The wealth of genome sequence information that has been produced in recent years allows for an accurate analysis of the distribution of OL biosynthesis genes. Genes coding for OlsB have a high predictive value, and it should be possible to predict the capacity of an organism to synthesize OL from the presence of the olsB gene. In many cases, where the olsB gene is phylogenetically less well conserved, the fact that olsB often occurs in an operon with olsA is of help. For the purpose of predicting the distribution of OLs, we analyzed all sequenced bacterial genomes for the presence of a gene encoding an OlsB homolog. BLAST searches with OlsB sequences from S. meliloti and B. cenocepacia pick up OlsB homologs in about 25% of the sequenced bacterial species which belong to the Alpha-, Beta-, Gamma-, Deltaproteobacteria, Actinomycetales, spirochetes, green nonsulfur bacteria, verrucomicrobia, firmicutes, Aquificales, and cyanobacteria (Table S1). Within the class Alphaproteobacteria, OlsB homologs can be detected in most sequenced species belonging to the orders Rhizobiales, Rhodobacterales, and Rhodospirillales, but are generally absent from species belonging to the orders Caulobacterales, Rickettsiales, and Sphingomonadales. OlsB can also be detected in the majority of sequenced Betaproteobacteria, including most Burkholderiales and many Neisseriales, but are absent from the Nitrosomonales. In the Gammaproteobacteria, OlsB homologs are absent from Enterobacterales, Vibrionales, Pasteurellales, Legionellales, and Aeromonadales. Many organisms presenting OlsB homologs belong to the orders Acidithiobacillales, Chromatiales, Pseudomonadales, Methylococcales, and Thiotrichales. In this context, it has to be mentioned that OLs have been described in Serratia marcescens, which belongs to the Enterobacteriaceae (Miyazaki et al., 1993). Unfortunately, no complete genome sequence of S. marcescens has been published so far. Within the Deltaproteobacteria, OlsB homologs are encoded in the genomes of Stigmatella aurantiaca, Bacteriovorax marinus, and Bdellovibrio bacteriovorus. Interestingly, OLs have been detected in the Deltaproteobacterium Sorangium cellulosum So ce56 (Keck et al., 2011), but no gene encoding an OlsB homolog is present in the genome. The best hit when searching the S. cellulosum genome with OlsB from B. cenocepacia is the gene rimI1, which is predicted to encode a ribosomal protein alanine acetyltransferase (sce1382). This suggests that a second unrelated family of N-acyl transferases might be responsible for LOL formation in S. cellulosum and possibly in other bacteria. Among the actinomycetes are several species encoding OlsB homologs. Most of them can be classified into the families Gordoniaceae, Micromonosporaceae, Mycobacteriaceae, Nocardiaceae, Pseudonocardiaceae, and Streptomyceteae. Among the spirochetes, several species from the genus Leptospira present a gene encoding an OlsB homolog. Only very few species belonging to other taxonomical groups present a gene encoding an OlsB homolog in their genomes. Compared to the large number of bacterial species that have been shown to form OL or that are predicted to be able to form OL, only few bacterial species have the now known OL-modifying enzymes. The identified OL hydroxylases belong either to the Fe2+/O2/α-ketoglutarate-dependent superfamily of hydroxylases (OlsC and OlsD) or to the di-iron fatty acid hydroxylase superfamily (OlsE) (Table S1). The phylogenetic distribution of these OL hydroxylases is described in the sections The OL hydroxylase OlsC'The OL hydroxylase OlsC ', The OL hydroxylase OlsD'The OL hydroxylase OlsD ' and The OL hydroxylase OlsE'The OL hydroxylase OlsE ' and in Table S1. The 2-hydroxylase from Burkholderia species has not been isolated yet, so it is not known whether it belongs to the already mentioned superfamilies or to yet another superfamily such as the cytochrome P450-dependent enzymes (Matsunaga et al., 2000; Lee et al., 2003; Girhard et al., 2007; Fujishiro et al., 2011). As possible OL modifications might occur only under specific stress conditions, it is possible that additional modifications with their respective responsible enzymatic activities and genes will be found in the future in other organisms.