TheVibrio fischeri sapABCDF locus is required for normal growth, both in culture and in symbiosis

Abstract

Inactivation of thesapABCDF genes results in a loss of virulence in several bacterial pathogens of animals and plants. The role of this locus in the growth physiology ofVibrio fischeri, and in the symbiotic colonization of the squidEuprymna scolopes was investigated. In rich medium, aV. fischeri sapA insertion mutant grew at only 85% the rate of its wild-type parent. While a similar effect has been attributed to a potassium-transport defect insap mutants of enteric bacteria, theV. fischeri mutant grew more slowly regardless of the potassium concentration of the medium. Similarly, the growth-rate defect was independent of the source of either carbon, nitrogen, or phosphorous, indicating that theV. fischeri sap genes do not encode functions required for the transport of a specific form of any of these nutrients. Finally, while a delay in colonizing the nascent light organ of the squid could be accounted for by the lower growth rate of the mutant, a small but statistically significant reduction in its final population size in the host, but not in medium, suggests that thesap genes play another role in the symbiosis. All of these phenotypic defects could be genetically complementedin trans by thesapABCDF genes, but not by thesapA gene alone, indicating that the insertion insapA is polar to the four downstream genes in the locus. Thus, while thesap locus is important to the normal growth ofV. fischeri, it plays different physiological roles in growth and tissue colonization than it does in enteric pathogens.