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The genus Enhydrobacter, first reported as a member of the family Vibrionaceae, has been placed in the family Moraxellaceae, but as a genus incertae sedis in Bergey's Manual of Systematic Bacteriology 2nd edition. During our taxonomic investigation of Enhydrobacter-like organisms, we observed that the 16S rRNA sequences of E. aerosaccus-type strain versions NCIMB 12535T, ATCC 27094 T and CCUG 58314T were very different from the accessible data (accession no. AJ550856). Phylogenetic analysis of our 16S rRNA sequence data revealed that these organisms were located within the family Rhodospirillaceae. The genera Inquilinus, Oceanibaculum, Skermanella and Nisaea were closely related (sequence similarities were 88.3∼87.0%), but Enhydrobacter could be distinguished from these genera by growth characteristics, fatty acid profiles (C19:0 cyclo ω8c; 38.4% C18:1ω7c; 32.2%, and C16:0; 8.9% were major components), in being non-flagellated, and differing in enzymatic activities, including trypsin and β-glucosidase. From these data, we conclude that the genus Enhydrobacter should be recognized as an independent genus of the family Rhodospirillaceae within the class Alphaproteobacteria.
The genus Enhydrobacter with its single species Enhydrobacter aerosaccus had been proposed by Staley et al. (1) in 1987 as a gas-vacuolated facultative anaerobic, heterotrophic rod. The genus Enhydrobacter was allocated tentatively to the family Vibrionaceae based on phenotypic features most closely resembling members of the genus Aeromonas in growing on a defined medium with D-glucose as the sole carbon source and using ammonium salts as the sole nitrogen source for growth. Enhydrobacter could use some amino acids such as L-alanine, L-serine, and L-arginine as carbon sources for growth, a feature that was also found in some Aeromonas sp. However, E. aerosaccus differs from Aeromonas sp. because of the high G+C mol% of DNA (66.3% buoyant density for E. aerosaccus versus 57–63% for Aeromonas sp.). E. aerosaccus did not grow on some routine media, including nutrient agar, King A and B media, methyl red-Voges Proskauer medium (Clark Lubs), and blood agar. Gas vacuoles were a characteristic of E. aerosaccus, a feature not observed in any Aeromonas sp.
In 2003, the 16S rRNA sequence of the type strain of E. aerosaccus LMG 21877 was determined (DDBJ/EMBL accession no. AJ550856). Phylogenetic analysis indicated that this organism was not closely related to the family Vibrionaceae, but was highly related to the type strain of Moraxella osloensis CCUG350T (FR726160), with almost 100% sequence homology (2). However, many of the biochemical features of E. aerosaccus, described above differed from those of the genus Moraxella. In Bergey's Manual of Systematic Bacteriology 2nd edition, this genus was described as ‘Family III incertae sedis, Genus I Enhydrobacter’ in the family Moraxellaceae (3). Consequently, the taxonomic position of the genus Enhydrobacter has not yet been resolved.
In the present study, we conducted a polyphasic approach in order to determine the taxonomic position of E. aerosaccus.
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We used four versions of the type strain of Enhydrobacter aerosaccus, each from different culture collections (NCIMB, ATCC, CCUG, and LMG). The phenotypic and chemotaxonomic features of one version PAGU 1623T (= NCIMB 12535T), are according to the original description of E. aerosaccus (1) and we therefore consider this strain to be the type strain of E. aerosaccus. Versions PAGU 1624T (= ATCC27094T) and PAGU 1700T (= CCUG 58314T) also share identical 16S rRNA sequences with version PAGU 1623T (= NCIMB 12535T) and are probably the same strain. However, LMG 21877 is clearly not a strain of E. aerosaccus, differing in growth properties, G+C mol% (43%), 16S rRNA sequence and other features. Unfortunately, the 16S rRNA sequence of this strain (Acc. No. AJ550856) was first published as a representative strain of E. aerosaccus and, as a consequence, confusion may easily arise. Several studies relating to descriptions of the genus Moraxella have linked these sequence data to closely related organisms (15–18). In Bergey's Manual of Systematic Bacteriology 2nd edition, this organism was described within the family Moraxellaceae, but as a genus incertae sedis (3). In addition, the whole genome sequence has been determined for strain SK60 and submitted as an example of E. aerosaccus (GenBank, Acc. No. ACYI01000000); however, the total G+C mol% was 43%, indicating that this strain might belong to the genus Moraxella.
These data demonstrate that strain LMG 21877 is not a strain of E. aerosaccus, and is possibly related to M. osloensis. However, some biochemical reactions of LMG 21877 differ from those of M. osloensis, such as being positive for nitrate reduction (by API 20NE), and being negative for valine arylamidase and cystine arylamidase (by API-ZYM). Clearly, further investigations are needed to fully clarify the situation.