Phylogenetic relationships between Bacillus species and related genera inferred from 16s rDNA sequences

Relações filogenéticas entre espécies de Bacillus e gêneros relacionados baseadas em sequencias 16S rDNA

GENERAL MICROBIOLOGY

Phylogenetic relationships between Bacillus species and related genera inferred from 16s rDNA sequences

Relações filogenéticas entre espécies de Bacillus e gêneros relacionados baseadas em sequencias 16S rDNA

Wei Wang; Mi Sun^*

Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao, China

ABSTRACT

Neigh-borjoining, maximum-parsimony, minimum-evolution, maximum-likelihood and Bayesian trees constructed based on 16S rDNA sequences of 181 type strains of Bacillus species and related taxa manifested nine phylogenetic groups. The phylogenetic analysis showed that Bacillus was not a monophyletic group. B. subtilis was in Group 1. Group 4, 6 and 8 respectively consisted of thermophiles, halophilic or halotolerant bacilli and alkaliphilic bacilli. Group 2, 4 and 8 consisting of Bacillus species and related genera demonstrated that the current taxonomic system did not agree well with the 16S rDNA evolutionary trees. The position of Caryophanaceae and Planococcaceae in Group 2 suggested that they might be transferred into Bacillaceae, and the heterogeneity of Group 2 implied that some Bacillus species in it might belong to several new genera. Group 9 was mainly comprised of the genera (excluding Bacillus) of Bacillaceae, so some Bacillus species in Group 9: B. salarius, B. qingdaonensis and B. thermcloacae might not belong to Bacillus. Four Bacillus species, B. schlegelii, B. tusciae, B. edaphicus and B. mucilaginosus were clearly placed outside the nine groups.

Key words: Bacillus phylogeny; Bayesian inference; Evolutionary trees; 16S rDNA

RESUMO

Árvores utilizando os métodos de neighbor-joining, máxima parcimônia, evolução mínima, máxima verossimilhança e bayesiana, construídas baseadas em seqüências de rDNA 16S de 181 linhagens-tipo de espécies de Bacillus e taxa relacionados, mostraram a formação de nove grupos filogenéticos. A análise filogenética mostrou que Bacillus não é um grupo monofilético. B. subtilis se colocou no Grupo 1. Grupos 4, 6 e 8, respectivamente, consistiram de bacilos termofílicos, halofílicos ou halotolerantes e alcalifílicos. Grupos 2, 4 e 8 consistindo de espécies de Bacillus e gêneros relacionados demonstraram que o sistema taxonômico corrente não concorda perfeitamente com as árvores evolucionárias por rDNA 16S. A posição de Caryophanaceae e Planococcaceae no Grupo 2 sugere que estes podem ser transferidos para Bacillaceae, e a heterogeneidade do Grupo 2 implica em que algumas espécies de Bacillus neste grupo podem pertencer a vários novos gêneros. O Grupo 9 foi principalmente composto de gêneros de Bacillaceae (excluindo Bacillus), portanto algumas espécies de Bacillus no Grupo 9: B. salarius, B. qingdaonensis e B. thermcloacae podem não pertencer a Bacillus. Quatro espécies de Bacillus, B. schlegelii, B. tusciae, B. edaphicus e B. mucilaginosus foram claramente colocadas fora dos nove grupos.

Palavras-chave: filogenia de Bacillus, inferência Bayesiana, árvores evolucionárias, 16S rDNA

INTRODUCTION

In recent years, numerous new species of genus Bacillus were reported and at the same time, many new genera of Bacillaceae were established. According to List of Prokaryotic names with Standing in Nomenclature (LPSN, http://www.bacterio.net ) (6), of more than 140 Bacillus species, there were only 54 species reported before 2000; and of more than 30 genera of Bacillaceae, only six genera were established before 2000. Bacillus has long been regarded as a phylogenetic heterogeneous group (1). However, the phylogeny of the new Bacillus species and the new genera of Bacillaceae have not been roundly studied.

Numerical classification based on a series of phenetic characters was used for classification of 368 Bacillus strains into 79 clusters (23). After 1990, 16S rDNA has been successfully applied in determining phylogenetic relationships of the aerobic, endospore-forming bacteria, which played an important role in the creation of several families and genera of Bacillales (7).

Nowadays 16S rDNA is a vital standard for taxonomy of the bacteria. Goto et al (9) used partial 16S rDNA sequence for rapid identification of Bacillus species. Then Xu and Côté (34) used 3' end 16S rDNA and 5' end 16S-23S ITS nucleotide sequences to infer phylogenetic relationships among Bacillus species and related genera. However, the two phylogenetic trees from the above two papers did not seem to be convincing because of less DNA sequences (69 and 40, respectively) and short sequence lengths (1057 bp and 220 bp, respectively). Almost complete 16S rDNA sequences with high quality from recently reported Bacillus species are accessible in GenBank, which become ideal data for phylogenetic analyses. Moreover, new softwares (25, 28) executing Bayesian or ML algorithm (11) and personal computer hardwares with high computing capability facilitate further study on phylogeny.

The primary aim of the current investigation was to establish phylogenetic relationships between Bacillus species and related genera by reconstructing 16S rDNA phylogenetic trees using several algorithms.

MATERIALS AND METHODS

Bacillus species, type species of the genera of Bacillaceae and type species of some families in Bacillales were selected for the phylogenetic study (LPSN updated date September 04, 2007). The 16S rDNA sequences of the type strains of the bacteria mentioned above were downloaded from the GenBank. If several 16S rDNA sequences from the type strain(s) of the same species were available, the longest one with the least non-AGTC characters would be selected. All the rectifiable ambiguous nucleotides in the selected sequences were corrected according to the homologous sequences searched by BLAST (http://130.14.29.110/BLAST/) and/or other 16S rDNA sequences of the type strain(s).

Nucleotide sequence alignments were made using ClustalX 1.83 (32) and optimized using Tune ClustalX (Hall 2004, http://homepage.mac.com/barryghall/TuneClustalX.html) by modifying multiple alignment parameters. Then Bioedit 7 (10) was used for refining the entire alignment by eye. Calculations of pairwise 16S rDNA sequences similarity were achieved using the EzTaxon server (http://www.eztaxon.org/) (4). Escherichia coli was used as the outgroup. The optimal models of nucleotide substitutions were estimated by the program Modeltest 3.7 (22), using hierarchical likelihood ratio tests (hLRT) and the Akaike Information Criterion (AIC).

Neighbor-joining (NJ), maximum-parsimony (MP) and minimum-evolution (ME) analyses were performed with MEGA 4 (29). NJ and ME analyses were performed using the maximum composite likelihood method and 1000 bootstrap replications. Maximum likelihood (ML)-based phylogenetic analyses were performed with RAxML-VI-HPC 4 (http://phylobench.vital-it.ch/raxml-bb/) using default parameters (bootstrap=100). Bayesian trees were inferred using MrBayes 3.1.2 (25) according to the MrBayes 3.1 Manual (2005). All the Markov chain Monte Carlo searches were run with four chains for 4, 300, 000 generations, with trees being sampled every 100 generations. The first 30000 trees were discarded as "burnin", keeping only trees generated well after those parameters stabilized.

RESULTS AND DISCUSSION

182 16S rDNA sequences were selected but the 16S rDNA sequence of B. mycoides (AB021192) was identical to that of B. weihenstephanensis (19 to 1531 bp of AB021199), so the former was omitted. TrN+I+G (30) and GTR (General time reversible (31)) +I+G models were selected according to hLRT and AIC of Modeltest 3.7, respectively. The Bayesian tree (Fig. 1.) was inferred by the GTR+I+G model while NJ (Fig. 2.) and ME (Fig. 3.) trees were inferred by the TrN+I+G model (Gamma distribution shape parameter = 0.5854).

 

Figure 4

Although the phylogeny of some bacteria was different among the trees (Fig.1. to Fig. 5.), the phylogeny of most bacteria studied was consistent. Therefore, nine groups could be set up from 181 taxa. Holder and Lewis (11) held that ML and Bayesian approaches were more advantageous than NJ, ME or MP methods, which was supported by the comparison of the five evolutionary trees. The Bayesian analysis of the 16S rDNA data set (181 taxa, 1603 sites) yielded a tree that supported with weak posterior probability (PP=0.5) the monophyly of nine groups (Fig. 1a). In general, the supports for the different groups were stronger in the Bayesian tree than other trees. This was particularly noticeable for Group 6 (PP=1) and Group 7 (PP=0.98) (Fig. 1e). Only in Bayesian tree were the supports (PP) for every group except Group 6 and Group 8 more than 0.5. We did not find any case where the other trees provided much stronger supports than the Bayesian tree for a given node in agreement with the general trend observed in the comparisons among these measures of statistical supports (16). The topology of the ML tree was similar with that of the Bayesian tree and the bootstrap supports of the ML tree were higher than those of NJ, ME or MP trees.

The Bayesian tree demonstrated that Bacillus was not a monophyletic group. The species in Group 4, 6 and 9 had similarities in their respective phenotypes while the species in other groups differed much in their phenotypes, which were in agreement with the results of Goto et al. (9) and Xu and Côté (34).

Stackebrandt and Swiderski (27) suggested Bacillus RNA group 1 (1) should be divided into at least four subgroups; while Fig. 1. demonstrated that the previous RNA group 1 harbored Group 1, 2, 3 and 5. (Note Group 2 also contained the previous RNA group 2). These four groups constituted the core of Bacillus, which embraced approximately 65% of the Bacillus species.

Group 1 (28 species) contained B. subtilis, the type species of Bacillus, which was confirmed in other phylogenetic trees (Fig. 1b). Group 2 could be clearly divided into two clusters (Fig. 1c). B. cereus cluster included 14 Bacillus species (B. mycoides not shown in Fig. 1) while B. insolitus cluster, i.e. Bacillus RNA group 2, contained nine Bacillus species and seven other genera including non-spore-forming Kurthia and Caryophanon. Stackebrandt and Swiderski called this cluster "evolutionary enigma" and "interesting taxonomic problem" (27). If this cluster were not present, Group 1, 2, 3 and 5 would be united. The presence of the complex heterogeneous cluster was consistent with the results of Rheims et al. (24); La Duc et al. (13) and Zhang et al. (37). The heterogeneity of the cluster implied that nine Bacillus species might belong to several potential genera in order to make classification consistent with phylogeny. For example, B. insolitus and B. silvestris would have to be described as novel genera (27).

Group 3 (32 species) contained more species than any other groups (Fig. 1d), but it was not present in MP tree and less supported in NJ or ME tree (bootstrap proportion, BP<0.1). Group 5 was a minor group including eight species, which was accommodated in Group 1 in the NJ or MP trees. In the ME tree, Group 5 was closely related to Group 1 while in the ML or Bayesian tree, it was related to Group 2. It followed that Group 5 was an individual group that could not be merged into Group 1 or 2.

Group 4 consisted of thermophilic bacteria (Fig. 1b). The 16S rDNA sequences of B. thermantarcticus and Geobacillus thermoglucosidasius ATCC 43742^T (X60641) showed a similarity of 98.3%. According to the temperature and pH range suitable for growth and G + C content (17, 18), B. thermantarcticus might be transferred to Geobacillus. However, other four Bacillus species in this group, B. methanolicus, B. aeolius, B. alveayuensis and B. smithii could be obviously separated from Geobacillus by the low 16S rDNA similarities of 92%. The presence of this Group 4 was consistent with the rRNA group 5 (1).

Group 6 (18 species) (Fig. 1e) was composed of halophilic or halotolerant Bacillus species except B. macyae (26). The presence of this Group 6 was consistent with the rRNA group 6 (19), which was also supported by the result of Ghosh et al. (8). Group 7 (7 species) was entirely composed of new published Bacillus species. The MP, ME and NJ trees dfemonstrated the close relationship between Group 6 and 7, but the two groups were entirely separated in the Bayesian or ML tree (Fig. 1e, Fig. 5). Not all the species in Group 7 were halotolerant except B. hwajinpoensis and B. decolorationis, which were included in Group 6 according to Yoon et al. (36). Nevertheless, our Bayesian tree and ML tree confirmed the position of the two halotolerant species was in Group 7 in agreement with the result of Nowlan et al. (21).

The 16S rDNA sequences of the type strains in Group 8 and 9, which had distinct insert sequences between 89 to 90 bp (B. subtilis AB042061 numbering), showed marked differences from those in other groups. Group 8 consisted of alkaliphilic and halotolerant bacteria (Fig. 1e) except that B. mannanilyticus was not halotolerant (20). Caldalkalibacillus thermarum was a thermophile and a peculiar member of Group 8, of which the 16S rDNA showed similarity of less than 92% to those of other Bacillus species in Group 8. Group 9 mainly consisted of the genera (excluding Bacillus) of Bacillaceae (Fig. 1f), but it had four Bacillus species: B. taeanensis, B. salarius, B. qingdaonensis and B. thermcloacae. The former three species were included in Group 8 using NJ methods by Lim et al. (14, 15) and Wang et al. (33), for the feasible reason that the authors did not use enough 16S rDNA sequences of the type strains of Group 9 for phylogenetic analysis. Therefore, the taxonomic positions of the four species were doubtful. The low 16S rDNA similarities (less than 92.5%) between the latter three species and their respective closest relatives in Bacillus suggested the latter three species were worthy to be reclassified.

There were four Bacillus species outside the nine groups in the 181 taxa phylogenetic trees (Fig. 1a). B. schlegelii and B. tusciae were thermophilic and facultatively chemolithoautotrophic bacteria with high G + C content of the genome (2). Their 16S rDNA sequences respectively showed a very low level of similarity (less than 90%) with respect to other 180 sequences, which demonstrated that B. schlegelii and B. tusciae might well belong to two as-yet-undescribed new genera. This opinion showed agreement with the views expressed by Stackebrandt and Swiderski (27) who held that B. schlegelii, B. tusciae and B. thermcloacae were potential new genera. B. edaphicus and B. mucilaginosus were always clustered with Paenibacillus polymyxa, and their relatively high 16S rDNA sequence similarity values (96-97%) to the nearest relatives (P. elgii and P. chinjuensis, respectively) indicated that B. edaphicus and B. mucilaginosus might be transferred into Paenibacillus. The facts mentioned above revealed that at least nine Bacillus species (one in Group 4, four in Group 9 and four outside 9 groups) might not be really Bacillus species. Their phylogenetic positions would not be determined pending further polyphasic taxonomic studies.

In the NJ and ME trees, Ureibacillus, B. thermcloacae and Exiguobacterium formed a clade with a bootstrap value of less than 50%; while in the ML, MP and Bayesian trees, Ureibacillus was in Group 2 and the latter two were in Group 9, where their phylogenetic positions were supported by BP or PP of more than 50%. The phylogeny of five species was uncertain (Table 1). Low 16S rDNA sequences similarities (<93%) were found between Sporolactobacillus inulinus and other 181 taxa. S. inulinus showed scanty growth in air, was catalase-negative and not thermophilic (12), which manifested different phenotypes from the species of Group 7 or Group 4. Moreover, in the Bayesian and ML trees, BP or PP (both less than 0.5) could not ensure its position between Group 7 and 9. B. infernus, B. thermoamylovorans, B. coagulans and B. azotoformans were in their respective groups of the Bayesian tree with PP of more than 0.5 but their positions could not be supported by BP of less than 50% in the NJ, ME, MP or ML tree. For example, the former three thermophilic bacteria were included in Group 4 with BP of less than 35%. Therefore, their positions in the Bayesian tree were more credible. Originally, B. infernus was clustered with B. methanolicus (3) and B. thermoamylovorans was grouped with Geobacillus and Saccharococcus (5), but bootstrap analyses were not done in the studies.

The evolutionary relationships among the nine groups differed much in the five phylogenetic trees. However, it was noteworthy that Group 8 was always an outlier of the other eight groups. Because of relatively high PP of more than 0.5, the phylogenetic relationships illustrated by the Bayesian tree, which agreed well with the ML tree, were more believable. In the five evolutionary trees, the phylogeny of the genera in Bacillales did not conform to the current well-known taxonomic system (7) (Table 2). The genera in Bacillaceae except Bacillus were mainly in Group 9, 2 and 4, while Vulcanibacillus was outside the nine groups, suggesting that it should be removed from Bacillaceae. Caryophanaceae and Planococcaceae (represented by Kurthia) were clustered in Group 2, confirming the result of Yoon et al. (35) and implying that they might be accommodated in Bacillaceae.

ACKNOWLEDGEMENTS

We thank Gao Qiang, Ji Cunpeng and Hao Jianhua for collaboration on computation. This work was supported by the National High Technology Research and Development Program of China (863 Program) in Marine Technology Area during the 11th Five-year Plan Period (2007AA091602).

Submitted: July 14, 2008; Returned to authors for corrections: October 06, 2008; Approved: May 04, 2009.

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Publication Dates

• Publication in this collection
  18 Aug 2009
• Date of issue
  Sept 2009

History

• Reviewed
  06 Oct 2008
• Received
  14 July 2008
• Accepted
  04 May 2009