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Applied and Environmental Microbiology, December 2002, p. 6429-6434, Vol. 68, No. 12
0099-2240/02/$04.00+0 DOI: 10.1128/AEM.68.12.6429-6434.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Nestlé Research Center, Vers-Chez-Les-Blanc, 1000 Lausanne 26, Switzerland
Received 22 April 2002/ Accepted 16 September 2002
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Bifidobacterium lactis is used as a probiotic in dairy products (e.g., yoghurt) or supplied in infant food (7, 10). This species was previously described by Meile et al. (8) and characterized by its high oxygen resistance and production of considerable amounts of formate. The phylogenetic position of B. lactis DSM 10140, defined by sequence similarity and sequence analysis of the ldh gene (13), revealed that B. animalis ATCC 25527 is the most closely related strain. Recently, Cai et al. (2) reinvestigated the taxonomic position of B. lactis DSM 10140 and proposed that B. lactis should be considered a junior subjective synonym of B. animalis.
In view of the widespread interest in B. lactis and B. animalis because of the expanding application of these species as probiotics, it is highly desirable to develop a molecular approach to clear and reliable species identification. The rRNA gene (rDNA) has been used widely to infer phylogenetic relationships among bacteria (23). However, as evolutionary distances decrease, the diversity found in the 16S rDNA is often insufficient and genetic relationships of closely related species cannot be accurately defined (1, 11). It has been recently suggested that sequencing of the internally transcribed spacer (ITS) region could overcome this problem since ITS regions might be under less evolutionary pressure and could provide greater genetic variation. Indeed, analysis of this region has already successfully differentiated strains and species of many bacterial groups (1, 3, 4, 9, 17).
We investigated 106 bifidobacterial strains isolated from different environments (Table 1) with recently published B. lactis species-specific primers (20). Six strains gave a specific B. lactis PCR amplicon. By applying the species-specific amplified ribosomal DNA restriction analysis method to all of the strains, it was possible to allocate them only to the B. animalis-B. lactis cluster (19). The 16S-23S ITS region from each strain was amplified by using primers that annealed to conserved regions of the 16S and 23S genes (primer 16S-for, 5'-GCTAGTAATCGCGGATCAG-3'; primer 23Si, 5'-CATTCGGACACCCTGGGATC-3'). Chromosomal DNA was PCR amplified in accordance with the manufacturer's (Gibco BRL) instructions. Amplicons were purified with the QIAquick PCR Purification Kit (Qiagen). Nucleotide sequences of PCR products were determined by using the fluorescently labeled primer cycle-sequencing kit (Amersham) and the LI-COR sequencer (MWG). Sequence alignments were done with the Multi-align program, and the ClustalW dendrogram was drawn with the ClustalX program.
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TABLE 1. Bacterial strains used to evaluate the specificity of B. animalis-specific PCR primers by a Multiplex-PCR approach
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To specifically detect B. animalis strains, we applied a PCR amplification strategy for the ITS regions with primers based on specific B. animalis sequences (see Fig. 5). All PCRs were in a total of 50 µl of a solution containing 10 mM Tris-HCl, 50 mM KCl, 1.5 mM MgCl2, 200 µM each deoxynucleoside triphosphate (Gibco BRL), 10 pmol (each) of Lm3 and Lm26 (6), 50 pmol of B. animalis-specific primer Ban2 (5'-CATATTGGATCACGGTCG-3'), 50 pmol of primer 23Si, 25 ng of template DNA, and 2.5 U of Taq DNA polymerase (Gibco BRL). Amplifications were performed with a DNA thermocycler (Perkin-Elmer Cetus 9700) as follows: 1 cycle of 95°C for 5 min; 30 cycles of 95°C for 30 s, 54°C for 1 min, and 72°C for 1.5 min; and 1 cycle of 72°C for 7 min. Amplicons were analyzed by 1.5% (wt/vol) agarose gel electrophoresis in Tris-acetate-EDTA buffer at a constant voltage of 7 V/cm and visualized with ethidium bromide (0.5 µg/ml) under UV light at 260 nm.
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FIG. 5. (A) Primers and amplification systems used to investigate the species B. animalis. (B) PCR products of various Bifidobacterium species obtained by a Multiplex-PCR approach with B. animalis species-specific PCR primers and bifidobacterial genus-specific primers. (A) The position numbering and expected product sizes are given in accordance with the numbering of the B. animalis 16S-23S spacer region (1). (B) Lanes: 1, B. animalis ATCC 25527; 2, B. animalis ATCC 27672; 3, B. lactis DSM 10140; 4, B. animalis ATCC 27673; 5, B. animalis ATCC 27674; 6, B. animalis ATCC 27536; 7, B. lactis NCC 363; 8, B. lactis NCC 387; 9, B. lactis NCC 402; 10, B. lactis NCC 311; 11, B. lactis NCC 383; 12, B. lactis NCC 239; 13, B. bifidum ATCC 29521; 14, B. breve ATCC 15700; 15, B. longum ATCC 15707; 16, B. infantis ATCC 15679; 17, B. adolescentis ATCC 15703; 18, B. catenulatum ATCC 27539. m, 1-kb DNA ladder (Gibco-BRL).
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Use of the ERIC-PCR technique resulted in clear identification at the bifidobacterial species level (15; data not shown). ERIC-PCR fingerprints demonstrated that B. lactis NCC 363, NCC 383, NCC 402, and NCC 311 and B. animalis ATCC 27536, ATCC 27673, and ATCC 27674 are not comparable to any other B. animalis strain (e.g., ATCC 25527, ATCC 27672), but they are highly similar to the neotype of the species B. lactis (Fig. 1). B. lactis strains were characterized by DNA restriction patterns with low-frequency cleavage endonucleases and PFGE. Overall, the size estimates for all B. lactis strains were compared with the size estimates for B. animalis ATCC 27673, ATCC 27674, and ATCC 27536 (12). These values were clearly different and unique for each of the B. animalis strains. Only B. lactis NCC 363, NCC 383, NCC 402, and NCC 311 displayed a rather different and unique PFGE profile (Fig. 2). Interestingly, the other two B. lactis isolates (NCC 239 and NCC 387) resulted in SpeI/XbaI PFGE patterns highly similar to that of B. lactis DSM 10140 (data not shown). This indicates that these organisms are very closely related. Indeed, it was demonstrated earlier that these strains are different with additional molecular typing assays such as randomly amplified polymorphic DNA PCR and triplicate arbitrary primed PCR (data not shown).
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FIG. 1. DNA fingerprint analysis of B. lactis and B. animalis strains by ERIC-PCR. Lanes: 1, B. lactis DSM 10140; 2, B. lactis NCC 363; 3, B. lactis NCC 311; 4, B. animalis ATCC 25527; 5, B. animalis ATCC 27672; 6, B. animalis ATCC 27674; 7, B. animalis ATCC 27536; 8, B. animalis ATCC 27673; 9, B. lactis NCC 383; 10, B. lactis NCC 402; 11, B. lactis NCC 239; M, 1-kb DNA ladder (Gibco-BRL).
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FIG. 2. PFGE patterns of genomic DNAs of B. lactis strains after SpeI digestion. Lanes: 1, B. lactis NCC 383; 2, B. lactis NCC 363; 3, B. lactis DSM 10140; 4, B. lactis NCC 402; 5, B. lactis DSM 10140; 6, B. lactis NCC 239; 7, B. lactis NCC 387; 8, B. lactis NCC 311; 9, B. lactis DSM 10140; M, DNA ladder (Bio-Rad).
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FIG. 3. (A) Dendrogram based on 16S-23S ITS sequences and showing the relationships of members of the genus Bifidobacterium related to B. animalis and B. lactis strains. The bar at the top indicates 0.02% sequence divergence. The 16S-23S ITS sequences used to draw the phylogenetic tree of the following strains were retrieved from the GenBank database: B. infantis ATCC 15697 (accession no. U09792), B. breve CIP 6470 (accession no. U09521), B. adolescentis CIP 6460 (accession no. U09513), B. adolescentis CIP 6459 (accession no. U09512), B. lactis DSM 10140 (accession no. X89513), B. animalis ATCC 25527 (accession no. U09858), B. choerinum CIP 103378 (accession no. L36968), B. pseudolongum ATCC 25526 (accession no. U09879), and B. globosum ATCC 25865 (accession no. U09524). (B) Overall 16S-23S rRNA spacer region sequence similarities of B. lactis strains.
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FIG. 4. Multiple alignment of the ITS sequences of six B. lactis strains, B. lactis DSM 10140 (accession no. X89513), and B. animalis ATCC 25527 (accession no. U09858). Nucleotides that differ between B. lactis strains and B. animalis ATCC 25527 are shaded in dark grey. Nucleotides that differ among B. lactis strains are shaded in light grey.
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Notably, two insertions of eight nucleotides each were identified in B. animalis but not in B. lactis and provided a PCR target for rapid separation of these two species. The Ban2 primer was designed to anneal to this conserved B. animalis region (Fig. 5A). The reverse primer, 23Si, was derived from the sequences encompassing the 3' end of ITS and the 5' end of the 23S rDNA and was complementary overall to all of the ITS sequences of B. animalis and B. lactis strains. Moreover, because of an often-occurring variability of PCR conditions, the lack of any amplicons must be attributed not only to the absence of any target DNA but also to an overall failure of the amplification reaction. To distinguish between these two events, we included primers Lm3 and Lm26 (6), which target conserved regions within the 16S rDNA of the genus Bifidobacterium, in the same PCR mixtures. When a Multiplex-PCR was performed with this mixture of four primers in the same reaction mixture, two amplicons of 467 and 1,350 bp were only detectable in the presence of DNA isolated from B. animalis (the sizes of all achievable PCR amplicons were calculated from databases and estimated by gel electrophoresis). The amplification of all other Bifidobacterium DNAs produced only one amplicon of 1,350 bp. The results of these PCR assays, on the basis of the ITS sequence data, are depicted in Fig. 5B. The 467-bp amplicon that occurred was apparent for two B. animalis strains but not for any of the B. lactis strains. Furthermore, we confirmed the overall absence of PCR amplicons for all other bifidobacterial type strains (data not shown).
B. lactis identification based on molecular sequence data has been hampered so far by the lack of detectable sequence variation between closely related B. animalis strains. Genetic information derived from 16S-23S rRNA spacer regions can be used to differentiate closely related organisms (1, 17).
More recently, Cai et al. (2) showed that the DNA of B. animalis ATCC 27536 and ATCC 27674 had a high level of overall DNA relatedness with B. lactis DSM 10140, ranging from 94.2 to 94.8%, which is significantly higher than their relatedness to B. animalis ATCC 25527. Since the DNA-DNA homology between B. lactis DSM 10140 and B. animalis ATCC 25527 has already been demonstrated to be 85.5% (2) and is therefore too high to allow separation of these strains into two separate species, no decision has been made by the International Committee on Systematic Bacteriology about whether to continue to treat B. lactis as a species separate from B. animalis (14).
Roy et al. (12, 13) also reported that B. animalis ATCC 27536 has an ldh nucleotide sequence identical to that of B. lactis DSM 10140 and a PFGE pattern more similar to that of the type strain of B. lactis than to that of the type strain of B. animalis. Analysis of a 60-kDa heat shock protein (HSP60) gene showed that the DNA sequence similarity between B. lactis DSM 10140 and B. animalis ATCC 25527 is high (98%), as is that among B. infantis, B. longum, and B. suis, but lower than that between strains of the same species (5). Therefore, the high level of 16S rDNA sequence similarity of 98.6%, (8) between the B. animalis and B. lactis type strains is additional evidence that these two taxa might, in fact, be combined. Thus, our 16S-23S ITS sequence data and ERIC-PCR data are consistent with above-mentioned data and suggest that these two species might be designated two subspecies of the same species. In fact, 16S rDNA sequence analysis is a good tool with which to explore intergenic relationships, while 16S-23S rDNA spacer region comparison provides information concerning intraspecific links and allows the detection of recently divergent species, as proposed by Bourget (1). Considering the new B. lactis strains described here and comparing them with the few B. animalis strains available, we suggest the use of the names B. animalis subsp. lactis, which includes strains DSM 10140, ATCC 27536, ATCC 27674, and ATCC 27673 and the new strains reported in this study, and B. animalis subsp. animalis, which includes, e.g., strains ATCC 25527 and ATCC 27672. Notably, all of these strains exhibit individual PFGE and/or randomly amplified polymorphic DNA PCR, and/or triplicate arbitrary primed PCR profiles (12; unpublished results), justifying separate strain designations.
Physiologic characteristics based on their different abilities to grow on milk-based medium give additional support for a potential taxonomic separation of strains of B. lactis from strains of B. animalis. In fact, B. lactis strains and very few B. animalis strains (ATCC 27536, ATCC 27674, and ATCC 27673) are able to grow in pure milk whereas B. animalis ATCC 25527 and ATCC 27672 both grow poorly in pure milk. Potentially, for this reason, all strains found in dairy products (e.g., yoghurt and infant formula), including products purported to contain B. animalis and/or B. lactis strains, do not at all resemble B. animalis but are, in fact, only strains of the B. lactis taxon. According to Bergey's Manual of Systematic Bacteriology (16), division of a species into subspecies is based on phenotypic variations (e.g., growth on milk-based medium) or on genetically determined clusters of strains within the species. On the basis of the above criterion, we suggest that B. lactis should not be considered a junior subjective synonym of B. animalis but rather be reclassified as a subspecies of B. animalis.
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