By using a new method for quantification of the different hopanoid derivatives, a total hopanoid content of about 30 mg/g (dry cell weight) was observed in Zymomonas mobiis. This value is the highest reported for bacteria so far. The major hopanoids in Z. mobiis were the ether and glycosidic derivatives of tetrahydroxybacteriohopane, constituting about 41 and 49% of the total hopanoids. Tetrahydroxybacteriohopane itself, diplopterol, and hopene made up about 6, 3, and 1%, respectively. Only minor changes in hopanoid composition were observed with changes in growth conditions. Earlier reports on a correlation between hopanoid content and ethanol concentration in the medium could not be confirmed. Over a wide range of ethanol concentrations (5 to 60 g/liter), growth rates (0.08 to 0.25 h-1), and temperatures (25 to 37C), the molar ratio of hopanoids to phospholipids in the cells amounted to about 0.7. Only at growth rates of >0.30 h-1 did the molar ratio increase to about 1.The gram-negative, obligately fermentative bacterium Zymomonas mobilis is well-known for its high specific rates of ethanol production of up to 5 g of ethanol per g of dry cell weight per h. In continuous culture, Z. mobilis can accumulate up to 6% (wt/vol) ethanol, and in batch culture it can even accumulate up to 13%. Thus, its ethanol tolerance is remarkably high and comparable to that of yeasts (24).Several studies have indicated that ethanol decreases the degree of membrane organization, increases membrane leakage, and influences activities of membrane-associated proteins and transport systems (16,19). In ethanol-tolerant microorganisms, adaptation to ethanol stress involves an increase in the mean chain length of incorporated fatty acids, thereby thickening the hydrophobic membrane core. Often, the proportion of monounsaturated fatty acids (e.g., cisvaccenic acid) also increases (15). The extremely ethanoltolerant Lactobacillus homohiochii even incorporates over 97% unsaturated fatty acids in the membrane when growing in the presence of 10% (wt/vol) ethanol (15). Furthermore, sterols enhance the alcohol endurability of the cell membrane in yeasts, as was demonstrated with anaerobic cultures of Saccharomyces sake, which depended on supplementation with sterol ester to acquire ethanol tolerance (12).For Z. mobilis it was shown that more than 70% of the fatty acids in the phospholipid fraction is cis-vaccenic acid, which appears to be suitable for an ethanol-tolerant microorganism (7, 30). The most striking result, however, was the observation of considerable amounts of neutral hopanoids (6). Several different hopanoid derivatives were detected in Z. mobilis (10) (Fig. 1). Structural similarities to sterols, found in eukaryotic membranes, are apparent (20). Both classes of compounds are amphiphilic and have planar, rigid carbocyclic structures. Moreover, physicochemical studies on model membranes have shown that, like sterols, hopanoids influence membrane fluidity and stability (8,17,18).Recently a new method has been developed which enab...