Food and Feed Research (Jan 2015)
RDAR morphotype: A resting stage of some Enteroba cteriaceae
Abstract
Bacteria of the genus Salmonella, some Escherichia coli strains as well as other species of the family Enterobacteriaceae, manifest the ability to express a specific phenotype, which is commonly termed as the 'rdar' (red, dry and rough) morphotype. Both rdar and non-rdar phenotypes are commonly found in the nature; however, rdar is of essential importance for bacterial survival in the environment, outside of the host organisms. In that respect, it is comparable with the process of spore formation in Gram-positive bacteria. In laboratory conditions, rdar morphotype demonstrates growth of aforementioned organisms on agar containing diazo dye Congo Red, appearing as dark red (violet), rough and irregularly margined colonies. A coordinated expression of the number of genes mediated by major transcriptional regulators CsgD (curli subunit gene D) is the prerequisite for the formation of rdar morphotype colonies. The key point in this process is the activation of csgBAC operon transcription, which encodes the synthesis of curli fimbriae and activates the AgfD regulated gene (AdrA) involved in the cellulose biosynthesis. Curli fimbriae and cellulose are two basic structural components of the rdar morphotype. Curli fimbriae enable initial surface adhesion and intercellular aggregation of bacteria, whilst the cellulose (and other exopolysaccharides) promotes intercellular interactions. CsgD affects the overall cell physiology towards a 'bacterial multicellular behavior' pattern, such as biofilm formation. The colony growth on Congo Red agar is a widely accepted laboratory method used for bacteria from the family Enterobacteriaceae and highly conserved between Salmonella. However, repeated laboratory sub-passaging results in the substantial loss of rdar morphotype and the formation of smooth mutants, suggests the necessity for using wild isolates. In this article, the characteristics of rdar and non-rdar morphotypes of diverse Salmonella serotypes isolated from animal feed, as well as Escherichia coli isolates originating from cow's milk with clinical mastitis, were presented. The research indicates substantial importance of the rdar phenotype for the persistence of Salmonella in the environment and its entering the food chain as well as the potential role of rdar phenotype of Escherichia coli in the pathogenesis of recurrent coli mastitis in dairy cows.
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