Since the pH of the RF-preparations used in this study did not reach extreme acidic levels, the Gad system may not have been induced. In the Arg system, decarboxylation (speA) of arginine via proton consumption resulting in the formation of agmatine stabilizes the cytoplasmic pH. Agmatine is either

exported via the arginine-agmatine antiporter (aidC) or converted (speB) to putresceine as part of the polyamine biosynthetic pathway. Considering that O157 is exposed to heat-shock, starvation and stationary-phase-like growth in the rumen, it is possible that these factors enhance acid-tolerance in the bacteria through other mechanisms such as outer membrane changes and synthesis of proton Selleck BMN 673 transport-related protective proteins, as well [49, 50]. Several stress (acid, low oxygen, osmolites, stationary phase)-responsive genes were expressed by O157 in this study, and included genes associated

with the metabolism of arginine (speA, speB), lysine (lysU), formate (hyC), tryptophan (tnaA) and maltoporin (lamB), catalase (katG), DNA polymerase-1 (polA) and AidA-1 adhesin-like protein (aidA) [49–51]. Flagellar genes are differentially Selleckchem SN-38 expressed under varying acid-stress conditions [51–53], and in our study, these genes were up-regulated in dRF and fRF but not uRF, suggesting less pH variation in the course of growth in uRF and limiting the role of flagella to motility alone. Stressed bacteria have been shown to be more adherent [35, 40, 53]; EPZ015938 research buy proteins associated with adherence (AidA-1 adhesin-like) and biofilm formation (BssR, CsgG, CsgB) were identified after 48 h incubation and not after longer incubation periods. Interestingly, several ‘resistance’ related proteins were up-regulated in RF-preparations, a subset of which (tellurite resistance, serine protease) have also been shown to contribute towards O157 adherence

[54, 55]. This suggests that adherence may be critical during the initial phase of O157 colonization and although LEE is suppressed, the bacteria rely on other mechanisms to adhere or form biofilms in the rumen. It has been observed that bacteria and protozoa in the rumen tend to adhere to the fibrous mat layers comprising of plant material to remain in the rumen and assist in the digestion of insoluble feed materials Mirabegron [56]. While this may not be in the case of O157, initial adherence to or biofilm formation on available surfaces may give the bacteria time to adapt and survive the rumen environment [34]. It appears that much of the adaptive changes are initiated early in colonization as reflected in more stress-induced, structural integrity-related outer membrane proteins (AsmA, LptE, Lpp, NagA, SlyB, OmpA, BamA, BamD, TolC, OmpW, ElaB, YbjP, LppC, YqjD), and cell division and growth proteins, being expressed at 48 h. This supports the observation that O157 is maintaining slow growth in the RF-preparations as well.