Alpha-keto acids are novel siderophores in the genera Proteus, Providencia, and Morganella and are produced by amino acid deaminases

Abstract

Growth promotion and iron transport studies revealed that certain at-keto acids generated by amino acid deaminases, by enterobacteria of the Proteus-Providencia-Morganella group (of the tribe Proteeae), show significant siderophore activity. Their iron-binding properties were confirmed by the chrome azurol S assay and UV spectra. These compounds form ligand-to-metal charge transfer bands in the range of 400 to 500 nm. Additional absorption bands of the enolized ligands at 500 to 700 nm are responsible for color formation. Siderophore activity was most pronounced with ce-keto acids possessing an aromatic or heteroaromatic side chain, like phenylpyruvic acid and indolylpyruvic acid, resulting from deamination of phenylalanine and tryptophan, respectively. In addition, cK-keto acids possessing longer nonpolar side chains, like a-ketoisocaproic acid or a-ketoisovaleric acid and even a-ketoadipic acid, also showed siderophore activity which was absent or negligible with smaller a-keto acids or those possessing polar functional groups, like pyruvic acid, ce-ketobutyric acid, or a-ketoglutaric acid. The fact that deaminase-negative enterobacteria, like Escherichia coli and Salmonella spp., could not utilize a-keto acids supports the view that specific iron-carboxylate transport systems have evolved in members of the tribe Proteeae and are designed to recognize ferric complexes of both ce-hydroxy acids and oe-keto acids, of which the latter can easily be generated by L-amino acid deaminases in an amino acid-rich medium. Exogenous siderophores, like ferric hydroxamates (ferrichromes) and ferric polycarboxylates (rhizoferrin and citrate), were also utilized by members of the tribe Proteeae.

Siderophores in marine coastal waters and their relevance for iron uptake by phytoplankton: experiments with the diatom Phaeodactylum tricornutum

“…The α-keto and α-hydroxy bidentate ligands, the MGF we used in the laboratory experiments, are weak iron binders compared to other types of siderophores produced by bacteria (Drechsel et al 1993). This is consistent with our findings.…”

“…In bacteria, production and utilization of MGF are induced by low iron concentrations in the medium (Drechsel et al 1993, Reissbrodt et al 1997. However, at least for Phaeodactylum tricornutum, utilization of MGF-bound iron is not influenced by low iron conditions.…”

“…Palenik & Morel (1990) found that marine phytoplankton can produce α-keto acids as a by-product of amino acid utilization. However, not all α-keto acids have siderophore properties (Drechsel et al 1993) and we have found in a separate study using several species of marine microalgae that these are not capable of producing such siderophores.…”

“…Trick (1989) found hydroxamic acid production in several strains of marine free-living bacteria. Drechsel et al (1993) reported that the type of MGF produced by bacteria belonging to the Providencia-Proteus-Morganella group depends on the culture medium in which they were grown. In a 'minimal medium' preference for α-hydroxy acid synthesis was observed, whereas in a medium with high concentrations of amino acids, α-keto acids were produced.…”

Discovery of a Nonclassical Siderophore, Legiobactin, Produced by Strains ofLegionella pneumophila

“…For a number of reasons, we strongly believe that this CAS-reactive substance represents a bona fide siderophore. First, the CAS assay has reliably identified and characterized a range of siderophores encompassing the iron chelators of over 20 bacterial genera (2,3,5,12,18,20,21,27,33,36,37,38,47,48,51,58,66,68,74,80,82,85,98). Second, the presence of CAS-reactive material correlated with enhanced aerobic growth in an iron-depleted defined medium (68).…”