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1 IRD, UR 101 Extrêmophiles, IFR-BAIM, Universités de Provence et de la Méditerranée, ESIL, Marseille, France
2 Laboratorio de Tratamiento de Aguas Residuales, Universidad Autónoma Metropolitana, México, DF, Mexico
Correspondence
Bernard Ollivier
ollivier{at}esil.univ-mrs.fr
| ABSTRACT |
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| MAIN TEXT |
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Several strains were isolated from oil/water mixtures taken from production well heads (21-D) in Tabasco (Gulf of Mexico). Samples were collected in sterile plastic bottles and kept at room temperature until used. P. mobilis DSM 10674T and P. miotherma DSM 10691T were obtained from the DSMZ, whereas P. olearia DSM 13574T and P. sibirica DSM 13575T were obtained from the Institut Universitaire Européen de la Mer, Plouzané, France. Enrichment cultures were performed in 120 ml serum bottles containing 20 ml medium and inoculated with 2 ml water/oil sample. The medium contained (l-1 distilled water): 1 g NH4Cl, 0·3 g K2HPO4, 0·3 g KH2PO4, 0·1 g KCl, 0·1 g CaCl2, 0·2 g MgSO4.7H2O, 0·5 g cysteine hydrochloride, 1 mg resazurin, 30 g NaCl and 10 ml trace mineral element solution (Balch et al., 1979
). The pH was adjusted to 7 with 10 M KOH. The medium was boiled under a stream of O2-free N2 gas and cooled to room temperature. Aliquots (20 ml) were dispensed into serum bottles under a stream of N2/CO2 (80 : 20, v/v) and the sealed vessels were autoclaved for 45 min at 110 °C. Prior to inoculation, Na2S.9H2O, NaHCO3, peptone (Difco), yeast extract and sodium thiosulfate were injected from sterile stock solutions to respective final concentrations of 0·04, 0·2, 0·5 and 0·02 % (w/v) and 20 mM. H2 pressure in the gas phase was 2 bars. The bottles were incubated at 60 °C and three enrichment series were performed in the same culture medium before isolation. Strains were isolated by repeated use of the Hungate roll-tube technique (Hungate, 1969
), with medium solidified with 0·8 % phytagel (Difco). Serial dilution in roll tubes was repeated at least twice to purify the cultures. Growth experiments were performed in duplicate, using a medium differing from the enrichment medium by the absence of peptone and hydrogen in the gas phase. In addition to the basal medium, MgSO4.7H2O (0·2 g l-1) was replaced by MgCl2.6H2O (1 g l-1) and the concentration of yeast extract was increased to 0·1 %.
The pH, temperature and NaCl and MgCl2 concentration ranges for growth were determined using basal medium supplemented with 20 mM glucose. The pH of the medium in Hungate tubes was adjusted by injecting 1 M HCl, NaHCO3 or Na2CO3 from 10 % sterile anaerobic stock solutions. The characterized strain was subcultured twice under the same experimental conditions before growth rates were determined. All substrates were tested at a final concentration of 20 mM except for formate (80 mM), cellulose, starch and xylan (all 10 g l-1). Elemental sulfur (1 %, w/v), sulfate (20 mM), thiosulfate (20 mM), sulfite (5 mM), nitrate (10 mM) and nitrite (5 mM) were tested as electron acceptors. H2S production was determined photometrically as colloidal CuS by using the method of Cord-Ruwisch (1985)
. Sensitivity to ampicillin, chloramphenicol, kanamycin, rifampicin and vancomycin was tested at 10, 25, 50 and 100 µg ml-1. Controls with ethanol and DMSO (solvents used for chloramphenicol and rifampicin) were performed. Growth was monitored by measuring OD580 and by microscopic observations. Morphological characteristics were determined with an Optiphot (Nikon) phase-contrast microscope. For electron microscopic studies, preparations were negatively stained with sodium phosphotungstate as described by Koussémon et al. (2001)
. Growth was monitored by measuring OD580 using a UV/visible spectrophotometer 50 Scan (Varian). Xylanolytic activity was measured as described previously (Fardeau et al., 1997a
); the assay was performed by adding dinitrosalicylic acid (Miller, 1959
) and the xylose released from xylan degradation was determined from the A540. The presence of L-alanine was determined enzymically at 340 nm as described previously (Fardeau et al., 1997a
). Fermentation products were determined by HPLC using a pump (Spectra Series P100; Thermo Separation Products), an automatic sampler (Spectra Series AS100), an Aminex HPX 87H (300x7·8 mm) column (Bio-Rad), a differential refractometer detector (Spectra System RI-150) and an integrator (Azur Microsoft). An aliquot of 20 µl cell-free supernatant was injected into the column, which was maintained at 37 °C. A 2·5 mM H2SO4 solution was used as solvent with a flow rate of 0·6 ml min-1. The effect of O2 on growth was determined in Hungate tubes containing anaerobic basal medium supplemented with 20 mM glucose. Tubes were inoculated and 20 ml sterile air was added to the gas phase. Cultures were incubated at 55 °C under agitation (150 r.p.m.). Growth was monitored by turbidity measurements (580 nm) and by microscopic observations. Experiments were conducted in duplicate and repeated at least twice. The presence of spores was determined by microscopic observation of cultures and pasteurization tests at 80 and 90 °C for 10 and 20 min.
The G+C content of DNA was determined at the DSMZ, using HPLC as described by Mesbah et al. (1989)
. DNADNA hybridization studies were carried out by the DSMZ as described by De Ley et al. (1970)
, with the modifications described by Huß et al. (1983)
and Escara & Hutton (1980)
, using a model 2600 spectrophotometer equipped with a model 2527-R thermoprogrammer and plotter (Gilford Instruments). Renaturation rates were computed with the TRANSFER.BAS program (Jahnke, 1992
). The 16S rDNA of strain MET12T was amplified by adding 1 µl cell culture to a thermocycler microtube containing 5 µl 10x Taq buffer, 0·5 µl 100 nM primers Fd1 and Rd1, 4 µl 25 mM MgCl2.6H2O, 0·5 µl 25 mM dNTPs, 0·5 µl Taq polymerase (5 U µl-1) and 38 µl sterilized distilled water. The primers used (Fd1, 5'-AGAGTTTGATCCTGGCTCAG-3'; Rd1, 5'-AAGGAGGTGATCCAGCC-3') were described by Winker & Woese (1991)
. The sample was placed in a hybrid thermal reactor thermocycler (GeneAmp PCR System 2400), denatured by heating for 3 min at 96 °C and subjected to 30 cycles of 20 s at 95 °C, 30 s at 52·9 °C and 2 min at 72 °C; this was followed by a final elongation step of 5 min at 72 °C. PCR products were cloned using the pGEM-T-easy cloning kit (Promega) according to the manufacturer's protocol. Clone libraries were screened by direct PCR amplification from a colony using the vector-specific primers SP6 (5'-ATTTAGGTGACACTATAGAA-3') and T7 (5'-TAATACGACTCACTATAGGG-3') and the following reaction conditions: 2 min at 96 °C, 40 cycles of 30 s at 94 °C, 1 min at 55 °C, 3 min at 72 °C, and a final extension of 10 min at 72 °C. Plasmids containing inserts of the expected length were isolated using the Wizard Plus SV Minipreps DNA purification system (Promega), according to the manufacturer's protocol. Purified plasmids were sent for sequencing to Genome Express (Grenoble, France). Sequence data were imported into the sequence editor BioEdit version 5.0.9 (Hall, 1999
); base calling was examined and a contiguous consensus sequence was obtained. The full sequence was aligned using the RDP Sequence Aligner program (Maidak et al., 2001
). The consensus sequence was then manually adjusted to conform to the 16S rDNA secondary structure model (Winker & Woese, 1991
). A non-redundant BLASTN search (Altschul et al., 1997
) of the full sequence through GenBank (Benson et al., 1999
) identified its closest relative. Sequences used in the phylogenetic analysis were obtained from the RDP (Maidak et al., 2001
) and GenBank (Benson et al., 1999
). Positions of sequence and alignment ambiguity were omitted and pairwise evolutionary distances were calculated using the method of Jukes & Cantor (1969)
. A dendrogram was constructed using the neighbour-joining method (Saitou & Nei, 1987
). Confidence in the tree topology was determined by using 100 bootstrapped trees (Felsenstein, 1985
). rDNA accession numbers of reference organisms are included in Fig. 2
. DNADNA hybridizations were performed at the DSMZ as described by Koussémon et al. (2001)
.
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Moderate to extreme thermophilic anaerobes belonging to the Bacteria and Archaea inhabit oil reservoirs originating from terrestrial or marine environments (Magot et al., 2000
). However, more attention has been paid so far to micro-organisms of the domain Bacteria belonging to the families Thermoanaerobiaceae and, particularly, Thermotogaceae, being well-known inhabitants of these terrestrial- and marine-subsurface ecosystems (Davey et al., 1993
; Fardeau et al., 1997a
; Grassia et al., 1996
; L'Haridon et al., 2001
, 2002
; Lien et al., 1998
; Magot et al., 2000
; Ravot et al., 1995a
; Stetter et al., 1993
). The order Thermotogales and family Thermotogaceae comprise members of the genera Thermotoga, Fervidobacterium, Thermosipho, Marinitoga, Petrotoga and Geotoga (Alain et al., 2002
; Reysenbach, 2001
; Wery et al., 2001
). Amongst these genera, strains belonging to Petrotoga and Geotoga have been isolated so far only from oil reservoirs (Davey et al., 1993
; L'Haridon et al., 2002
; Lien et al., 1998
). Based on phylogenetic studies, we provide evidence of the existence of a novel isolate (strain MET12T) of the genus Petrotoga that inhabits an oil reservoir in the Gulf of Mexico. This confirms the close ecological relationship between micro-organisms of this genus and the oilfield ecosystem. Strain MET12T has P. miotherma and P. olearia as its closest phylogenetic relatives, but DNADNA hybridization studies (values<70 %) with both these species and with P. sibirica and P. olearia allowed its assignment to a novel bacterial species (Wayne et al., 1987
). In addition, strain MET12T possesses the highest G+C content within the genus Petrotoga (Table 1
).
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It is noteworthy that thiosulfate can be used as an electron acceptor and that L-alanine is a minor end product of glucose metabolism by P. mexicana. L-Alanine production together with thiosulfate reduction have been found as quite common metabolic features within members of the order Thermotogales (Ravot et al., 1995b
, 1996
). Because of the position of these micro-organisms in the deepest branches of the phylogenetic tree, L-alanine production from glucose fermentation has been interpreted as a remnant of an ancestral metabolism (Ravot et al., 1996
). A thiosulfate-reducing activity shared by several members of this order might (i) help them when oxidizing hydrogen or amino acids such as leucine, isoleucine and valine (Fardeau et al., 1993
, 1997b
) or (ii) have an undesirable influence on biocorrosion due to sulfide production (Magot et al., 2000
). The occurrence of Petrotoga species only in oil reservoirs so far raises questions about (i) their survival in this deep subsurface environment and (ii) their possible indigenicity to this peculiar ecosystem. Further microbial and ecological studies are needed to appreciate their impact in the oilfield environment.
Description of Petrotoga mexicana sp. nov.
Petrotoga mexicana (me.xi.ca'na. N.L. fem. adj. mexicana Mexican, describing the site of sampling of the type strain).
Cells are motile rods, 0·50·7x1·45·0 µm, with an outer, sheath-like structure (toga), occurring singly, in pairs or in chains up to 30 µm long. Electron microscopy of cell sections exhibits a Gram-negative-type cell wall. No spore formation is detected. Growth occurs from 25 to 65 °C (optimum 55 °C). Grows in the presence of 10200 g NaCl l-1 (optimum 30 g l-1) and 0200 g MgCl2.6H2O l-1 (optimum 1·5 g l-1). Growth occurs between pH 5·8 and 8·5 (optimum 6·6). Ferments D-arabinose, cellobiose, fructose, galactose, glucose, lactose, maltose, mannose, raffinose, rhamnose, ribose, starch, sucrose, xylose, xylan and pyruvate. The xylanase activity is endocellular. Glucose is fermented into acetate, lactate, H2 and CO2, with L-alanine as a minor end product. Casamino acids and peptone are used in the presence of thiosulfate. Acetate, butyrate, formate, fumarate, lactate, malate, butanol, methanol, propanol, methylamine, trimethylamine and H2 are not used. Uses elemental sulfur, thiosulfate and sulfite but not sulfate, fumarate, nitrate or nitrite as electron acceptors. Grows in the presence of 10 µg chloramphenicol, kanamycin and vancomycin ml-1 and 25 µg ampicillin and rifampicin ml-1. The G+C content is 36·1 mol% (HPLC).
The type strain, MET12T (=DSM 14811T=CIP 107371T), was isolated from an oil well located in the Gulf of Mexico.
| ACKNOWLEDGEMENTS |
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