The osmolyte trehalose strongly limits protein aggregation both in vitro and in vivo. The addition of trehalose to the culture medium reduced the aggregation of recombinant proteins expressed in Escherichia coli in a concentration-dependent manner. Comparable positive effects were obtained when the host bacteria were engineered to overexpress the gene products of otsA and otsB, the two enzymes involved in trehalose synthesis. Apparently, the osmolyte preserves protein monodispersion rather than directly facilitating protein folding. However, the stabilization of the protein folding intermediate(s) resulted in higher yields of native proteins and aggregates of lower complexity. Other osmolytes have been tested in vitro in comparison with trehalose. Di-myo-inositol1,1'-phosphate (DIP) seems to be a good candidate to test in in vivo applications, although the opportunity of using otsA/B overexpressing cells is simpler and less expensive. (c) 2007 Elsevier Inc. All rights reserved.