The genetic and molecular characteristics that define a lipA small colony variant of Escherichia coli.

摘要

Small colony variants (SCVs) can be defined as a naturally occurring sub-population of bacteria characterized by their reduced colony size and distinct biochemical properties. SCVs of Staphylococcus aureus have been studied extensively over the past two decades due to their role in recurrent human infections. However, little work has been done on SCVs of Escherichia coli, and this work has focused on the physiology and morphology that define these colonies of E. coli such as small size and slow growth. E. coli strain JW0623, has a lipA mutation in lipoic acid synthase, and is a lipoic acid auxotroph. When the mutant is grown in LB to log phase, it showed remarkable resistance to acid (pH3), hydrogen peroxide, heat and osmotic stress compared to its parent BW25113. Using RTPCR and real time RT-PCR, the expression of certain genes was compared in the two strains in an attempt to create a molecular profile of Escherichia coli SCVs. These include genes involved in glycolysis, the TCA cycle, electron transport, iron acquisition, biofilm formation and cyclopropane fatty acid synthesis. It was also demonstrated that the addition of 5 mug/m1 of lipoic acid to LB medium allows for the phenotypic rescue of the mutant; reversing its slow growth, its resistance characteristics, and elevated gene expression. These results indicate that the mutation in lipA leads to an E. coli SCV that resembles an electron transport defective SCV of S. aureus. These strains are typically auxotrophs, have increased expression of glycolysis genes, down regulation of the TCA cycle and the electron transport chain, and are phenotypically rescued by adding the missing metabolite to the rich medium. There are global shifts in gene expression which are reversible and depend on whether the auxotrophic molecule is absent or present. Looking at the E. coli SCV from an evolutionary point of view, it becomes evident that its path to survival is to express genes that confer stress resistance, allowing it to survive in a hostile environment. Our results clearly expand the understanding of the physiological and molecular characteristics of an Escherichia coli SCV. We also propose that depending on the environment, the SCV can assume one of three states of existence.