The effects of low-shear modeled microgravity on Streptococcus pneumoniae and adherent-invasive Escherichia coli



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The effects of low-shear modeled microgravity (LSMMG) were investigated on Streptococcus pneumoniae global gene expression and on adherent-invasive Escherichia coli (AIEC) physiology and colonization properties. Habitation in space exposes both humans and microbes to microgravity conditions which are characterized by reductions in fluid shear forces. Areas of low-shear stress are also encountered in physiologically relevant regions of the body including the respiratory, gastrointestinal, and urogenital tracts. The LSMMG environment impacts both bacterial physiology and virulence properties and can be modeled using rotating-wall bioreactors known as high-aspect ratio vessels (HARVs). \r\nPrevious studies have evaluated the global transcriptional profiles of Gram-negative bacteria; however, no Gram-positive species have been examined. Microarray analysis of S. pneumoniae strain TIGR4 (serotype 4), after growth under LSMMG, revealed a dramatic down-shift in gene expression based on cluster analysis. Within this group of responsive genes, statistical analyses revealed that the expression of 81 genes was significantly altered. These genes were found to be associated with 7 different functional categories, including many which were uncharacterized. Several gene groups shared common functional operons and regulons such as those involved in competence induction, antimicrobial peptide production, and carbohydrate uptake. \r\nWhile previous studies examining the effects of LSMMG on bacteria have focused on well-characterized strains of both commensal and pathogenic species, there is limited information regarding the effects of LSMMG on clinical isolates associated with Crohn’s Disease, an inflammatory bowel pathology. Analysis of wild-type AIEC strain O83:H1 and an isogenic rpoS mutant (CAA001), after growth under LSMMG, revealed alterations in environmental stress resistance and increased adherence. Altered resistances to thermal and osmotic stresses were observed by LSMMG-grown AIEC O83:H1, while resistance to oxidative and acid stresses appeared to be rpoS-dependent. Further, CAA001 displayed a hyper-adherent phenotype while grown under LSMMG. TnphoA mutagenesis was used to abolish the hyper-adherent phenotype of CAA001 under LSMMG, and the insertion was mapped within the tnaB gene, encoding tryptophan permease. Complementation of the tnaB gene in the rpoS tnaB double-mutant restored adherence capabilities. These findings extend our understanding of how mechanical forces (e.g. LSMMG) can affect the functions of Gram-positive and Gram-negative species.\r\n