Dr. Krassi R. Hristova

Dr. Krassi Hristova
Dr. Krassi R. HristovaMarquette University

Wehr Life Sciences, 208

MilwaukeeWI53201United States of America
(414) 288-5120


Microbial ecology of fresh water ecosystems, antimicrobial resistance


M.S., 1987, Sofia University "St. Kliment Ohridski" , Bulgaria 
Ph.D., 1994, Sofia University "St. Kliment Ohridski", Bulgaria 
Post-doctoral Fellow, 1998-1999, University of Illinois at Urbana-Champaign, IL 
Postdoctoral Scientist, 1999-2001, Department of Land, Air, and Water Resources, University of California, Davis
Professional Researcher/Research Assistant Professor, 2002-2008, Department of Land, Air, and Water Resources, University of California, Davis            
Professional Researcher/Research Associate Professor, 2008-2010, Department of Land, Air, and Water Resources, University of California, Davis 

Courses Taught

BIOL3802 Experimental Microbiology
BIOL 8803 Microbial Diversity and Ecology
CORE 1929 Science, Policy, and People

Research Interests

My research interests are in microbial ecology of contaminated freshwater ecosystems and the link between impaired ecosystem services and human health. 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, antimicrobial resistance, and biodegradation of pollutants.

Current areas of research include:

Study the impact of non-source pollution from agricultural practices on water quality and antibiotic resistance

The ecology of environmental antibiotic resistance has recently become an important area of research as antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are recognized as emerging biological contaminants. Characterizing the impact of agricultural non-point source pollution on ARG dissemination and abundance in the environment is useful for monitoring how large-scale farming impacts environmental reservoirs of antimicrobial resistance. Currently, we are working on characterizing environmental factors and spatial and temporal drivers on dissemination of ARGs and ARB, including human pathogens, in regions of elevated antibiotic use, such as concentrated agriculture. Models of the sources of these exposures are needed to better manage AMR as a global health concern. We are collaborating with Dr. Marc Serre, UNC Chapel Hill on implemented Euclidean and river distance metrics in land-use regression models and models accounting for in-river flow to understand the relative impacts of different sources on AMR in rivers in agricultural area dominated by dairy CAFOs in Kewaunee County. Our work is vital to determine methods to mitigate the spread of environmental ARB and ARGs.

In collaboration with the Kewaunee County Community, we are aiming to establish a functioning citizen driven surface water quality monitoring program for testing and analyzing water parameters associated with agricultural consolidation and manure practices. In collaboration with Dr. Jill Birren (College of Education) and Dr. Amber Wichowsky (Political Sciences), we are exploring social and political dimensions of contamination problems and potential solutions in the Kewaunee County community.


Studying the mechanisms and interactions of heavy metals and engineered nanoparticles with eukaryotic 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 cell’s 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 was evaluated 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.


Characterizing the emergence and dissemination of antibiotic resistant bacteria

The increase use of antimicrobials has resulted in a surge of antibiotic resistance infections worldwide. Of which of these, the increased prevalence of extended spectrum beta-lactamases is cause for growing concern. Extended spectrum beta-lactamase producing Enterobacteriaceae have the ability to break down common beta-lactam antibiotics like penicillin and cephalosporins (4). This high level of resistance leaves only a couple effective antibiotics including carbapenems which are one of the last line of antibiotics to treat these resistant strains. The prevalence of extended beta-lactamase producing bacteria and multi-drug resistance has increased over time with Escherichia coli predominating amongst ESBL producing bacteria. In ongoing project, we have determined the prevalence of ABR among E. coli isolates from hospital versus urban wastewater, and throughout the wastewater treatment process. Results showed that hospital wastewater has over an 8-fold higher prevalence of multi-drug resistant E. coli isolates compared to urban wastewater influent and effluents. ESBL producers were more common in urban wastewater influent and effluents compared to hospital wastewater and appear plasmid-mediated.  The presence of the ESBL CTX-M was primarily plasmid-mediated and its absence was associated with increased susceptibility to ceftazidime.

Professional Affiliations

Society for Environmental Toxicology and Chemistry, 2013-present
International Water Association, 2013-present
International Society for Microbial Ecology; 1998-present
American Society for Microbiology; 2000 -present
American Chemical Society, 2014-present


Wiesner-Friedman, C., R. Beattie, J. Stewart, K.R. Hristova, and M. Serre. 2022. Characterizing differences in sources and contributions to fecal contamination of sediment and surface water with the microbial FIT framework. Env. Sci. Technol., Vol 56 (7), 4231-4240, doi.org/10.1021/acs.est.2c00224..

Beattie, R., Su, B., Thill, R*., and K. R. Hristova. 2022. Recycled concrete aggregates are an economic form of urban riparian erosion management with limited impacts on freshwater chemistry and microbial diversity. J. Hazardous Materials, Vol 434, 15 July 2022, 128934.

Beattie, R., Skwor, T., and K.R. Hristova. 2020. Survivor microbial populations in post-chlorinated wastewater are strongly associated with untreated hospital sewage and include ceftazidime and meropenem resistant populations. Science of the Total Environment, in press.

Beattie, R. and K. R. Hristova. 2021. Manure derived nutrients alter microbial community composition and increase the presence of potential pathogens in freshwater sediment. Journal of Applied Microbiology, 132 (1):747-757.

Beattie, R., Skwor, T., and K.R. Hristova. 2020. Survivor microbial populations in post-chlorinated wastewater are strongly associated with untreated hospital sewage and include ceftazidime and meropenem resistant populations. Science of the Total Environment, in press, 

Beattie, R.*, E. Bakke, N. Konopek, R. Thill, E. Munson and K. R. Hristova. 2020. Antimicrobial Resistance Traits of Escherichia coli Isolated from Dairy Manure and Freshwater Ecosystems Are Similar to One Another but Differ from Associated Clinical Isolates. Microorganisms, 8:747. 

Beattie, R., M. Walsh, M. Cruz, L. R. McAliley, L. Dodgen, W. Zheng, K. R. Hristova. 2018. Agricultural contamination impacts antibiotic resistance gene abundances in river bed sediment temporally. FEMS Microbial Ecology vol. 94, 9, https://doi.org/10.1093/femsec/fiy131

Mashock, M., T. Zenon, A. Kappell, L. Petrella, E. Andersen, and K.R. Hristova. Copper oxide nanoparticles impact several toxicological endpoints and cause neurodegeneration in Caenorhabditis elegans. PLoS One, First published online November 11, 2016,  http://dx.doi.org/10.1002/etc.3159

Honors and Awards

Way Klingler Young Scholar Award, Marquette University, 2013


Investigating the effects of disinfection on the persistence of surface-associated pathogens. Department of Defense; PI

Electroremediation of PFAS. Department of Defense, co-PI

Optimization of Bacteria Quantification on Ceramic Surfaces. Kohler Inc., PI

Shear Stress Required to Remove Microorganisms from Common Bath Surfaces Using Moving Air. Kohler Inc, PI

Why Marquette

Exciting research opportunities and a good balance of teaching and research


Richard Melton (Ph.D. student)

Dr. Hristova is not currently accepting new Ph.D. students


Former Students

Kyle Leistikow, Ph.D.

Faculty Directory

Faculty Directory

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Emeriti & Former Faculty


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Department of Biological Sciences

Wehr Life Sciences, 109
1428 W. Clybourn St.
Milwaukee, WI 53233

(414) 288-7355

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