Introduction to decontam
Identifying contaminants in marker-gene and metagenomics data
Introduction to the
decontam R package. User support, feature requests, comments and
suggestions are welcome at the decontam issues
tracker. The preprint
introducing decontam includes performance benchmarking, and
demonstrates the benefits of decontam-inating your data for
more accurate profiling of microbial communities.
Introduction
The investigation of environmental microbial communities and microbiomes has been transformed by the recent widespread adoption of culture-free high-throughput sequencing methods. In amplicon sequencing a particular genetic locus is amplified from DNA extracted from the community of interest, and then sequenced on a next-generation sequencing platform. In shotgun metagenomics, bulk DNA is extracted from the community of interest and sequenced. Both techniques provide cost-effective and culture-free characterizatoins of microbial communities.
However, the accuracy of these methods is limited in practice by the
introduction of contaminating DNA that was not truly present in the
sampled community. This contaminating DNA can come from several sources,
such as the reagents used in the sequencing reaction, and can critically
interfere with downstream analyses, especially in lower biomass
environments. The decontam package provides simple
statistical methods to identify and visualize contaminating DNA
features, allowing them to be removed and a more accurate picture of
sampled communities to be constructed from marker-gene and metagenomics
data.
Necessary Ingredients
The first decontam ingredient is a feature table derived
from your raw data, i.e. a table of the relative abundances of sequence
features (columns) in each sample (rows). These “sequence features” can
be any of a wide variety of feature types, including amplicon sequence
variants (ASVs), operational taxonomic units (OTUs), taxonomic groups or
phylotypes (e.g. genera), orthologous genes or
metagenome-assembled-genomes (MAGs) – anything with a quantitative
abundance derived from marker-gene or metagenomics data.
The second decontam ingredient is one of two types of
metadata:
DNA quantitation data recording the concentration of DNA in each sample. Most often this is collected in the form of fluorescent intensities measured prior to mixing samples into equimolar ratios for sequencing (and after PCR in amplicon sequencing), but sometimes it is collected via qPCR or other approach on extracted DNA.
A defined set of “negative control” samples in which sequencing was performed on blanks without any biological sample added. Extraction controls are preferred, and in amplicon sequencing the negative controls should also be carried through the PCR step, as each step in the workflow has the potential to introduce new contaminants.
Finally, this data needs to be imported into R. The feature table
should be imported as a sample-by-feature matrix and the
sample metadata as a vector with length the number of
samples. Alternatively, you can use the phyloseq package to
import the data as a phyloseq object.
Setting up
The decontam package works with feature tables in the
form of a standard R matrix, but it is even easier to work
with phyloseq objects from the phyloseq
package, which is designed to ease the analysis of marker-gene and
metagenomics datasets.
In this introductory tutorial, we’ll start by reading in a
phyloseq object to work with:
## [1] '1.51.0'## [1] '3.5.1'## [1] '1.27.0'## phyloseq-class experiment-level object
## otu_table() OTU Table: [ 1951 taxa and 569 samples ]
## sample_data() Sample Data: [ 569 samples by 6 sample variables ]
## tax_table() Taxonomy Table: [ 1951 taxa by 6 taxonomic ranks ]This phyloseq objects has a table of 1951 amplicon sequence variants
(ASVs) inferred by the DADA2
algorithm from amplicon sequencing data of the V4 region of the 16S
rRNA gene. The phyloseq objects also includes the sample metadata
information needed to use decontam.
## X.SampleID PlateNumber Subject Habitat quant_reading
## P1101C01701R00 P1101C01701R00 3 1101 Tongue 2829
## P1101C01702R00 P1101C01702R00 3 1101 Cheek_Right 5808
## P1101C01703R00 P1101C01703R00 3 1101 Cheek_Left 5052
## P1101C08701R00 P1101C08701R00 3 1101 Tongue 1227
## P1101C08702R00 P1101C08702R00 3 1101 Cheek_Right 3872
## P1101C08703R00 P1101C08703R00 3 1101 Cheek_Left 5872
## Sample_or_Control
## P1101C01701R00 True Sample
## P1101C01702R00 True Sample
## P1101C01703R00 True Sample
## P1101C08701R00 True Sample
## P1101C08702R00 True Sample
## P1101C08703R00 True SampleThe key sample variables are quant_reading, which is the
DNA concentration in each sample as measured by fluorescent intensity,
and Sample_or_Control which tells us which samples are the
negative controls.
Inspect Library Sizes
Let’s take a quick first look at the library sizes (i.e. the number of reads) in each sample, as a function of whether that sample was a true positive sample or a negative control:
df <- as.data.frame(sample_data(ps)) # Put sample_data into a ggplot-friendly data.frame
df$LibrarySize <- sample_sums(ps)
df <- df[order(df$LibrarySize),]
df$Index <- seq(nrow(df))
ggplot(data=df, aes(x=Index, y=LibrarySize, color=Sample_or_Control)) + geom_point()
The library sizes of the positive samples primarily fall from 15,000 to 40,000 reads, but there are some low-read outliers. The negative control samples have fewer reads as expected. Note: It is important keep the low-read samples for now, because we want to use those negative controls to help identify contaminants!
Identify Contaminants - Frequency
The first contaminant identification method we’ll use is the “frequency” method. In this method, the distribution of the frequency of each sequence feature as a function of the input DNA concentration is used to identify contaminants.
In our phyloseq object, "quant_reading" is the sample
variable that holds the concentration information:
## freq prev p.freq p.prev p contaminant
## Seq1 0.323002694 549 1.000000e+00 NA 1.000000e+00 FALSE
## Seq2 0.098667396 538 1.000000e+00 NA 1.000000e+00 FALSE
## Seq3 0.003551358 160 1.135975e-18 NA 1.135975e-18 TRUE
## Seq4 0.067588419 519 9.999998e-01 NA 9.999998e-01 FALSE
## Seq5 0.045174743 354 1.000000e+00 NA 1.000000e+00 FALSE
## Seq6 0.040417101 538 1.000000e+00 NA 1.000000e+00 FALSEThis calculation has returned a data.frame with several
columns, the most important being $p which containts the
probability that was used for classifying contaminants, and
$contaminant which contains
TRUE/FALSE classification values with
TRUE indicating that the statistical evidence that the
associated sequence feature is a contaminant exceeds the user-settable
threshold. As we did not specify the threshold, the default value of
threshold = 0.1 was used, and
$contaminant=TRUE if $p < 0.1.
##
## FALSE TRUE
## 1893 58## [1] 3 30 53 80 152 175Just 58 out of the 1901 ASVs were classified as contaminants, but this includes some very abundant sequences, including the third (!) most abundant ASV.
Let’s take a look at what a clear non-contaminant (the 1st ASV), and a clear contaminant (the 3rd ASV), look like:
plot_frequency(ps, taxa_names(ps)[c(1,3)], conc="quant_reading") +
xlab("DNA Concentration (PicoGreen fluorescent intensity)")
In this plot the dashed black line shows the model of a noncontaminant sequence feature for which frequency is expected to be independent of the input DNA concentration. The red line shows the model of a contaminant sequence feature, for which frequency is expected to be inversely proportional to input DNA concentration, as contaminating DNA will make up a larger fraction of the total DNA in samples with very little total DNA. Clearly Seq3 fits the red contaminat model very well, while Seq1 does not.
Let’s inspect a couple more of the ASVs that were classified as contaminants to ensure they look like what we expect:
set.seed(100)
plot_frequency(ps, taxa_names(ps)[sample(which(contamdf.freq$contaminant),3)], conc="quant_reading") +
xlab("DNA Concentration (PicoGreen fluorescent intensity)")
Those all look like contaminants!
Now that we have identified likely contaminants, let’s remove them from the phyloseq object:
## phyloseq-class experiment-level object
## otu_table() OTU Table: [ 1951 taxa and 569 samples ]
## sample_data() Sample Data: [ 569 samples by 6 sample variables ]
## tax_table() Taxonomy Table: [ 1951 taxa by 6 taxonomic ranks ]## phyloseq-class experiment-level object
## otu_table() OTU Table: [ 1893 taxa and 569 samples ]
## sample_data() Sample Data: [ 569 samples by 6 sample variables ]
## tax_table() Taxonomy Table: [ 1893 taxa by 6 taxonomic ranks ]And off we go with analysis of the contaminant-filtered data!
See the phyloseq web site for documentation of the many powerful analyses available through the phyloseq R package.
Identify Contaminants - Prevalence
The second contaminant identification method we’ll use is the “prevalence” method. In this method, the prevalence (presence/absence across samples) of each sequence feature in true positive samples is compared to the prevalence in negative controls to identify contaminants.
In our phyloseq object, "Sample_or_Control" is the
sample variable that holds the negative control information. We’ll
summarize that data as a logical variable, with TRUE for
control samples, as that is the form required by
isContaminant.
sample_data(ps)$is.neg <- sample_data(ps)$Sample_or_Control == "Control Sample"
contamdf.prev <- isContaminant(ps, method="prevalence", neg="is.neg")
table(contamdf.prev$contaminant)##
## FALSE TRUE
## 1827 124## [1] 30 80 152 175 176 193Prevalence-based contaminant identification has identified a larger
number of contaminants, 124, than did the frequency-based method, 58, in
this dataset, but also missed the very clear and highly abundant
contaminant Seq3, because Seq3 was present in
almost all samples - negative and positive.
Note that as before, the default threshold for a contaminant is that
it reaches a probability of 0.1 in the statistical test being performed.
In the prevalence test there is a special value worth knowing,
threshold=0.5, that will identify as contaminants all
sequences thare are more prevalent in negative controls than in positive
samples. Let’s try using this more aggressive classification threshold
rather than the default.
contamdf.prev05 <- isContaminant(ps, method="prevalence", neg="is.neg", threshold=0.5)
table(contamdf.prev05$contaminant)##
## FALSE TRUE
## 1809 142Let’s take a look at the number of times several of these taxa were observed in negative controls and positive samples.
# Make phyloseq object of presence-absence in negative controls and true samples
ps.pa <- transform_sample_counts(ps, function(abund) 1*(abund>0))
ps.pa.neg <- prune_samples(sample_data(ps.pa)$Sample_or_Control == "Control Sample", ps.pa)
ps.pa.pos <- prune_samples(sample_data(ps.pa)$Sample_or_Control == "True Sample", ps.pa)
# Make data.frame of prevalence in positive and negative samples
df.pa <- data.frame(pa.pos=taxa_sums(ps.pa.pos), pa.neg=taxa_sums(ps.pa.neg),
contaminant=contamdf.prev$contaminant)
ggplot(data=df.pa, aes(x=pa.neg, y=pa.pos, color=contaminant)) + geom_point() +
xlab("Prevalence (Negative Controls)") + ylab("Prevalence (True Samples)")
Taxa seem to split pretty cleanly into a branch that shows up mostly in positive samples, and another that shows up mostly in negative controls, and the contaminant assignment (at default probability threshold) has done a good job of identifying those mostly in negative controls.
Putting It All Together
The two basic methods implemented are the frequency and
prevalence methods shown above, but a number of additional
ways to utilize those methods are available. The combined,
minimum and independent modes all use both the
frequency and prevalence methods to identify
contaminants, but combine the results of the two methods in different
ways (see ?isContaminant for more information). There is
also a batch functionality that allows contaminants to be
identified independently in different batches (e.g. sequencing runs, or
different studies) that are expected to have different contaminant
profiles.
Conclusion
We all know that contaminants are a problem in marker-gene and
metagenomics data. Right now, it is widespread practice to address
contamition by sequencing negative controls, and then doing nothing, in
large part because there haven’t been easy to use tools available to
identify contaminants in this kind of data. That’s where the
decontam package comes in. decontam provides a
simple interface that takes in your table of sequence features (ASVs,
OTUs, MAGs, genera, etc), and classifies each sequence feature as a
contaminant based on signatures of contamination that have been
demonstrated over many previous studies.
Removing contaminants makes the characterizations provided by marker-gene and metagenomics sequencing more accurate. It prevent false positives in exploratory analyses. It reduces batch effects between different studies and sequencing runs. It increases statistical power by reducing the additional hypotheses spent on contaminant sequencing features.
Removing contaminants makes your data better, and that’s always worth doing.