Max Denies (Summer Research Program 2011)



Past Graduate Students at UC Davis

Geetika Joshi (PhD 2014), Tee Prapakorn (PhD 2014), Reef Holland (MS 2011), Jennifer Bradford (MS 2008), Adriana Ortegon (MS 2008), Kristin Hicks (PhD 2007), Vincent Battaglia (MS 2006), Stephanie Smith (MS 2005), Banu Inceoglu (MS 2004).



Biological Sciences Department

Marquette University, Wehr Life Sciences
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(414) 288-7355
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