Natural transformation in Aeromonas species

Date

2008-05

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

Members of the genus Aeromonas are ubiquitous aquatic bacteria that can be causative agents of diseases in fish, amphibians, reptiles and mammals, including humans. Aeromonads are capable of natural genetic transformation, a process whereby genetic information is horizontally transferred between bacterial cells. This study described the optimal physiological parameters whereby transformation occurs. Competence was induced in 20% nutrient broth during the stationary phase of growth. Optimal transformation assay conditions for one chosen isolate were in Tris buffer with magnesium or calcium, pH 5 to 8, and a saturating concentration of 0.5 µg of DNA per assay (3.3 ng DNA µl-1) at 30ºC. Sodium was also required and could not be replaced with ammonium, potassium, or lithium. The maximal transformation frequency observed was 1.95 × 10-3 transformants (recipient cell)-1. Transformation was evaluated among environmental aeromonads by assaying auxotrophic recipients with point mutations (n = 46) with donor DNA from other wild-type, prototrophic aeromonads under optimal conditions. The results demonstrated that 57% of the isolates were able to act as recipients, and 100% were able to act as donors to at least some other aeromonads. Aeromonas species only took up DNA from closely related species, and not all species became competent under the same conditions. Four genes essential for transformation were identified—tapY1 (type IV pili biogenesis protein), ptsI (phosphoenolpyruvate phosphotransferase protein I), recBC (helicase/exonuclease involved in recombination), and clpA/clpS (caseinolytic protease). These genes indicate mechanisms of competence development through starvation (ptsI) and stress responses (clpA/clpS), DNA binding (tapYI), and recombination (recBC). To better understand the potential for Aeromonas transformation under natural conditions, assays were done using playa water. The water from three Lubbock, Texas lakes—Maxey, Higinbotham, and Stevens—was collected monthly for one year and filter-sterilized prior to assay. Transformation frequencies were analyzed with respect to levels of bicarbonate, boron, calcium, carbonate, chloride, conductivity, magnesium, nitrate nitrogen, pH, phosphorus, potassium, sodium, sulfate, sodium absorption ratio, and total dissolved solids measured in each lake. Transformation frequencies were lower in the water (maximum 1.90 × 10-5) when compared to optimal transformation buffer, yet still took place, indicating the process could occur in the lakes and serve as a natural environment for gene exchange.


This dissertation won 1st Place in the Texas Tech University Outstanding Thesis and Dissertation Award, Biological Life Sciences, 2009.

Description

Keywords

Natural transformation, Aeromonas, PtsI (protein), TapY1 (protein), Caseinolytic protease (clpA/clpS)

Citation