Professor
Microbial Molecular Genetics and Pathogenesis
Gene Transfer Mechanisms
Department of Microbiology
Oklahoma State University
Stillwater, OK 74078
(405)744-7730
E-mail: nirmal@okway.okstate.edu
Fax: (405) 744-6790
My research program has two objectives involving the genetics of multiple antibiotic resistance in streptococci and pathogenesis of Helicobacter pylori.
The primary one revolves around the genetics and molecular biology of antibiotic resistance in streptococci, particularly, pneumococci. Streptococcus pneumoniae, a severe human pathogen and the etiologic agent of lobar pneumonia, is surprisingly devoid of plasmids. Even though it can receive and stably maintain plasmids from other streptococci, natural isolates of this bacterium are not generally known to carry any extra-chromosomal elements. However, multiple antibiotic resistance began to appear in this species in the 1970s and the resistance genes were identified to reside on heterologous insertions in the host genome. These novel elements termed "conjugative transposons" are capable of intracellular as well as intercellular transposition by a process requiring cell-to-cell contact and that is independent of general homologous recombination functions of the host. We have been studying a 65 kb conjugative transposon, Tn5253, originally discovered in the chromosome of S. pneumoniae BM6001. By a variety of genetic and biochemical criteria, we showed that Tn5253 was a composite structure of two independent conjugative transposons, Tn5251 (Tc, 18 kb) and Tn5252 (Cm, 47 kb).
For the last several years, we have focused our efforts on studying the structural and biological properties of Tn5252. Following in vitro site-specific mutagenesis and introduction of mutations within the element via transformation, we were able to localize several transfer-related regions. DNA sequencing of the element revealed the presence of several open reading frames. Genbank analysis showed that the predicted amino acid sequences of these ORFs were significantly similar to site-recombinases, excisionases, DNA relaxases, cytosine mcthyltransferases, and proteins involved in the intercellular transport of DNA across the cytoplasmic membrane. The significance and functional roles of these and other unidentified genes and their products would be the subject of our future investigations. Conjugative transposons seem to functionally replace plasmids in pneumococci. Tn5252 provides a model system to study unusual genomic rearrangements predicted to occur and play a role in the evolution of heterogeneity such as capsular types in this species.
The second project that was initiated recently focuses on the genetics of H. pylori, the causative agent of peptic ulcer in humans. This bacterium has also been implicated in the development of duodenal cancer. Our aim is to investigate the biological and genetic properties common to the clinical specimens of H. pylori isolated from cancer patients in order to obtain clues concerning the possible origins of oncogenesis. This line of study is being carried out in collaboration with a research group in Turkey.