In a recent paper, Chiou et al. (1) described ethanol-mediated changes in the fatty acid composition of two different strains of Escherichia coli O157:H7.

Surprisingly, although the original literature was cited, the fatty acid composition of the bacteria shown in Table 2 of this paper is contradictory to many previous scientific publications on the fatty acid composition of E. coli. The fatty acid content of E. coli has been investigated intensively. This bacterium was shown to contain the saturated fatty acids 12:0, 14:0, 16:0, and 18:0; the unsaturated fatty acids 16:1Δ9cis and 18:1Δ11cis; and the cyclopropane fatty acids 17cyclo and 19cyclo (6, 8). Among these, the three fatty acids palmitic acid (16:0), palmitoleic acid (16:1Δ9cis), and cis-vaccenic acid (18:1Δ11cis) are the dominant ones that make up more than 90% of the fatty acid content in exponentially growing cells. Very recently, this fatty acid composition was also measured for E. coli strain O157:H7 (9, 10). Therefore, I have several concerns regarding the data and conclusions presented in the paper by Chiou et al. (1). (i) In E. coli, fatty acids are synthesized by the so-called anaerobic pathway of fatty acid biosynthesis (3, 8). There are no fatty acids present in this bacterium that are synthesized by the so-called aerobic pathway of fatty acid biosynthesis. Therefore, it is confusing that the authors report the presence of oleic acid (18:1Δ9cis). Instead, E. coli is expected to contain cis-vaccenic acid (18:1Δ11cis) as C₁₈ unsaturated fatty acid (7). (ii) No odd-numbered fatty acids such as 17:0 or 19:0 have been detected in E. coli (3, 6). Therefore, it can be assumed that the peaks found in the chromatogram and presented in Table 2 (1) were the two cyclopropane fatty acids (17cyclo and 19cyclo) (4). (iii) No cis-trans isomerization of unsaturated fatty acids has been described in E. coli. This is substantiated by both genomic and physiological studies (2, 5, 7). Therefore, the reported presence of this isomerase in E. coli O157:H7 may be questioned.

In conclusion, errors caused by misidentification of fatty acid methyl ester chromatograms and maybe by the erroneous allocation of the single peaks to fatty acid standards may have contributed to the fatty acid composition reported in the paper of Chiou et al. (1).