|SAN FRANCISCO, MICHAEL|
Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2007
Publication Date: 7/15/2007
Citation: Dowd, S.E., Killinger-Mann, K., Blanton, J., San Francisco, M., Brashears, M. 2007. Positive adaptive state: Microarray evaluation of gene expression in Salmonella enterica Typhimurium upon exposure to sub-therapeutic levels of nalidixic acid. Foodborne Pathogens and Disease. 4(2):187-200.
Interpretive Summary: The livestock industry and the economy of the United States benefit tremendously from the use of subtherapeutic levels of antibiotics in production facilities. Recently, there have arisen concerns related to the development of antibiotic resistance in certain pathogens due of this practice. To begin to understand how subtherapeutic levels of antibiotics could induce resistance and generate a scientific foundation for future efforts to reduce this potential, the current study was performed. Using cutting edge methods that allow ARS scientists to study the entire genome of a bacterium and the genetic changes that occur during exposure to subtherapeutic levels of antibiotics, Salmonella enterica, a noted foodborne pathogen, was exposed to low levels of Nalidixic acid, a common antibiotic. The results revealed that the subtherapeutic levels of antibiotics induced in the bacteria a state of stress in which the bacteria actually promotes the mutation of its own genes. Because of this, the bacteria is more prone to acquiring mutations that promote resistance to the antibiotic studied. Most importantly, however, this study has also revealed genes that can be targeted to prevent these mutations through antibiotic adjuvant technologies. Studies such as this may extend the usefulness of subtherapeutic antibiotic, contribute to the development of new antibiotics, and the development of safer usage of current antibiotics.
Technical Abstract: In order to evaluate changes in gene expression that occur upon exposure to sub-therapeutic levels of a quinolones antibioitic, Salmonella enterica Typhimurium ATCC# 14028 was exposed to 1.6 ug/L of nalidixic acid (NA). Microarray analysis of the expression profile during exposure to NA was compared to control bacteria grown without this selective pressure. The results showed significant (p < 0.05) difference (fold expression differences > 2.0) in the expression of 277 genes. Notably, Salmonella Pathogenicity islands 1 and 2 (SPI1 and SPI2) and their encoded type III secretion systems were found to be repressed. On the other hand, a broad range of efflux pumps that included members from all five families of multidrug-resistance efflux pumps were observed to be induced, suggesting a broad spectrum response is initiated, facilitating export of NA from the cytoplasm. Genes involved in lipopolysaccharide chain length were also induced, suggesting membrane adaptations to NA. Finally, increased expression of the error-prone DNA repair mechanisms were also observed to be induced. From these data, we show a highly integrated response of S. enterica Typhimurium to NA that places the bacterium into a positive adaptive state. This state is conducive to the types of modifications necessary to confer actual resistance to the selective pressure of NA. Evaluation of expression profile changes that occur during exposure to sub-therapeutic levels of antibiotics may improve our understanding of how antibiotic resistance mechanisms develop.