Aerobic Gram-positive heterotrophic bacteria Exiguobacterium mexicanum and Microbacterium sp. in the gut lumen of Artemia franciscana larvae under gnotobiotic conditions

Abstract

In a previous study, Exiguobacterium mexicanum strain 8N (DSM 16483 T) and Microbacterium sp. strain 8L (DSM 16485) in gnotobiotic cultures of the brine shrimp, Artemia franciscana indicated beneficial effects on the development of Artemia larvae. In this study, Artemia larvae with and without autoclaved yeast as food, were challenged with strains 8N and 8L under monoxenic condition to determine ingestion, location and viability of bacterial cells in Artemia gut lumen. The results of light and scanning electron microscopy demonstrated that the bacterial cells of both strains were ingested by Artemia metanauplii. Furthermore, both strains were detected by fluorescence microscopy as live or dead cells along the Artemia gut lumen. However, the band profile in denaturing-gradient gel electrophoresis of the 16S rDNA sequence, retrieved from Artemia gut lumen exposed to both bacteria in monoxenic and dixenic conditions, showed only a band corresponding to strain 8N, suggesting that a differential bacterial ingestion or a differential generation of PCR products occurred. In all Artemia experiments, the cells of the strains 8N and 8L were confined within the chyme in the space limited by the perithrophic membrane. No evidence was found that the bacteria adhered or colonized the intestinal epithelium.