Strain L5 Research Articles

Isolation and identification of mycorrhizal helper bacteria of Vaccinium uliginosum and their interaction with mycorrhizal fungi.

Mycorrhizal helper bacteria (MHB) can promote mycorrhizal fungal colonization and form mycorrhizal symbiosis structures. To investigate the effect of interactions between mycorrhizal beneficial microorganisms on the growth of blueberry, 45 strains of bacteria isolated from the rhizosphere soil of Vaccinium uliginosum were screened for potential MHB strains using the dry-plate confrontation assay and the bacterial extracellular metabolite promotion method. The results showed that the growth rate of mycelium of Oidiodendron maius 143, an ericoid mycorrhizal fungal strain, was increased by 33.33 and 77.77% for bacterial strains L6 and LM3, respectively, compared with the control in the dry-plate confrontation assay. In addition, the extracellular metabolites of L6 and LM3 significantly promoted the growth of O. maius 143 mycelium with an average growth rate of 40.9 and 57.1%, respectively, the cell wall-degrading enzyme activities and genes of O. maius 143 was significantly increased. Therefore, L6 and LM3 were preliminarily identified as potential MHB strains. In addition, the co-inoculated treatments significantly increased blueberry growth; increased the nitrate reductase, glutamate dehydrogenase, glutamine synthetase, and glutamate synthase activities in the leaves; and promoted nutrient uptake in blueberry. Based on the physiological, and 16S rDNA gene molecular analyses, we initially identified strain L6 as Paenarthrobacter nicotinovorans and LM3 as Bacillus circulans. Metabolomic analysis revealed that mycelial exudates contain large amounts of sugars, organic acids and amino acids, which can be used as substrates to stimulate the growth of MHB. In conclusion, L6 and LM3 and O. maius 143 promote each other's growth, while co-inoculation of L6 and LM3 with O. maius 143 can promote the growth of blueberry seedlings, providing a theoretical basis for further studies on the mechanism of ericoid mycorrhizal fungi-MHB-blueberry interactions. It laid the technical foundation for the exploitation of biocontrol strain resources and the development of biological fertilizer.

Two chronically misdiagnosed patients infected with Nocardia cyriacigeorgica accurately diagnosed by whole genome resequencing.

Nocardiosis is a rare but life-threatening infection particularly affecting immuno-compromised hosts, causing localized or systemic suppurative disease usually in human beings. Nocardia species, as the pathogen of nocardiosis, are difficult to differentiate because of their complex colony morphological features. In this study, we describe two patients who had been misdiagnosed for a long time infected with Nocardia cyriacigeorgica with completely different morphology were accurately diagnosed. Single colonies were analyzed by Gram staining, acid-fast stain, mass spectrometry and whole genome resequencing (WGRS). These two bacterial, strains L5.53 and L5.54, were found to be Gram-negative and acid-fast-weak positive. Blood sample culturing of strain L5.53 yielded white colonies, which were like a layer of hoarfrost, while colonies of L5.54 were yellow, rough, slightly convex. The two strains were identified as Nocardia sp. by mass spectrometry, and WGRS accurately determined them as N. cyriacigeorgica. After medical treatment, one patient was cured and the other was still receiving treatment in the hospital. It can be seen that Nocardia sp. cannot be accurately classified and identified only by phenotypic tests such as bacterial morphological differences, so it is necessary to identify Nocardia spp. with phenotypic tests in combination with other molecular biology technologies, such as WGRS.

English

For production and preservation of traditional fermented foods, the genera, lactic acid bacteria (LAB) have been used. This study was carried out to determine the characteristics and the antimicrobial activities of lactic acid bacteria isolated from selected Nigerian traditional fermented foods. Changes in pH and titratable acidity (TA) of the samples were investigated for a period of four days (96 h). Eleven tentative LAB from fermented maize and cassava (Ogi and Fufu, respectively) were isolated and characterized. The spoilage organisms from fish were aseptically identified and the antimicrobial activity was determined by agar well diffusion method against three isolated food spoilage organisms (Pseudomonas aeruginosa, Enterobacter aerogene and Bacillus cereus). The isolates were selected and further identified as Lactobacillus amylolyticus strain L6, Lactobacillus plantarum strain ci-4w and Lactobacillus sakei strain MLS1 by the aide of genotypic characteristics (16S rRNA gene sequences). These strains were screened for their EPS producing activity, resistance to low pH and bile salts as well as bacteriocin activity. These strains can be used as starter culture or protective cultures to improve the hygiene, quality and increased safety of the food products by inhibiting the food borne pathogens and spoilage microorganisms.   Key words: Lactic acid bacteria, fermented foods, exopolysaccharides, antimicrobial activity, ogi, fufu.

Characterization of a lactic acid bacteria using branched-chain amino acid transaminase and protease from Jinhua Ham and application in myofibrillar protein model

This study was to screen a strain with both branched-chain amino acid transaminase (BCAT) and protease activities for producing methyl-branched flavor compounds in a myofibrillar protein extract model. Lactic acid bacteria (LAB) was isolated from Jinhua ham and screened with BCAT activity by an electrochemical sensor and protease activity by the agar plate method. In the culture medium, strain L6 was selected with high utilization rate and characteristic metabolites content of branched-chain amino acids (BCAAs), and identified as Lactobacillus fermentum YZU-06 (L. fermentum). Compared with the previously reported L. plantarum, L. fermentum exhibited an excellent capacity of hydrolyzing myofibrillar protein with the higher contents of free amino acids, peptides and small molecular weight proteins. Moreover, L. fermentum group presented more BCAAs metabolites of 2-methylbutanal and 3-methylbutanal than that of L. plantarum group. In conclusion, L. fermentum YZU-06 is a promising starter culture to improve the flavor of fermented meat products.

Effective biodeterioration of a common endocrine disruptor 17β-estradiol using mixed microbial cultures isolated from waste water

Twelve bacteria were isolated from wastewater for the degradation of 17β-estradiol (strain K1, K2, K3, K4, L5, L6, L7, L8, M9, M10, M11 and M12) by enrichment method. Initial screening revealed maximum degradation (78 ± 1.1%, and 86 ± 4.2%) of 17β-estradiol by strains L7, and M10, respectively. The production of manganese peroxidase by bacteria gets involved in the degradation of various contaminants. Almost all strains isolated from the enriched medium produced manganese peroxidase. Based on initial screening on 17β-estradiol degradation and manganese peroxidase production, two bacteria (Flavobacterium longum L7 and Pseudomonas aeruginosa M10) were selected to analyze 17β-estradiol degradation in co-culture system. The co-cultured strains could rapidly remove 17β-estradiol (91.3%) from the culture medium at 10 mg/L 17β-estradiol initial concentration. Moreover, degradation of 17β-estradiol was only 82% and 75% when the strains L7 and M10 were treated individually. Initial 17β-estradiol concentration was 10 mg/L and 0.3% inoculum were optimum and achieved >90% degradation. Alkaline pH was optimum (pH 7.5) and reached 98.4 ± 7.5% degradation in co-culture. Glucose and trehalose improved the growth and degradation of 17β-estradiol. The organic nitrogen sources such as, yeast extract and peptone improved the removal of 17β-estradiol. The selected bacterial strains were resistant against penicillin, erythromycin, oxytetracycline and tetracycline. The present findings demonstrated the application of F. longum L7 and P. aeruginosa M10 co-culture for the removal of steroid hormones in the wastewater.

Analysis of the Taxonomy and Pathogenic Factors of Pectobacterium aroidearum L6 Using Whole-Genome Sequencing and Comparative Genomics.

Soft rot pectobacteria are devastating plant pathogens with a global distribution and a broad host range. Pectobacterium aroidearum L6, previously isolated from leaves of Syngonium podophyllum, is a pectolytic bacterial pathogen that causes typical soft rot on S. podophyllum. There is a shortage for genome data of P. aroidearum, which seriously hinders research on classification and pathogenesis of Pectobacterium. We present here the complete genome sequence of P. aroidearum L6. The L6 strain carries a single 4,995,896-bp chromosome with 53.10% G + C content and harbors 4,306 predicted protein-coding genes. We estimated in silico DNA–DNA hybridization and average nucleotide identity values in combination with the whole-genome-based phylogeny from 19 Pectobacterium strains including P. aroidearum L6. The results showed that L6 and PC1 formed a population distinct from other populations of the Pectobacterium genus. Phylogenetic analysis based on 16S rRNA and genome sequences showed a close evolutionary relationship among Pectobacterium species. Overall, evolutionary analysis showed that L6 was in the same branch with PC1. In comparison with 18 Pectobacterium spp. reference pathogens, strain L6 had 2,712 gene families, among which 1,632 gene families were identified as orthologous to those strains, as well as 1 putative unique gene family. We discovered 478 genes, 10.4% of the total of predicted genes, that were potentially related to pathogenesis using the Virulence Factors of Pathogenic Bacteria database. A total of 25 genes were related to toxins, 35 encoded plant cell-wall degrading enzymes, and 122 were involved in secretion systems. This study provides a foundation for a better understanding of the genomic structure of P. aroidearum and particularly offers information for the discovery of potential pathogenic factors and the development of more effective strategies against this pathogen.

Biocontrol Potential of Some Entomopathogenic Fungal Strains Against Bean Aphid Megoura japonica (Matsumura)

This research reported the in vitro pathogenicity of Verticillium lecanii strains, L2 and L5, and Beauveria bassiana strains, B76 and B252, against Megoura japonica using leaf-dip method. The virulence potential of these four entomopathogenic fungal strains of V. lecanii and of B. bassiana were compared between fungi conidia (concentrations 1 × 106, 1 × 107, and 1 × 108 conidia mL−1) and culture filtrate. Moreover, binary combination of four different fungal strains (L2 + B76, B76 + L5, L2 + B252, and L2 + B76 + B252 + L5) were evaluated against M. japonica under control condition. Aphid mortality was recorded after two, four, six, and eight days of post-treatment. In the conidial concentration bioassay, strain B76 showed maximal mortality (85.3%) against bean aphid, and strain L5 showed the lowest effect (60.0%) at the highest concentration (1 × 108 conidia mL−1) at eight days post-treatment. Whereas, binary combinations of B76 and L2 strains showed the highest effect against M. japonica (90.5%) than other combinations. Moreover, in comparison with the effect of filtrate and conidia bioassay, 91.4% and 84.1% were achieved in strain B76, and the lowest effect (63.8% and 55.1%) was recorded in strain L5.

Characterization of Fe (III)-reducing enrichment culture and isolation of Fe (III)-reducing bacterium Enterobacter sp. L6 from marine sediment

To enrich the Fe (III)-reducing bacteria, sludge from marine sediment was inoculated into the medium using Fe (OH)3 as the sole electron acceptor. Efficiency of Fe (III) reduction and composition of Fe (III)-reducing enrichment culture were analyzed. The results indicated that the Fe (III)-reducing enrichment culture with the dominant bacteria relating to Clostridium and Enterobacter sp. had high Fe (III) reduction of (2.73±0.13) mmol/L-Fe (II). A new Fe (III)-reducing bacterium was isolated from the Fe (III)-reducing enrichment culture and identified as Enterobacter sp. L6 by 16S rRNA gene sequence analysis. The Fe (III)-reducing ability of strain L6 under different culture conditions was investigated. The results indicated that strain L6 had high Fe (III)-reducing activity using glucose and pyruvate as carbon sources. Strain L6 could reduce Fe (III) at the range of NaCl concentrations tested and had the highest Fe (III) reduction of (4.63±0.27) mmol/L Fe (II) at the NaCl concentration of 4g/L. This strain L6 could reduce Fe (III) with unique properties in adaptability to salt variation, which indicated that it can be used as a model organism to study Fe (III)-reducing activity isolated from marine environment.

Complete Genome Sequence of Micromonospora Strain L5, a Potential Plant-Growth-Regulating Actinomycete, Originally Isolated from Casuarina equisetifolia Root Nodules

Micromonospora species live in diverse environments and exhibit a broad range of functions, including antibiotic production, biocontrol, and degradation of complex polysaccharides. To learn more about these versatile actinomycetes, we sequenced the genome of strain L5, originally isolated from root nodules of an actinorhizal plant growing in Mexico.

Effect of Lactobacillus plantarum isolated from digestive tract of wild shrimp on growth and survival of white shrimp (Litopenaeus vannamei) challenged with Vibrio harveyi

Two hundred and two strains of lactic acid bacteria (LAB) isolated from digestive tracts of cultivated and wild adult shrimp, including Litopenaeus vannamei, Metapenaeus brevicornis and Penaeus merguiensis were selected based on their antibacterial activity against Vibrio harveyi. LAB strain of MRO3.12 exhibiting highest reduction of V. harveyi was identified as Lactobacillus plantarum MRO3.12 based on the nucleotide sequence of its 16S rDNA, which showed 99% (780/786 bp) homology to L. plantarum strain L5 (GenBank accession number DQ 239698.1). Co-cultivation of V. harveyi and L. plantarum MRO3.12 showed complete reduction of V. harveyi at 24 h under aerobic and anaerobic conditions, whereas L. plantarum increased from 5.29 to 9.47 log CFU ml −1. After 6-week feeding trial with L. plantarum supplemented diet, white shrimp ( L. vannamei) exhibited significant differences ( p < 0.05) in relative growth rate (% RGR), feed conversion ratio (FCR) and survival compared to the control group fed with non-supplemented diet. LAB-fed group showed 98.89% survival, whereas only 68.89% survival was observed in the control group. LAB from the digestive tract of probiotic-fed shrimp showed higher level of 5.0 ± 0.14 log CFU/g than the non-supplemented ones (3.34 ± 0.21 log CFU/g). However, total bacterial and non-fermenting vibrios counts decreased in shrimps fed on L. plantarum. Ten days after infection with V. harveyi (5.3–5.5 log CFU ml −1), significant survival ( p < 0.05) of 77% was observed in LAB supplemented shrimp, while only 67% survival was observed in the control.

Experimental study on prevention and treatment of Helicobacter pylori infection by Lactobacillus acidophilus in C57BL/6 mice

Objective To investigate the potential effects of Lactobacillus acidophilus strain L6 on prevention and treatment of Helicobacter pylori (H. pylori) infection in animal models. Methods A total of 200 hundred C57BL/6 mice were used in the study. ① Sixty mice were divided into control group (infected with H. pylori) and prevention group (previously treated with L6 and followed by infection with H. pylori) with 30 each. The incidence of H. pylori infection was compared between two groups.② Sixty mice were divided into control group (infected with H. pylori) and treatment group (infected with H. pylori for 4 weeks and followed by treatment with L6) with 30 each. The changes of H. pylori infection was compared between tow groups. ③ Eighty mice previously infected with H. pylori were orally administrated with L6 in the water supply over a period of 9 months. Of which, 40 H. pylori negative mice were either continuously or discontinuously treated with L6. The re-infection of H. pylori was compared between two groups. The H. pylori infection, gastric mucosal inflammatory and serum anti-H. pylori-lgG titer was measured by using urea breath test, histopathology and ELISA, respectively. Results The positive rate of H. pylori infection was 100% in control group and 20% in prevention group (pre-treated with L6 for 1 week). Whereas there was no H. pylori infection in the rest mice of prevention group pre-treated with L6 for 2 or 4 weeks. There was significant difference in H. pylori infection between two groups (P<0. 05). The serum anti-H. pylori IgG titer was lower in treatment group than in control group (P<0. 05). The reinfection of H. pylori in mice continuously treated with L6 was significantly lower than those discontiously treated with L6 (P<0. 05). Conclusions Preventively feeding L6 can significantly reduce the H. pylori infection in C57BL/6 mice, whereas the inhibition of H. pylori re-infection can also be achieved with long-term administration of L6. Key words: Lactobacillusacidophilus; Helicobacter pylori; Animal experimentation

The influence of probiotic Enterococcus faecium strain L5 on the microbiota and cytokines expression in rats with dysbiosis induced by antibiotics

The animal model of intestinal dysbiosis induced by antibiotics was created. Dysbiotic condition was confirmed by the changes in titre of the indigenous microbiota (excessive growth of opportunistic microorganisms and reduced number of lactobacilli, bifidobacteria and enterococci) and the appearance of dyspeptic symptoms. Consumption of the fermented milk product with probiotic strain Enterococcus faecium L5 led to the rapid disappearance of dysbiosis symptoms, normalisation of the microbiota, increase in expression of IL-10 and decrease in IL-8 expression.

Screening of potential infants' lactobacilli isolates for amino acids production

A total of 9 lactic acid bacteria (LAB) isolated from Egyptian infants, belonging to lactobacilli strains, were screened and selected according to their amino acids production. The isolate strains produced different essential amino acids in fermentation of M17 medium. Lactobacillus strain L1 produced the highest yield of histidine 92.5 mg/l, L2 strain produced the highest amount of asparagine 59.61 mg/l, cysteine 32.96 mg/l, phenylalanine 17.49 mg/l and tyrosine 244.91 mg/l. On the other hand, Lactobacillus strain L3 is the most potential culture for the production of citrulline 61.12 mg/l and glutamic 68.78 mg/l. This strain also produced the highest amount of lysine, ornithine and proline, which were 57.04, 95.58 and 87.06 mg/l, respectively. While the strain L4 produced the highest amount of glycine 32.12 mg/l, strain L7 produced the highest amount of alanine 147.55 mg/l and strain L5 produced the highest amount of asparatic acid 218.09 mg/l. The present study concluded that the Lactobacillus strains have the potential to produce different amino acids with variance concentrations. Key words: Lactobacilli, Lactobacillus, amino acids, screening, production.

Transcriptional Gene Silencing Reveals Two Distinct Groups of Entamoeba histolytica Gal/GalNAc-Lectin Light Subunits

The Entamoeba histolytica cell surface Gal/GalNAc-inhibitable lectin is a heterodimer between a heavy (170 kDa) subunit linked via a disulfide bond to a light (31 to 35 kDa) subunit. Five light subunit genes with high homology have been identified (Ehlgl1 to -5). We have previously shown that silencing of the expression of Ehlgl1, in the G3 trophozoites which had already been silenced in the amoebapore gene (Ehap-a), also suppressed the transcription of Ehlgl2 and -3 (strain RBV). The total absence of the lgl1 to -3 subunits in the RBV trophozoites affected their ability to cap the surface Gal-lectin molecules to the uroid region. We have now found that in the RBV trophozoites, the lgl4 and -5 subunits (31 kDa) are overexpressed and appear to compensate for the missing lgl1 to -3 in the heterodimer complex. Transcriptional silencing of Ehlgl5 was achieved by transfection of G3 trophozoites with a plasmid containing the open reading frame of Ehlgl5 ligated to the 5' promoter region of the Ehap-a gene. The transfected trophozoites (strain L5) were silenced in Ehlgl5 and the closely related Ehlgl4, while the expression of the larger lgl1 to -3 subunits was upregulated. L5 trophozoites retained their ability to cap the Gal-lectin molecules. Attempts to simultaneously silence all of the Ehlgl genes have failed so far, possibly due to their crucial importance to the Gal-lectin functions. Our ability to silence part of the genes belonging to the same family can serve as a tool to study the relationships and functions of the members of other gene families.

The effect of rhizobiophages on Sinorhizobium meliloti - Medicago sativa symbiosis

Using Tn5 mutagenesis of lysogenic Sinorhizobium meliloti strain L5–30 (φL54), three neomycin-resistant transconjugants differing in phage-resistance profiles were isolated. Two of them increased the dry weight of plants and were capable of establishing root nodules, whereas the third one was ineffective. The bacterium-phage interaction did not have observable consequences in the Medicago sativa - S. meliloti symbiosis because it did not affect the number of nodules on M. sativa or plant dry weight.

Potential for misidentification of a spore-forming Paenibacillus polymyxa isolate as an endophyte by using culture-based methods.

While Paenibacillus polymyxa strain Pw-2 has been identified as an endophyte of lodgepole pine (M. Shishido, B. M. Loeb, and C. P. Chanway, Can. J. Microbiol. 41:707-713, 1995), P. polymyxa strain L6 has not, a distinction that could be explained by the differential abilities of these isolates to form spores, rather than the differential abilities to colonize the interior tissues of lodgepole pine. Chemical disinfection was used to destroy bacteria on the root exterior, but bacterial endospores are known for their ability to withstand chemical disinfection, and strain Pw-2 was found to produce 300 to 11,000 times more germinating endospores than strain L6 under the experimental conditions used by Shishido et al. (Can. J. Microbiol. 41:707-713, 1995). Attempts to identify strain Pw-2 within lodgepole pine root tissues by using confocal microscopy techniques failed. We discuss the possibility that spore-forming bacteria can be mistakenly identified as endophytes when culture-based methods alone are used.

Variation in symbiotic characters of alfalfa cultivars inoculated with Sinorhizobium meliloti strains

In this paper the interactive effects of different Sinorhizobium meliloti strains and alfalfa cultivars on the host plant growth and nodulation were evaluated. In the experiments, six alfalfa cultivars (Giza-1, Ismailia-94, Kometa, Radius, Luzella and Legend) as well as five S. meliloti strains (ARC-1, ARC-2, A2, TAL and L5-30) were included. The alfalfa cultivars, Giza-1 and Ismailia-94, exhibited the highest average dry forage weight with all studied strains. Generally, plants inoculated with the S. meliliti ARC-2 strain produced significantly higher shoot and root dry weights over all evaluated cultivars. The strain L5–30 of S. meliloti produced the highest nodule number with all the studied alfalfa cultivars whereas, among tested cultivars, N2-fixing Luzella showed the highest nodulation with the majority of rhizobial strains. Analysis of correlation coefficients for the studied alfalfa-rhizobium associations revealed positive and significant relationships of shoot dry weight with root dry weight. In contrast, nodule number per plant correlated negatively with forage dry weight of plants. It was also found that nodule number per plant is not a promising trait with which to select the most effective N2-fixing rhizobium-legume systems; selection of such symbiotic associations should be done on the basis of shoot and root dry weights.

Degradation of 3-chlorobenzoate under low-oxygen conditions in pure and mixed cultures of the anoxygenic photoheterotroph Rhodopseudomonas palustris DCP3 and an aerobic Alcaligenes species.

The presence or absence of molecular oxygen has been shown to play a crucial role in the degradability of haloaromatic compounds. In the present study, it was shown that anaerobic phototrophic 3-chlorobenzoate (3CBA) metabolism by Rhodopseudomonas palustris DCP3 is oxygen tolerant up to a concentration of 3 microM O2. Simultaneous oxidation of an additional carbon source permitted light-dependent anaerobic 3CBA degradation at oxygen input levels which, in the absence of such an additional compound, would result in inhibition of light-dependent dehalogenation. Experiments under the same experimental conditions with strain DCP3 in coculture with an aerobic 3CBA-utilizing heterotroph, Alcaligenes sp. strain L6, revealed that light-dependent dehalogenation of 3CBA did not occur. Under both oxygen limitation (O2 < 0.1 microM) and low oxygen concentrations (3 microM O2), all the 3CBA was metabolized by the aerobic heterotroph. These data suggest that biodegradation of (halo)aromatics by photoheterotrophic bacteria such as R. palustris DCP3 may be restricted to anoxic photic environments.

The growth-promoting effects of a bacterial endophyte on lodgepole pine are partially inhibited by the presence of other rhizobacteria

To test the hypothesis that rhizobacteria naturally present in soils may interfere with the extent of root colonization and plant growth promotion by plant growth-promoting rhizobacteria (PGPR), we studied two lodgepole pine PGPR (Bacillus polymyxa strains L6 and Pw-2) when inoculated singly and when coinoculated with a non-PGPR competitor (Curtobacterium flaccumfaciens PF322). Bacillus polymyxa Pw-2 and Curtobacterium flaccumfaciens PF322 were consistently found as endophytes, while Bacillus polymyxa L6 was never found within the root interior. Strains Pw-2 and L6 differed in the rate and type of growth promotion. Strain Pw-2 increased root growth (branching and elongation) and shoot biomass accumulation 6 and 9 weeks, respectively, after inoculation, while strain L6 increased primary root elongation and root biomass accumulation after 12 weeks. Seedlings coinoculated with Pw-2 and PF322 had decreased shoot biomass and primary root lengths when compared with seedlings inoculated only with Pw-2. This effect was not linked to a decrease in the population size of Pw-2 in the rhizosphere or in the root interior of coinoculated treatments. In contrast, strain L6-mediated growth promotion was not impaired by coinoculation with PF322. Strain L6 did interfere to some degree with the growth-promoting capability of strain Pw-2. These results indicate that endophytic PGPR may be less adapted to microbial competition than external root-colonizing PGPR, and that the efficacy of endophytic PGPR may be reduced by the presence of other bacteria on external or internal root tissues.Key words: PGPR, endophytes, colonization, coinoculation, competition.

Competition for oxygen and 3-chlorobenzoate between two aerobic bacteria using different degradation pathways

In nature a significant part of the microbial activity is concentrated at or near oxic/anoxic interfaces, where oxygen concentrations are often low. Bacteria possessing different kinetic characteristics for oxygen and employing distinct metabolic pathways for the degradation of (halo)aromatic substrates for which oxygen is needed as co-substrate may have to compete with each other in such environments. In this study the competitiveness of Pseudomonas sp. strain A3 relative to Alcaligenes sp. strain L6 was tested in batch and in continuous cultures. While both of these strains are able to metabolise 3-chlorobenzoate (3CBA), the former was isolated under air saturating conditions and employs the catechol pathway, whereas the latter was isolated under reduced partial pressures of oxygen and was capable of metabolising 3CBA via the gentisate pathway. Competition experiments in batch culture resulted in pure cultures of Pseudomonas sp. strain A3 under air saturating conditions. However, if reduced partial pressures of oxygen (2%) were used, Alcaligenes sp. strain L6 remained present in substantial numbers after three transfers. Continuous culture experiments demonstrated that Alcaligenes sp. strain L6 was able to outcompete Pseudomonas sp. strain A3 under oxygen- as well as under carbon-limiting conditions as long as the dilution rate remained below 0.136 h −1 (low oxygen) and below 0.178 h −1 (high oxygen). These results support the hypothesis that organisms metabolising chlorobenzoate via the gentisate pathway may play a significant role in natural ecosystems where xenobiotic compounds and naturally produced aromatics occur at very low concentrations and in combination with limiting oxygen tensions.