Friday, May 4, 2012

Creating a bee-specific database


Arguably, 454 pyrosequencing has revolutionized the field of microbial ecology.  Where it was once costly to generate libraries of a few hundred 16S rRNA gene sequences, 454 pyrosequencing allows researchers to deeply probe a microbial community at relatively little cost per sequence.  The ultimate goal of 454 pyrosequencing amplicon studies is to characterize a microbial community, either in terms of composition (DNA) or activity (RNA).  A large number of groups have been using the Ribosomal Database Project's Naïve Bayesian Classifier (RDP-NBC) to achieve this goal (Wang et al., 2007). The advantages are numerous but I'll list a few of the practical ones here: classification is straightforward (putting sequences in their taxonomic context), efficient (especially when considering tens of thousands of sequences) and does not require full length 16S sequences (making it an appropriate tool for pyrosequencing studies).  However, the NBC relies on an accurate training set – on reference sequences used to train the model and generate the classification results.  In a publication by Werner et al. (2011), the training set had a significant impact on classification, improving the classification of previously “unclassified” sequences and increasing the number classified to genus [1].

For environments that lack cultured isolates or are relatively unexplored, it can be difficult to find the appropriate training set to reveal the true taxonomic identity of the sequences extracted.  However, if previous clone libraries have generated full length, high-quality 16S sequences these can be added to the seed alignment and the taxonomy framework.  This is what I've aimed to do for the honey bee gut, using Mothur. In Mothur you can “tweak” the alignment seed for any particular environment, creating a custom database that will more accurately classify sequences of interest.  

To create a bee-specific alignment compatible with Mothur you need two files: a reference database and a taxonomy file for each of those sequences. To generate the database I downloaded all sequences that corresponded to accession numbers published in analyses of bee-associated microbiota and that were near full length (1250 bp) (A total of 5,713 sequences were downloaded and 5,158 passed the length threshold).  These sequences were clustered at 99% identity, reducing the dataset to 276 representatives.  This set of sequences were aligned using the SINA aligner (v 1.2.9, [2]) to the arb-silva SSU database (SSURef_108_SILVA_NR_99_11_10_11_opt_v2.arb) and visually inspected using ARB [3].  To generate a phylogeny I used this aligned sequence set as input to RAxML (GTR+g with 1000 bootstrap replicates) using a maximum likelihood framework [4].  Taking a quick look at this taxonomy, it is clear that we've got the majority of sequences falling into either the Firmicutes, Actinobacteria, or Proteobacteria.  Also, specific clades identified by previous groups are clearly marked on the tree (based on the literature). Now, it's difficult to classify novel sequences so you might be asking your self, how could you taxonomically classify them based on this tree?  Fine scale taxonomic placement (below phylum level) for relatively novel bacterial groups is difficult to accomplish and subject to some debate [5].  So, I queried the RDP for nearest cultured representatives.  If these cultured representative was >95% identical to the bee derived sequence then that novel sequence was placed in the genus of the cultured representative.  If, however, the sequence identity above 95% was not found for these sequences in the cultured isolates, but they claded with a cultured representative, they were placed in the same phylum, class, or order (depending on the group) and we noted incerte sedis in the taxonomy file.  In addition to this de novo generation of taxonomic information for these bee sequences, if phylogenetic information (as established by Cox-Foster et al., 2006) was associated with any of these Genbank submissions, that information was also included in the taxonomy.  

I then downloaded each of three pre-existing, Mothur-compatible training sets: 1) the RDP 16S rRNA reference v7 (9,662 sequences), 2) the Greengenes reference (84,414 sequences), and 3) the SILVA bacterial reference (14,956 sequences) each available on the Mothur WIKI page (http://www.mothur.org/wiki/Main_Page).  These datasets are comprised of both an unaligned sequence file and a taxonomy file.  To each of these I added the honey bee specific training set I generated.  Using each of these six alternative datasets (either with or without the honey bee specific sequences), I classified the honey bee gut microbiota generated in our recent publication [4] using the RDP-II Naive Bayesian Classifier [6] and a 60% confidence threshold.  


 Figure 1.  Phylogenetic relationships for the bacterial species included in the honey bee specific database (with bootstrap support indicated above branches if > 75%).  Class level designations are highlighted in red while lower taxonomic designations are indicated out using arrows on nodes.  Specific clades identified previously in honey bees are colored in blue while novel clades identified here, including cultured isolates and well-described genera (such as Wolbachia), are colored in yellow.

What I find most interesting about this analysis is how well the addition of the bee-specific sequences helps to create congruence among the datasets (the Orbus classification by RDP not withstanding).  Clearly, inclusion of environment specific sequences can increase the accuracy of the RDP-NBC.  I wanted to use this framework to explore fine-scale diversity (OTU level) within the gut.  



Figure 2. The effect of training set on the classification of sequences from the honey bee gut visualized by a heat map.   Unique sequences (4,480) were classified using the NBC trained on either RDP, GG, or SILVA (A) or three custom databases including near full length honey bee-associated sequences RDP+bees, GG+bees, SILVA+bees (B).  The effect of including custom sequences is most obvious in the classification discordance between RDP, GG and SILVA and their relative congruence when honey bee associated sequences are added to the training set (B).


Below I ask, how many individual unique sequences and how many likely "species" do we find in each of these families based on 97% clustering of operational taxonomic units (OTUs)? (Table 1). 

Table 1. For each family found with honey bee guts, the number of unique sequences and the number of 97% identity operational taxonomic units (OTUs) is shown.  The taxonomy shown here is based on classification by the RDP-NBC using the SILVA + honey bee sequences training set (available for anyone via email).  The most abundant families represent a large amount of fine-scale bacterial diversity.



Family
Num. unique sequences
OTUs
Enterobacteriaceae
1621
175
gamma-1
436
48
beta
532
35
Bifidobacteriaceae
363
32
firm-5
929
32
firm-4
253
21
alpha-2.1
90
15
alpha-1
65
13
Lactobacilliaceae
86
12
Flavobacteriaceae
2
2
Leuconostocaceae
2
2
Moraxellaceae
6
2
Sphingomonadaceae
2
2
Xanthomonadaceae
2
2
Actinomycetaceae
1
1
Aeromonadaceae
1
1
alpha-2.2
10
1
Clostridiaceae
2
1
Corynebacteriaceae
1
1
Cytophagaceae
1
1
Enterococcaceae
9
1
Incertae_Sedis_XI
1
1
Kineosporiaceae
1
1
Nakamurellaceae
1
1
Oxalobacteraceae
1
1
Prevotellaceae
1
1


One central goal of our previous study (see [7]) was to determine if there was a difference between colonies generated from promiscuous honey bee queens and those that were relatively chaste.  When we compared the OTU content between these two colony types, we found that the genetically diverse colonies host more diverse microflora [7].  This difference, based on number of 97% identity clusters found within each colony, is independent of classification and was recapitulated using the SILVA + honey bee  taxonomic classification (the 95% confidence interval (CI) for mean difference between species diversity compared between colony types exceeded 0; 95% CI = 110, 102; mean = 106.25). 

The next question is, is this difference in microbiota composition attributable to any specific taxonomic group?  That is, within specific bacterial families, do we see differences between genetically diverse and genetically uniform colonies with regards to their OTU content?  I used the bootstrapped confidence interval analysis used in [7] to answer this question.  The difference in genetic diversity between colony types was found to effect the OTU-level diversity of specific bacterial groups (Table 2).  This suggests that fine-scale diversity within these honey-bee specific families may be ecologically relevant, and shouldn’t be ignored.  

Table 2. Total number of operational taxonomic units (97% ID) in either genetically uniform or genetically diverse colonies and classified as one of the honey bee specific taxonomic groups (mean number used in CI calculation in parentheses).  Statistically significant differences between colony types was observed for most of these families and their OTU content (indicated by an asterisk).

Taxon
Genetically Diverse
Genetically Uniform
Bootstrap 95% CI of mean difference
Firm-4*
44 (36.10)
25 (25.04)
(11.22, 10.90)
Firm-5*
56 (45.13)
46 (46.05)
(0.74,1.09)
Alpha-2.1*
21(16.03)
21 (21.04)
(4.92, 5.09)
Alpha-2.2
4 (4.05)
4 (4.01)
(0.09, -0.005)
Alpha-1*
16 (12.01)
13 (13.06)
(0.96, 1.14)
Beta*
60 (48.98)
38 (37.99)
(11.12, 10.85)
Gamma-1*
66 (52.73)
51 (50.99)
(1.94, 1.55)

Which brings me to a final point – that’s more of a rant really.  This is about lumping and splitting – you say “toma-toe” and I say “tomah-toe” – and what one calls a bacterial “species” (I’m not stepping into that mine field).   We don’t yet know what the observed % divergence at the 16S rRNA gene means in the honey bee gut microbiota.  Could these differences be primarily attributable to diversity between operons within a single strain?  This is unlikely; within the majority of bacterial genomes these operons evolve by concerted evolution and show <1% divergence between gene copies [8].  We are using the 16S rRNA gene as a marker for diversity – as a taxonomic tag.  This short tag is just that – a marker.  It could represent an enormous diversity at the genome level, we don’t yet know.  What we do know is that the microbial world is vast, that diversity is the norm – very few environments are characterized by low species abundance or “clonal” strains.  The fact that we are able to pick up a statistically significant signal between honey bee colonies based on OTUs suggests to me that there is more to investigate here.




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