Dr. Krassimira Hristova

Dr. Krassimira Hristova
Dr. Krassimira HristovaMarquette University

Wehr Life Sciences, 208

MilwaukeeWI53201United States of America
(414) 288-5120

Associate Professor

Microbiology, microbial ecology



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

Research Interests

My research focuses on studying the functional role of microbial communities in contaminated environments and seeks to understand the impact of pollution on ecosystem, organismal, cellular and molecular levels.  I use variety of classical microbiology, molecular genetics and modern genomic approaches to investigate microbial diversity and function in different ecosystems, such as Lake Michigan, Gulf of Mexico beach sand, and groundwater. The overarching research question is: How does human pollution with gasoline or new emerging contaminants such as pharmaceuticals, nanoparticles, and personal care ‘antibacterial’ products impact environmental and human health.


Specific projects of interest are: 

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 bacteria with multiple resistance to several different classes of antibiotics. 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 project, 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. 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: A second project in the lab is aiming to understand the toxic effects of metal nanooxides 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 in relation to their physicochemical properties have not been well defined. The main objective is to identify the pathways and mechanisms underlying interactions between metal oxide NPs and eukaryotic cells. Spherical, 24 nm CuO nanoparticles (NPs) showed significant inhibition of Saccharomyces cerevisiae cells’ metabolic activity, whereas 62 nm CuO NPs with irregular morphology showed much less effect. Aged NPs in the cells growth media were more inhibitory than fresh NPs dispersions implicating the importance of the NPs-media components’ (organic molecules and salts) interactions on their toxicity. Ongoing research focuses on understanding how CuO NP exposure impacts cellular copper homeostasis and determining what the key factors of protection against Cu-induced stress are under elevated intracellular Cu concentrations utilizing whole transcriptome analyses. Toxicity endpoints and the possible neurodegenerative effect of CuO NPs were also investigated with using the nematode Caenorhabditis elegans as a genetic model. The lab strain and the four studied environmental strains of C. elegansshowed significantly greater sensitivity to CuO nanoparticles compared to equal molar concentrations of CuSO4 based on greater reduction in body length, feeding behavior, and reproduction.

Microbial Biodegradation of Organic Pollutants: On the ecosystem level, my research involves studies of the ability of native microbial communities to degrade organic pollutants such as polyaromatic hydrocarbons (PAHs) and methyl tertiary butyl ether in coastal beach sand, lake sediment, or groundwater. Recently, my lab studied the PAH biodegradation potential of coastal sand microbial communities in the Gulf of Mexico after the oil spill in two beach locations with different impacts from the spill.  Both the heavily oiled and the non-impacted coastal communities showed little variation in their biodegradation ability for low molecular weight PAHs. Next-generation sequencing of 16S rRNA genes from the controlled lab biodegradation experiments showed that known PAH degraders and genera frequently associated with oil hydrocarbon degradation represented a major portion of sand bacterial community. While PAH degrading bacteria are common in the environment, their activity is dependent on many environmental factors. Thus understanding how multiple factors interact and control pollutant degradation is critically important in bioremediation. In ongoing project, funded by the Milwaukee Metropolitan Sewage District, we are studying the microbial PAH degradation potential in Lake Michigan coastal environment.

A fourth 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 past, we used the bacterial strain Methylibium petroleiphilum PM1 in several bioremediation studies to clean up successfully contaminated groundwater aquifers. In this bacterium, 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. We have recently identified that gene mdpC plays a regulatory role in the MTBE degradation pathway of M. petroleiphilum PM1. In addition, the role of the co-pollutant ethylbenzene on the expression of key MTBE degradation genes in strain PM1 has been elucidated. 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.

Honors and Awards

Way Klingler Young Scholars Award (2013)


Rachelle Beattie (Ph.D. student)

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


Former Lab Members

Michael Mashock (Ph.D. May 2016)
Yin Wei (Postdoctoral Researcher)
Cory Leeways (undergraduate)
Julia Dabrowski (BS 2016)
Allison Driskill (BS 2016)
Kyle Antene (BS 2016)
Nicholas Callard (BS 2016)
Kyle Kohlwey (BS 2015)
Tylor Zenon (BS 2015)
Kylli Paavola (BS 2015)
Jose Nieves (BS 2015)
Neha Ahuja  (BS 2014)
Nadia Hallaj (BS 2012)
Katie Shelledy (Summer Research Program 2015)
Charlie Metcalf (Summer Research Program 2014)
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).

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