Anaerobic aniline degradation via reductive deamination of 4-aminobenzoyl-CoA in Desulfobacterium anilini

The initial reactions involved in anaerobic aniline degradation by the sulfate-reducing Desulfobacterium anilini were studied. Experiments for substrate induction indicated the presence of a common pathway for aniline and 4-aminobenzoate, different from that for degradation of 2-aminobenzoate, 2-hydroxybenzoate, 4-hydroxybenzoate, or phenol. Degradation of aniline by dense cell suspensions depended on CO₂ whereas 4-aminobenzoate degradation did not. If acetyl-CoA oxidation was inhibited by cyanide, benzoate accumulated during degradation of aniline or 4-aminobenzoate, indicating an initial carboxylation of aniline to 4-aminobenzoate, and further degradation via benzoate of both substrates. Extracts of alinine or 4-aminobenzoategrown cells activated 4-aminobenzoate to 4-aminobenzoyl-CoA in the presence of CoA, ATP and Mg²⁺. 4-Aminobenzoyl-CoA-synthetase showed a K _m for 4-aminobenzoate lower than 10 M and an activity of 15.8 nmol · min^-1 · mg^-1. 4-Aminobenzoyl-CoA was reductively deaminated to benzoyl-CoA by cell extracts in the presence of low-potential electron donors such as titanium citrate or cobalt sepulchrate (2.1 nmol · min^-1 · mg^-1). Lower activities for the reductive deamination were measured with NADH or NADPH. Reductive deamination was also indicated by benzoate accumulation during 4-aminobenzoate degradation in cell suspensions under sulfate limitation. The results provide evidence that aniline is degraded via carboxylation to 4-aminobenzoate, which is activated to 4-aminobenzoyl-CoA and further metabolized by reductive deamination to benzoyl-CoA.     

Anaerobic biodegradation ofPara-cresol under three reducing conditions

The anaerobic degradation ofp-cresol was studied with one sediment source under three reducing conditions—denitrifying, sulfidogenic, and methanogenic. Loss ofp-cresol (1 mM) in all the anaerobic systems took initially 3 to 4 weeks. In acclimated culturesp-cresol was degraded in less than a week.p-Cresol was completely metabolized under denitrifying, sulfidogenic, and methanogenic conditions, with formation of nitrogen gas, loss of sulfate, and formation of methane and carbon dioxide, respectively.p-Cresol metabolism proceeded throughp-hydroxybenzal-dehyde andp-hydroxybenzoate under denitrifying and methanogenic conditions. These compounds were rapidly degraded in cultures acclimated top-cresol under all three reducing conditions. These results suggest that the initial pathway ofp-cresol degradation is the same under denitryfying, sulfidogenic, and methanogenic conditions and proceeds via oxidation of the methyl substituent top-hydroxybenzaldehyde andp-hydroxybenzoate. The initial rate ofp-hydroxybenzaldehyde degradation was high in both the unacclimated cultures and in the cultures acclimated top-cresol, suggesting that this step is nonspecific. Benzoate was additionally detected as a metabolite followingp-hydroxybenzoate in the methanogenic cultures, but not in the denitrifying or sulfidogenic cultures. The degradation pathway therefore may diverge afterp-hydroxybenzoate formation depending on which electron acceptor is available.     

Fermentative degradation of monohydroxybenzoates by defined syntrophic cocultures

From anaerobic freshwater enrichment cultures with 3-hydroxybenzoate as sole substrate, a slightly curved rod-shaped bacterium was isolated in coculture with Desulfovibrio vulgaris as hydrogen scavenger. The new isolate degraded only 3-hydroxybenzoate or benzoate, and depended on syntrophic cooperation with a hydrogenoxidizing methanogen or sulfate reducer. 3-Hydroxybenzoate was degraded via reductive dehydroxylation to benzoate. With 2-hydroxybenzoate (salicylate), short coccoid rods were enriched from anaerobic freshwater mud samples, and were isolated in defined coculture with D. vulgaris. This isolate also fermented 3-hydroxybenzoate or benzoate in obligate syntrophy with a hydrogen-oxidizing anaerobe. The new isolates were both Gram-negative, non-sporeforming strict anaerobes. They fermented hydroxybenzoate or benzoate to acetate, CO₂, and, presumably, hydrogen which was oxidized by the syntrophic partner organism. With hydroxybenzoates, but not with benzoate, Acetobacterium woodii could also serve as syntrophic partner. Other substrates such as sugars, alcohols, fatty or amino acids were not fermented. External electron acceptors such as sulfate, sulfite, nitrate, or fumarate were not reduced. In enrichment cultures with 4-hydroxybenzoate, decarboxylation to phenol was the initial step in degradation which finally led to acetate, methane and CO₂.     

Activation of aromatic acids and aerobic 2-aminobenzoate metabolism in a denitrifying Pseudomonas strain

The initial reactions possibly involved in the acrobic and anaerobic metabolism of aromatic acids by a denitrifying Pseudomonas strain were studied. Several acyl CoA synthetases were found supporting the view that activation of several aromatic acids preceeds degradation. A benzoyl CoA synthetase activity (AMP forming) (apparent K _m values of the enzyme from nitrate grown cells: 0.01 mM benzoate, 0.2 mM ATP, 0.2 mM coenzyme A) was present in aerobically grown and anaerobically, nitrate grown cells when benzoate or other aromatic acids were present. In addition to benzoate and fluorobenzoates, also 2-amino-benzoate was activated, albeit with unfavorable K _m (0.5 mM 2-aminobenzoate). A 2-aminobenzoyl CoA synthetase (AMP forming) was induced both aerobically and anaerobically with 2-aminobenzoate as growth substrate which had a similar substrate spectrum but a low K _m for 2-aminobenzoate (<0.02 mM). Anaerobic growth on 4-hydroxybenzoate induced a 4-hydroxybenzoyl CoA synthetase, and cyclohexanecarboxylate induced another synthetase. In contrast, 3-hydroxybenzoate and phenyl-acetate grown anaerobic cells appeared not to activate the respective substrates at sufficient rates. Contrary to an earlier report extracts from aerobic and anaerobic 2-aminobenzoate grown cells catalysed a 2-aminobenzoyl CoA-dependent NADH oxidation. This activity was 10–20 times higher in aerobic cells and appeared to be induced by 2-aminobenzoate and oxygen. In vitro, 2-aminobenzoyl CoA reduction was dependent on 2-aminobenzoyl CoA NAD(P)H, and oxygen. A novel mechanism of aerobic 2-aminobenzoate degradation is suggested, which proceeds via 2-aminobenzoyl CoA.     

Biodegradation of phenol and <Emphasis Type="Italic">m</Emphasis>-cresol by <Emphasis Type="Italic">Candida albicans</Emphasis> PDY-07 under anaerobic condition

Strain Candida albicans PDY-07 was used to study the anaerobic biodegradation of phenol and m-cresol as single and dual substrates in batch cultures. The strain had a higher potential to degrade phenol than m-cresol. The cell growth kinetics of batch cultures with various initial m-cresol concentrations was investigated, and the Haldane kinetic model adequately described the dynamic behavior of cell growth on m-cresol. When cells grew on the mixture of phenol and m-cresol, substrate interactions were observed. Phenol inhibited the utilization of m-cresol; on the other hand, m-cresol also inhibited the degradation of phenol. However, the presence of low-concentration phenol enhanced m-cresol biodegradation; 100 mg/l m-cresol could be completely degraded within a shorter period of time than m-cresol alone in the presence of 150–300 mg/l phenol. The maximum m-cresol biodegradation rate was obtained at the existence of 200 mg/l phenol. Phenol was preferably utilized by the strain as a carbon and energy source. In addition, a sum kinetics model was used to describe the cell growth behavior in binary mixture of phenol and m-cresol, and the interaction parameters were determined. The model adequately predicted the growth kinetics and the interaction between the substrates.     

Anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) by a denitrifying bacterium

The anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) was studied in a denitrifying bacterium. Cells grown with 2-hydroxybenzoate were simultaneously adapted to degrade benzoate. Extract of these cells formed benzoate or benzoyl-CoA when incubated under reducing conditions with salicylate, MgATP, and coenzyme A, suggesting a degradation of 2-hydroxybenzoate via benzoate or benzoyl-CoA. This suggestion was supported by enzyme activity measurements. In extracts of 2-hydroxybenzoate-grown cells, the following enzyme activities were detected: two CoA ligases, one specific for 2-hydroxybenzoate, the other for benzoate, and two different enzyme activities catalyzing the reductive transformation of 2-hydroxybenzoyl-CoA. These findings suggest a degradation of salicylic acid by two new enzymes, 2-hydroxybenzoate-CoA ligase (AMP-forming) and 2-hydroxybenzoyl-CoA reductase (dehydroxylating), catalyzing (1) 2-hydroxybenzoate + MgATP + CoASH → 2-hydroxybenzoyl-CoA + MgAMP + PP_i (2) 2-hydroxybenzoyl-CoA + 2[H] → benzoyl-CoA + H₂O Benzoyl-CoA was dearomatized by reduction of the ring. This represents another case in which benzoyl-CoA is a central intermediate in anaerobic aromatic metabolism. Received: 1 February 1996 / Accepted: 24 February 1996     

Anaerobic degradation of p-cresol in batch and continuous cultures by a denitrifying bacterial consortium

Summary The anaerobic degradation of p-cresol under denitrifying conditions by a bacterial consortium was studied in batch and continuous cultures. Concentrations up to 3 mm were degraded within 5–6 days with 4-hydroxybenzyl alcohol, 4-hydroxybenzaldehyde and 4-hydroxybenzoate as intermediates. Steady states could be maintained at only one dilution rate, D=0.04 h^–1. A further increase in the dilution rate to 0.0 8 h^–1 resulted in culture wash-out. An estimation of the Saturation constant was made (<1 mg/l), taking the maximum specific growth rate as 0.045 h^–1, thus yielding a value of 0.125 mg p-cresol/l. Correspondence to: N. Khoury     

Anaerobic degradation of phenylacetic acid by mixed and pure cultures

Summary A phenylacetic acid-degrading mixed culture was enriched from effluent of an anaerobic reactor for the treatment of waste water from cellulose bleaching. From this consortium a phenylacetic acid-degrading pure culture, strain DSU3, was isolated and, due to its typical morphology and substrate spectrum, tentatively classified as a Desulfosarcina sp. It could grow on and degrade phenylacetic acid, cyclohexane carboxylate, cyclohexylacetate, benzoate, fumaric acid and several volatile fatty acids, while phenol, o-hydroxybenzoate, p-hydroxybenzoate and glucose were not utilized. Production of mandelic acid from phenylacetic acid by the enrichment culture and utilization of benzoate, an intermediate of the mandelic acid pathway, by strain DSU3 may presumably indicate degradation of phenylacetic acid via the mandelic acid pathway.     

Degradation of phenol via carboxylation to benzoate by a defined,obligate syntrophic consortium of anaerobic bacteria

Summary An obligate syntrophic culture was selected in mineral medium with phenol as the only carbon and energy source. The consortium consisted of a short and a long rod-shaped bacterium and of low numbers of Desulfovibrio cells, and grew only in syntrophy with methanogens, e. g. Methanospirillum hungatei. Under N₂/CO₂, phenol was degraded via benzoate to acetate, CH₄ and CO₂, while in the presence of H₂/CO₂ benzoate was formed, but not further degraded. When 4-hydroxybenzoate was fed to the mixed culture, it was decarboxylated to phenol prior to benzoate formation and subsequent ring cleavage. Isolation of pure cultures of the two rod-shaped bacteria failed. Microscopic observations during feeding of either 4-hydroxybenzoate, phenol or benzoate implied an obligate syntrophic interdependence of the two different rod-shaped bacteria and of the methanogen. The non-motile rods formed phenol from 4-hydroxybenzoate and benzoate from phenol, requiring an as yet unknown co-substrate or co-factor, probably cross-fed by the short, motile rod. The short, motile rodshaped bacterium grew only in syntrophy with methanogens and degraded benzoate to acetate, CO₂ and methane. Desulfovibrio sp., present in low numbers, apparently could not contribute to the degradation of phenol or 4-hydroxybenzoate.     

Initiation of anaerobic degradation of p-cresol by formation of 4-hydroxybenzylsuccinate in desulfobacterium cetonicum

The anaerobic bacterium Desulfobacterium cetonicum oxidized p-cresol completely to CO₂ with sulfate as the electron acceptor. During growth, 4-hydroxybenzylsuccinate accumulated in the medium. This finding indicated that the methyl group of p-cresol is activated by addition to fumarate, analogous to anaerobic toluene, m-xylene, and m-cresol degradation. In cell extracts, the formation of 4-hydroxybenzylsuccinate from p-cresol and fumarate was detected at an initial rate of 0.57 nmol min⁻¹ (mg of protein)⁻¹. This activity was specific for extracts of p-cresol-grown cells. 4-Hydroxybenzylsuccinate was degraded further to 4-hydroxybenzoyl-coenzyme A (CoA), most likely via β-oxidation. 4-Hydroxybenzoyl-CoA was reductively dehydroxylated to benzoyl-CoA. There was no evidence of degradation of p-cresol via methyl group oxidation by p-cresol-methylhydroxylase in this bacterium.     

An Uncultivated Nitrate-Reducing Member of the Genus Herminiimonas Degrades Toluene

Stable isotope probing (SIP) is a cultivation-free methodology that provides information about the identity of microorganisms participating in assimilatory processes in complex communities. In this study, a Herminiimonas-related bacterium was identified as the dominant member of a denitrifying microcosm fed [¹³C]toluene. The genome of the uncultivated toluene-degrading bacterium was obtained by applying pyrosequencing to the heavy DNA fraction. The draft genome comprised ∼3.8 Mb, in 131 assembled contigs. Metabolic reconstruction of aromatic hydrocarbon (toluene, benzoate, p-cresol, 4-hydroxybenzoate, phenylacetate, and cyclohexane carboxylate) degradation indicated that the bacterium might specialize in anaerobic hydrocarbon degradation. This characteristic is novel for the order Burkholderiales within the class Betaproteobacteria. Under aerobic conditions, the benzoate oxidation gene cluster (BOX) system is likely involved in the degradation of benzoate via benzoyl coenzyme A. Many putative genes for aromatic hydrocarbon degradation were closely related to those in the Rhodocyclaceae (particularly Aromatoleum aromaticum EbN1) with respect to organization and sequence similarity. Putative mobile genetic elements associated with these catabolic genes were highly abundant, suggesting gene acquisition by Herminiimonas via horizontal gene transfer.     

Involvement of coenzyme A thioesters in anaerobic metabolism of 4-hydroxybenzoate by Rhodopseudomonas palustris.

The initial steps of anaerobic 4-hydroxybenzoate degradation were studied in whole cells and cell extracts of the photosynthetic bacterium Rhodopseudomonas palustris. Illuminated suspensions of cells that had been grown anaerobically on 4-hydroxybenzoate and were assayed under anaerobic conditions took up [U-14C]4-hydroxybenzoate at a rate of 0.6 nmol min-1 mg of protein-1. Uptake occurred with high affinity (apparent Km = 0.3 microM), was energy dependent, and was insensitive to external pH in the range of 6.5 to 8.2 Very little free 4-hydroxybenzoate was found associated with cells, but a range of intracellular products was formed after 20-s incubations of whole cells with labeled substrate. When anaerobic pulse-chase experiments were carried out with cells incubated on ice or in darkness, 4-hydroxybenzoyl coenzyme A (4-hydroxybenzoyl-CoA) was formed early and disappeared immediately after addition of excess unlabeled substrate, as would be expected of an early intermediate in 4-hydroxybenzoate metabolism. A 4-hydroxybenzoate-CoA ligase activity with an average specific activity of 0.7 nmol min-1 mg of protein-1 was measured in the soluble protein fraction of cells grown anaerobically on 4-hydroxybenzoate. 4-Hydroxybenzoyl-CoA was the sole product formed from labeled 4-hydroxybenzoate in the ligase reaction mixture. 4-Hydroxybenzoate uptake and ligase activities were present in cells grown anaerobically with benzoate, 4-hydroxybenzoate, and 4-aminobenzoate and were not detected in succinate-grown cells. These results indicate that the high-affinity uptake of 4-hydroxybenzoate by R. palustris is due to rapid conversion of the free acid to its CoA derivative by a CoA ligase and that this is also the initial step of anaerobic 4-hydroxybenzoate degradation.     

Aerobic and anaerobic catabolism of phthalic acid by a nitrate-respiring bacterium

A Bacillus sp., isolated by anaerobic enrichment on a o-phthalic acid-nitrate medium, grew either aerobically or anaerobically on phthalic acid. Cells grown anaerobically on phthalate immediately oxidized phthalate and benzoate with nitrate, whereas aerobic oxidation only occurred after a lag period and was inhibited by chloramphenicol. 2-Fluoro-and 3-fluorobenzoate were formed from 3-fluorophthalate by cells grown anaerobically on phthalate. Aerobically grown cells immediately oxidized phthalate, benzoate, 3-hydroxybenzoate and gentisate with oxygen. The aerobic and anaerobic route of catabolism of phthalate may thus share an initial decarboxylation to benzoate. This is the first report of the anaerobic dissimilation of phthalic acid by a pure bacterial culture.     

Anaerobic degradation of m-cresol by Desulfobacterium cetonicum is initiated by formation of 3-hydroxybenzylsuccinate

The anaerobic bacterium Desulfobacterium cetonicum oxidized m-cresol completely with sulfate as electron acceptor. During growth, 3-hydroxybenzylsuccinate (identified by gas chromatography/mass spectroscopy and by comparison of high-performance liquid chromatography retention time and UV spectrum with a chemically synthesized reference compound) accumulated in the medium. This finding indicates that the methyl group of m-cresol is activated by addition to fumarate as in the case of anaerobic toluene metabolism. In cell-free extracts of D. cetonicum, the formation of 3-hydroxybenzylsuccinate from m-cresol and fumarate was detected at an activity of 0.5 nmol min^–1 (mg protein)^–1. This reaction depended strictly on anoxic assay conditions. Treatment with air resulted in a complete loss of activity; however, some activity could be recovered after restoring anoxic conditions. The activity was slightly membrane-associated. 3-Hydroxybenzylsuccinate was degraded via CoA thioesterification and further oxidation to 3-hydroxybenzoyl-CoA as subsequent steps in the degradation pathway. Received: 20 May 1999 / Accepted: 19 July 1999     

Effect of fluorinated analogues of phenol and hydroxybenzoates on the anaerobic transformation of phenol to benzoate

The effects of fluorinated analogues on the anaerobic transformation of phenol to benzoate were examined. At 250 M 2- or 3-fluorophenol, phenol transformation was delayed. 2-Fluorophenol had no apparent effect on subsequent degradation of benzoate, but benzoate accumulated in the presence of 250 M 3-fluorophenol. In contrast, 4-fluorophenol at 2 mM had no effect on either phenol transformation or benzoate degradation. Phenol and 2-, or 3-fluorophenol were transformed simultaneously, but phenol was transformed more rapidly than either fluorophenol. Thus, fluorinated analogues of phenol did not prevent anaerobic transformation of phenol to benzoate. 2-Fluorophenol was converted to 3-fluorobenzoate, and phenol enhanced the rate and extent of its transformation. 3-Fluorophenol was transformed to 2-fluorobenzoate to a limited extent (3%) when phenol was present. 4-Fluorophenol was not transformed regardless of the presence of phenol. 3-Fluoro-4-hydroxybenzoate, a potential fluorinated intermediate product of para-carboxylation, was transformed rapidly to 2-fluorophenol and 3-fluorobenzoate, irrespective of the presence of phenol, indicating that both dehydroxylation and decarboxylation occurred. Initially, 2-fluorophenol and 3-fluorobenzoate were rapidly formed in an approximate molar ratio of 2 : 1. Once 3-fluoro-4-hydroxybenzoate was completely removed, the 2-fluorophenol, initially formed, was converted to 3-fluorobenzoate at a slower rate. Thus, phenol enhanced transformation of the fluorinated analogues, and the products of transformation suggested para-carboxylation. 3-Fluoro-2-hydroxybenzoate was not transformed in either the presence or absence of phenol, indicating that ortho-carboxylation did not occur.Abbreviations 3F4HB 3-fluoro-4-hydroxybenzoate - 3F2HB 3-fluoro-2-hydroxybenzoate (3-fluorosalicylate) Contribution No. 692, Environmental Research Laboratory, U.S. EPA, Gulf Breeze, FL. 32561, USA     

Uptake of phenol by the phenol-metabolizing bacteria of a stable,strictly anaerobic consortium

Phenol was absorbed unspecifically by active and by inactivated cells of a strictly anaerobic, phenol-degrading consortium to reach about twice the concentration of the medium. The absorption was temperature-dependent. A Q₁₀ of 1.7 was determined, indicating that accumulation was due to diffusion or facilitated diffusion and not to an active transport process. At increasing phenol concentration in the medium, concentrated cell suspensions adsorpted phenol proportionally until saturation was reached at about 25 nmol phenol/mg cell dry weight. At a phenol concentration in the medium of 2 mm, the washed cell pellet contained 3.5 mm phenol. Under conditions that allowed phenol metabolism (presence of CO₂), [¹⁴C]4-hydroxybenzoyl-coenzyme A and [¹⁴C]4-hydroxybenzoate were found as early intermediates of [U-¹⁴C]phenol degradation for the first time. [¹⁴C]Benzoate was excreted stoichiometrically if phenol degradation to acetate was prevented by H₂. Absolutely no ¹⁴C-label was found in the phenylphosphate peak after HPLC separation, which excluded phosphorylation of phenol during uptake or during degradation in the cells. Correspondence to: J. Winter     

Anaerobic degradation of toluene by pure cultures of denitrifying bacteria

Several denitrifying Pseudomonas spp., isolated with various aromatic compounds, were tested for the ability to degrade toluene in the absence of molecular oxygen. Four out of seven strains were able to degrade toluene in the presence of N₂O. More than 50% of the ¹⁴C from ring-labelled toluene was released as CO₂, and up to 37% was assimilated into cell material. Furthermore it was demonstrated for two strains that they were able to grow on toluene as the sole carbon and energy source in the presence of N₂O. Suspensions of cells pre-grown on toluene degraded toluene, benzaldehyde or benzoate without a lag phase and without accumulation of intermediates. p-Cresol, p-hydroxybenzylalcohol, p-hydroxybenzaldehyde or p-hydroxybenzoate was degraded much slower or only after distinct lag times. In the presence of fluoroacetate [¹⁴C]toluene was transformed to [¹⁴C]benzoate, which suggests that anaerobic toluene degradation proceeds through oxidation of the methyl side chain to benzoate.     

Ubiquinone biosynthesis in Escherichia coli K-12. Accumulation of an octaprenol, farnesylfarnesylgeraniol, by a multiple aromatic auxotroph

Cell extracts of a multiple aromatic auxotroph of Escherichia coli K-12, strain AB2830, grown in the absence of precursors of the quinone rings of the ubiquinone and menaquinone molecules, converted 4-hydroxy[U-¹⁴C]benzoate into a mixture of 3-octaprenyl-4-hydroxybenzoate and 2-octaprenylphenol. An octaprenol, farnesylfarnesylgeraniol, was isolated from such cell extracts and characterized by n.m.r. and mass spectroscopy. Neither the octaprenol, nor polyprenylation of 4-hydroxy[U-¹⁴C]benzoate, could be detected in cell extracts of strain AB2830 grown in the presence of 0.1mm-4-hydroxybenzoate. It was concluded that, in the biosynthesis of ubiquinone, the polyprenyl side chain is added to 4-hydroxybenzoate as a C₄₀ unit, the active form of which is converted by cell extracts into farnesylfarnesylgeraniol. The multiple aromatic auxotroph, when grown in the absence of 4-hydroxybenzoate but in the presence of 4-aminobenzoate, converted the latter compound into 3-octaprenyl-4-aminobenzoate. This compound was isolated from whole cells and characterized by n.m.r. and mass spectroscopy.     

Growth performance and pathway flux determine substrate preference of Alcaligenes eutrophus during growth on acetate plus aromatic compound mixtures

During batch growth of Alcaligenes eutrophus on various aromatic compounds in the presence of acetate, several distinct behaviour patterns were observed. The utilization of substrates of the meta pathway (phenol or p-cresol) was inhibited by acetate. When the aromatic was a substrate of the p-hydroxybenzoate branch of the ortho pathway, growth was mixotrophic, i.e. both substrates were consumed simultaneously. For the substrates of the gentisate pathway or the benzoate branch of the ortho pathway, substrate preference was governed by growth performance. Aromatic compounds enabling growth rate and yields higher than those obtained on acetate alone (i.e. benzoate, benzaldehyde, m-hydroxybenzoate and gentisate) inhibited acetate utilization, while acetate was the substrate consumed preferentially in mixtures containing aromatic compounds supporting only slow growth (i.e. benzoyl formate and 4-fluorobenzoate). Received: 18 April 1996 / Received revision: 9 July 1996 / Accepted: 15 July 1996