This dialectic was written by Scott Olesen, a graduate student in Biological Engineering at MIT.
One day in Athens, the microbiome scientist Everymeno encounters 16Socrates.
Everymeno: Hail, 16Socrates! I’ve been troubled by some results from my 16S amplicon sequencing data, and I was hoping to hear your wisdom.
16Socrates: I don’t know that I can answer your questions, but we can look for an answer together.
E: In a microbiome experiment, I collected samples, extracted the DNA, and amplified the 16S gene. I’m converting the raw 16S sequences into operational taxonomic units (OTUs). I know there’s a lot of discussion in the Microbiome Agora about what kinds of OTU-calling methods we should use, so I tried to avoid controversy by using all of them and seeing what comes out.
I started calling OTUs using different strategies supported by usearch, which seems to be a commonly-used program for microbiome analysis. Many people use usearch via QIIME, which calls usearch behind-the-scenes to cluster de novo or to do reference-based searches against databases.
16S: I think I’m following you so far, but I’m a little confused. When you say “usearch”, do you mean the program usearch, or do you mean the proprietary heuristic alignment algorithm USEARCH that’s implemented by the program usearch?
E: I see how that could be confusing. I’m mostly confused about the behavior of the algorithm USEARCH. I will say usearch only I am talking other things that the program usearch does—like call de novo OTUs—or when we I am talking explicitly about using the program to do USEARCH.
16S: We’ll try to keep it straight.
In this first discussion, Everymeno is confused about his OTU calling pipeline. He gets dramatically different numbers of OTUs depending on whether he uses de novo or reference-based OTU-calling. 16Socrates suggests a small in silico experiment that shows that usearch is not guaranteed to map similar sequences to the same reference OTU, thus increasing the number of OTUs that result from reference-based OTU-calling with usearch.
16S: So what’s been bugging you, Everymeno?
E: First, I found that I get a lot more OTUs if I call OTUs by USEARCH-ing against the 97%-identity reference OTUs in the Greengenes database than I do if I call de novo 97%-identity OTUs using the program usearch. I thought they should give the same results or, if anything, I would get more OTUs by clustering de novo, since there might be sequences in my samples that aren’t in the database.
16S: It sounds like you expected that using de novo or reference-based OTU-calling at a similar sequence-identity should lead to similar results.
E: Yes, but it doesn’t stop there. When I saw these results, I wondered what was happening, so I looked for sequences that got put into the same OTU in the de novo clustering but ended up in different OTUs in the reference-based clustering. These set of sequences are similar to one another, but the taxonomies assigned by using USEARCH and the Greengenes database include a lot of different genera and species! Why are these sequences not put into the similar OTUs when I cluster using the two different methods? And why should similar sequences have such different taxonomies assigned to them?
16S: That sounds like a difficult question. Maybe we can do a little experiment on a much simpler system to get a sense of what’s going on. Let’s look at the very first sequence from the trimmed, processed 16S data in the Human Microbiome Project data. It’s a 292-nucleotide sequence with header:
F48MJBB01DZOWV_cs_nbp_rc cs_nbp=29-315 sample=700014956 rbarcode=TCACAC primer=V3-V5 subject=158822939 body_site=Stool center=WUGSC
Let’s use USEARCH to find a 97% OTU in Greengenes that matches this sequence.
E: I found an OTU (4351920) whose representative sequence is a 97.3% match for my HMP sequence. I’m going to draw it like this: I put the OTU in the center and draw concentric circles indicating the sets of sequences that are ever-less-similar to the OTU. I’ll mark the HMP sequence somewhere just inside the 97% identity line.
16S: Let’s see what happens if we have another sequence very similar to this one. Does it end up in the same OTU?
E: I expect it would! To my mind, this is the whole point of OTU calling: it’s a way to put similar sequences together into fewer, more-easily-interpreted units. That’s what I take the “T” in OTU to mean.
16S: In other words, you expect that very closely-related organisms will have very closely-related 16S sequences, so you want to put those in the same taxonomic unit?
E: Yes, exactly.
16S: Well, let’s check that our OTU calling method puts very similar sequences into the same OTU. To generate very similar sequences, let’s enumerate all the sequences that are one nucleotide different from the HMP sequence. For each nucleotide in the original sequence, I’ll make three new sequences, replacing that nucleotide with the three other nucleotides. This will make 292 x 3 = 876 sequences that are each one nucleotide different from the original sequence.
E: So, since the original sequence starts GTG, the first new sequence will start ATG, the next CTG, the next TTG, and then the next GAG, and so on?
16S: That’s right. Let’s use the usearch_global command, which performs the USEARCH algorithm, to find which reference OTUs in the database these sequences belong with.
E: What sequence identity cut-off should I use? The reference OTUs are at 97% identity, so I think I should use a 97% identity cut-off.
E: Well, “97% OTUs” means that every sequence in an OTU is at least 97% similar to the OTU’s representative sequence. I expect that my query sequence is covered by this database, so my sequence should be in some OTU, meaning that it’s 97% similar to something.
16S: That sounds reasonable to me!
E: Then I’ll run the usearch command:
usearch -usearch_global my_hmp_sequence.fasta -db 97_otus.fasta -id 0.97 -blast6out output.b6
[Everymeno runs the command, and a worried look comes over his face.]
The results are confusing!
The first thing that bothers me is that only 16% of the 876 sequences we made even have matches with the database. I’ll draw a table, where I’ll write which OTU was hit, that OTU’s taxonomy, and the number of our 876 sequences that hit that OTU.
|21||4351920 (original match)||g__Sphingomonas; s__wittichii|
The second thing that bothers me is that the OTU that matched the greatest number of the one-nucleotide-different sequences (1121270) isn’t the one (4351920) that matched the original sequence! The third thing that bothers me is that, although all the OTUs in the table have the same phylogeny down to family (Sphingomonadaceae), they have different genus and species names. The sequences in this fake data set we made are at most two nucleotides different from one another. That’s less than 1% different. How can sequences that are 1% different from one another end up in different genera?
In Part I, Everymeno discovered that USEARCH isn’t guaranteed to map similar query sequences to the same database sequence. 16Socrates helps Everymeno understand how this unintuitive mapping works by conducting another in silico experiment on USEARCH, and he’s able to formulate an explanation for his results from Part I.
16S: I can see why you’re distressed by those results! You put in many similar sequences and USEARCH told you they mapped to reference OTUs, not all of which are even in the same genus. I have a hunch about what’s going on. To see if I’m right, I’ll ask a crazy question: what happens if you turn the minimum sequence identity requirement down?
E: You mean, allow worse matches between my query sequences and the database? I’m not sure why that’s a good idea, but I’ll try it. I’ll run the same command I did before with only a 90% identity cutoff, so I’ll replace
-id 0.97 with
[Everymeno runs the command.]
Now I’m even more confused! These hits are different than my first set of hits. Most of these hits are to an OTU (3937304) that didn’t even appear in my first search. Also, the sequence identities I got with a 97% cut-off ranged from 97.3% to 99.3%, but all of these hits are at least 99.3%! Why should allowing worse hits somehow produce only better hits? And different ones?
16S: I think I can resolve some of your confusion. USEARCH is a heuristic algorithm, meaning that it’s not guaranteed to find the best hit, it just finds a hit that matches your criteria.
E: So it sounds like, when we ran our first experiment with the slightly different query sequences, USEARCH could have reported that all those sequences mapped to the same OTU, since that would have met the identity threshold we set?
16S:That sounds right.
E: Well, why didn’t it? I suppose I should read some of usearch’s documentation to figure that out.
[He spends some time flipping through the manual.]
I have a little hypothesis: in usearch’s documentation, it says that usearch works by somehow organizing the database sequences by their k-mer content. When we changed the one nucleotide in our query sequences, we changed many k-mers.
16S: I suppose that’s possible, but I haven’t heard a lot about k-mers, so I’m not sure I can relate to how your thinking about this problem. Is there another way we can get insight into how USEARCH is making matches?
In Part II, Everymeno showed how USEARCH’s heuristics could lead to the weird results he saw in Part I, where similar query sequences mapped to different database sequences. At the end of Part II, 16Socrates challenged Everymeno to formulate a simple way to investigate what kind of mapping USEARCH is doing. Everymeno runs this experiment but runs into problems when he tries to apply the approach he devises to OTU calling.
E: One way to see what kinds of hits USEARCH can make would be to have it report all the matches it finds, not just the first one that meets its accepting criteria. I read in the documentation that you can do that by setting
-maxaccepts 0 -maxrejects 0. I expect that this will give a lot of hits, so let’s do something simple: I’ll use only the original HMP sequence and 97% identity cutoff, but I’ll have usearch show me all the hits. That command is:
usearch -usearch_global my_hmp_sequence.fasta -db 97_otus.fasta -id 0.97 -maxaccepts 0 -maxrejects 0
[Everymeno runs the command. For the first time, he looks less worried.]
Well, that makes me feel a little better. Of the 34 hits that are here, one is the hit we got in the very first search (4351920) and one is the best hit we got when we relaxed the sequence identity cutoff to 90% (3937304). I’ll write down the results in a table.
|OTU||hit identity||OTU||hit identity|
|3937304 (90% identity hit)||99.7||4476892||97.9|
|1121270 (matched many one-different-HMP)||99||4311514||97.6|
|4311513||98.3||4351920 (original hit)||97.3|
16S: Why don’t you think people normally take this show-me-everything approach?
E: Well, I know that showing all the hits is slower than just taking the first hit, but it didn’t seem that much slower: I think the heuristic speedup in USEARCH comes mostly from efficient look-ups in the reference database, not in the actual alignments, so reporting multiple possible hits doesn’t seem that bad.
I suppose it’s also confusing if you have a tie among your good hits. What should you do? Just assign that sequence randomly to one of the two OTUs? Or should you pick the OTU that most of your other sequences have fallen into? That sounds fraught.
But I think the worst part is the existential bellyache this is causing me: if the heuristic search can give me so many sequences to which my sequence is a good match, isn’t something going wrong?
At the end of Part III, Everymeno raised some questions about how to apply the lessons he learned, namely, how to see all the sorts of hits that usearch might report. He’s troubled by the number of possible hits usearch reports. In this part, 16Socrates challenges Everymeno to explain how one query sequence can appear to map to so many reference OTUs.
16S: Everymeno, I’m also confused by the number of “acceptable” hits usearch is giving us. I’m particularly troubled by one thing, and maybe you can explain. We’re using the 97% Greengenes OTUs, so I would think, because these OTUs were called at 97% identity, that each OTU’s representative sequence is at least 3% different from any other OTU’s representative sequence. Correct?
E: I can see how you’re trying to trick me! What you said might not be true. We said before that “97% OTUs” means that each sequence in an OTU is at most 3% different from the representative OTU sequence. It’s like this picture here: any sequence within the circle of at least 97% identity —that is, at most 3% difference— could be assigned to the OTU.
This doesn’t necessarily mean that every OTU is 3% different from every other one. That’s what this next picture shows.
I suppose it wouldn’t be a very good OTU-calling algorithm if we had a bunch of OTUs, all 1% different from one another, like in this third picture. You would have too many choices about how to assign sequences to OTUs while still keeping the 97% sequence-to-OTU identity rule: all of the sequences that fall inside both circles could be assigned to either OTU.
16S: I understand these pictures, but they’ve made me confused about something else. You said that you would want your 97% OTU calling algorithm to give OTUs who representative sequences are more like 6% different rather than 1% different?
E: That’s right. If OTUs are too similar to one another, you’ll have too many choices about how to assign sequences to them.
16S: So you’re saying that, if the Greengenes 97% OTUs were called according to this rule, then we would expect that most OTUs will be more like 6% different rather 1% different from one another? That is, that it’s unlikely —but not impossible— to find two OTUs that are 99% similar to one another?
E: Yes, I think that’s right, although I feel a little pedantic for making you say it that way!
16S: Then here’s my confusion: We said it’s unlikely to find two OTUs that are 99% similar, but in your last USEARCH, we saw that one query sequence hit multiple OTUs with 99% identity. How can one sequence be 1% different from two OTUs that should be around 6% different from one another?
E: I can see how you’re trying to trick me, 16Socrates, but I know about the Greengenes database! The sequences in the database are long —1,400 nucleotides is typical— while amplicon reads are short, like our 292 nucleotide HMP sequence. The OTUs in the database are at least 3% different across their entire sequence, so they could in theory be identical over the region that our short sequence is matching. Thus, our 292 nucleotide sequence is 1% different from a ~292 nucleotide subsequence in the Greengenes OTUs, while those OTUs may be more different from one another over their entire >1,000 nucleotide length.
16S: I’m not sure it was a trick, but that was tricky!
E: I guess this clears up what I was confused about in the very beginning: my original HMP sequence was a good fit over its short length to many reference OTUs, and USEARCH’s heuristics meant that slightly different query sequences got mapped to fairly dissimilar reference OTUs that were nevertheless good matches to the query sequences over their short length. When I did my OTU calling, I expected that USEARCH would map similar sequences in my samples to the same reference OTU, but I can see now that USEARCH isn’t at all guaranteed to do that!
In Part IV, Everymeno explained how a single, short query sequence can hit many reference OTU sequences —which are all fairly different from one another— with high identity, and he synthesizes the results of his experiments about USEARCH’s heuristic features to explain his original search results. In this part, he and 16Socrates puzzle about how to apply these lessons to their future OTU-calling efforts.
16S: Since you like tricky questions, I’ll ask, does all this confusing mess mean that USEARCH is bad? Should we not use USEARCH?
E: What I learned from talking with you is that USEARCH is good at what it does: it’s a heuristic search algorithm. It finds approximate matches for my sequences in the database. It’s not designed to give the single, best hit. I was expecting it to do that, and so I was disappointed.
16S: Bravo! And a final follow-up: What does this mean for OTU calling? How can we be sure that similar sequences in your experimental data will end up in similar OTUs and be assigned the same taxonomies?
E: I suppose I can’t rely on the USEARCH algorithm to do that, necessarily. I suppose that, if I want to be sure that similar sequences end up in the same OTU, I’ll either have to use a much slower, non-heuristic reference-based search or just cluster de novo first.
A tool that doesn’t rely as strongly on alignments might also work. When I use RDP on the 876 one-nucleotide-changed sequences, 868 of them are assigned one taxonomy (genus Sphingomonas), and only 8 get a different taxonomy (genus Sphingosinicella in the same family).
16S: Well, Everymeno, we worked through this idea so quickly, maybe next time we should take up a more difficult problem, like the definition of virtue!
Scott Olesen is a PhD candidate in Biological Engineering at MIT. His current project is about modeling the heterogeneity of stool transplant donors’ fecal material.