Characterization of Mechanisms of Antibiotic Resistance in Neisseria gonorrhoeae

Duncan, Margaret Elizabeth

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March 22, 2019

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Duncan, Margaret Elizabeth
- Affiliation: School of Medicine, Department of Pharmacology

Abstract

Neisseria gonorrhoeae is a Gram-negative diplococcus that causes the sexually transmitted infection, gonorrhea. As a naturally competent organism, N. gonorrhoeae can take up genetic material from outside the cell and incorporate it into its own DNA through homologous recombination. This has allowed the pathogen to generate antigenic diversity for escape from the human immune system and to develop and spread antibiotic resistance genes. This mode of resistance, chromosomally mediated resistance, is complex and requires at least five resistance determinants. Four of these determinants have been identified at the molecular level: penA (mutations in penicillin-binding protein 2 [PBP 2]), mtrR (overexpression of the MtrC-MtrD-MtrE efflux pump), penB (mutations in PorB1b), and ponA (mutation in PBP 1). These determinants can be readily transferred from a penicillin-resistant donor (FA6140) to a susceptible recipient strain (FA19). However, despite repeated attempts, transformation to high-level penicillin resistance equivalent to the donor strain has not been achieved. I initiated studies to further elucidate the complex mechanisms of chromosomally mediated antibiotic resistance in N. gonorrhoeae. First, I characterized unique mutations in mtrR and penB found in a group of clinical isolates from New Caledonia. These mutations proved to have weaker phenotypes than the more common mutations. I also identified a set of 67 genes found only in penicillin-resistant strains of N. gonorrhoeae. When transformed into a penicillin-sensitive strain, none of the genes increased resistance. Finally, I initiated studies to elucidate the mechanisms of high-level penicillin resistance and to characterize Factor X, the unknown gene(s) that is (are) responsible for high-level resistance. My results indicate that the phenotype of Factor X is independent of the other four resistance determinants and is expressed phenotypically even in the absence of other determinants. Additionally, Factor X plays a role in increasing resistance to bactericidal, but not bacteriostatic, antibiotics, reminiscent of studies in E. coli whereby bactericidal antibiotics, no matter what pathway they inhibit, kill bacteria through a common oxidative stress mechanism. Together, the results of these studies add to our knowledge about the interactions between N. gonorrhoeae and antimicrobials, and further our understanding of resistance mechanisms.

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