BackgroundUnderstanding the mechanisms that govern protein stability under poly-extreme conditions continues to be a major challenge. Xylanase (BSX) from Bacillus sp. NG-27, which has a TIM-barrel structure, shows optimum activity at high temperature and alkaline pH, and is resistant to denaturation by SDS and degradation by proteinase K. A comparative circular dichroism analysis was performed on native BSX and a recombinant BSX (R-BSX) with just one additional methionine resulting from the start codon. The results of this analysis revealed the role of the partially exposed N-terminus in the unfolding of BSX in response to an increase in temperature.MethodologyWe investigated the poly-extremophilicity of BSX to deduce the structural features responsible for its stability under one set of conditions, in order to gain information about its stability in other extreme conditions. To systematically address the role of the partially exposed N-terminus in BSX stability, a series of mutants was generated in which the first hydrophobic residue, valine (Val1), was either deleted or substituted with various amino acids. Each mutant was subsequently analyzed for its thermal, SDS and proteinase K stability in comparison to native BSX.ConclusionsA single conversion of Val1 to glycine (Gly) changed R-BSX from being thermo- and alkali- stable and proteinase K and SDS resistant, to being thermolabile and proteinase K-, alkali- and SDS- sensitive. This result provided insight into the structure-function relationships of BSX under poly-extreme conditions. Molecular, biochemical and structural data revealed that the poly-extremophilicity of BSX is governed by a partially exposed N-terminus through hydrophobic interactions. Such hitherto unidentified N-terminal hydrophobic interactions may play a similar role in other proteins, especially those with TIM-barrel structures. The results of the present study are therefore of major significance for protein folding and protein engineering.

<p class="abstract"><strong>Background:</strong> Probiotics which when administered in adequate amounts confer a health benefit on the host. The role of probiotics is the replacement of pathogenic species with non-pathogenic species. Dairy food like cheese, curd and milk are considered as useful vehicles to carry probiotic bacteria. Aim of the study was to compare and evaluate the efficacy of various probiotics against different Streptococcus species.</p><p class="abstract"><strong>Methods:</strong> Three probiotic products viz probiotic milk, probiotic yogurt and probiotic capsules were used. <em>Streptococcus</em> species i.e. <em>S. mutans, S. sanguinis</em> and <em>S. sobrinus</em> were isolated from the saliva of children with moderate to high caries. 0.2 ml of each probiotic product was transferred to the blood agar plates coated with <em>Streptococcus</em> species.  </p><p class="abstract"><strong>Results:</strong> The zone of inhibition was observed in all the test groups, for all the <em>Streptococcus</em> species against all the probiotics, highest for <em>S. mutans</em> against probiotic milk. The growth of <em>S. mutans, S. sanguinis</em> and <em>S. sobrinus</em> was inhibited by <em>L. casei shirota</em> present in probiotic milk (Yakult), <em>Lactobacilli acidophilus</em> present in probiotic yogurt (Actiplus nestle) followed by <em>Lactobacilli rhamnosus</em> present in pre and probiotic capsule (Inlife).</p><p><strong>Conclusions:</strong> The use of probiotic products that are readily available and cost effective like milk and yogurt can be inculcated in general population especially in children as a preventive tool for dental caries. </p>