Author ORCID Identifier

Date of Graduation


Document Type

Thesis (MS)

Program Affiliation

Biomedical Sciences

Degree Name

Masters of Science (MS)

Advisor/Committee Chair

Charles Darkoh

Committee Member

Steven Norris

Committee Member

Heidi Kaplan

Committee Member

David Volk

Committee Member

Xia Yang


Characterization of Metronidazole- and Vancomycin-Resistant Clinical Isolates of Clostridium difficile

Chioma Odo, MS.

Supervisory Professor: Charles Darkoh, Ph.D.


The incidence of C. difficile infections (CDI) has been increasing at an alarming rate. This was precipitated by the emergence of strains with increased virulence, disease severity, and high recurrence rates. These strains also exhibit high propensity for resistance to antibiotics such as fluoroquinolones and beta lactams, which has made the treatment of CDI very challenging. Currently, metronidazole and vancomycin are the most commonly used drugs for the treatment of primary CDI. Metronidazole is used for the treatment of mild to non-severe cases of CDI while vancomycin is reserved for severe CDI cases. In 25-30% of the patients treated with these antibiotics, the infection may recur, and this further complicates CDI treatment. Because C. difficile strains have an intrinsic ability to resist multiple antibiotics, it was hypothesized that there may be strains with high resistance to metronidazole and vancomycin circulating in the patient population. To investigate this hypothesis, 536 clinical CDI stool samples obtained from patients who presented with diarrhea at St. Luke’s Episcopal Hospital at the Texas Medical Center Houston, Texas, Kenyatta National Hospital, Nairobi, and Kisii Teaching and Referral Hospital, Kisii, Kenya were screened for resistant C. difficile strains. The stool samples were analyzed on C. difficile-specific differential medium containing either metronidazole (8 µg/ml) or vancomycin (4 µg/ml). These are concentrations designated by the Clinical and Laboratory Standards Institute to be the resistant breakpoint for each of the antibiotics. Stools that grew resistant colonies were identified and colonies were selected for further analysis. The minimum inhibitory concentration (MIC) of the isolates was determined by E-test and broth microdilution. The results showed that 33.1% (145/438) of the CDI patients from Texas had C. difficile strains in their stools that were resistant to both metronidazole and vancomycin. Remarkably, 93.9% of the CDI patient stools from Kenya had both metronidazole- and vancomycin-resistant C. difficile strains. The resistant strains from both patient populations also exhibit high level of tolerance for these antibiotics that far exceed the previously reported MICs (˃1024 µg/ml compared to 256 µg/ml for metronidazole and >1024 µg/ml compared to 16 µg/ml for vancomycin). All of the vancomycin-resistant strains isolated from the patients in both populations had the homologue of vanA gene, which has been shown to confer a high degree of vancomycin resistance in Gram-positive bacteria. Together, the results demonstrate high prevalence of metronidazole- and vancomycin-resistant C. difficile strains circulating in the patient populations from Texas and Kenya. The spread of C. difficile strains that are resistant to these two antibiotics of last resort may have serious public health implications and underscores the urgent need for a more in-depth analysis of the circulating resistant strains to help inform clinical decisions.


Clostridium difficile, Antibiotics, Vancomycin, Metronidazole, Resistant, Sensitive, Isolate, Strains, Prevalence

Included in

Microbiology Commons



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