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Chapter 3

Biological Sciences 55-237 Chapter 3: Microbiology Topic 3

Biological Sciences
Course Code
BIOL 2070

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Microbiology Topic 3
Microbial Species
What is a species?
Species definition for animals- a population of individuals that can interbreed under
normal conditions and produce fertile offspring
o Reproductively isolated from other populations
This does not work well for some of the other organisms, including Bacteria,
Archaea, and some eukaryotic microbes
Try to compare different types of diversity of different types of organisms by
comparing different species
o Bacteria and viruses do not seem to be prevalent
Bacterial Species
For a long time, bacterial species were defined by their shape, Gram stain result, and
diseases caused by the bacterium
Eventually, once biochemical tests were developed, those were added to the mix
Historically: Bacterial species defined as a distinct group of strains that have certain
distinguishing features and generally bear close resemblance to one another in
more essential features of categorization
o This definition is vague and not terribly objective criteria
More recently: Two bacterial or archaeal strains of the same species must have
similar molecular percent guanine plus cytosine content in genome (mol % G + C), at
least 97% similarity in 16s rRNA sequence, and must exhibit 70% or greater
DNA/DNA reassociation (Defined by relatedness at genetic levelphylogenetic
species concept)
Later, molecular biology tools made it possible to study the sequence of genes and
proteins, and make better determinations of relatedness
Although there is not a single definition of what makes a bacterial species, we are
getting toward the point where a phylogenetic species concept is being used
o This defines a microbial species as a group of strains that share certain
diagnostic traits, are genetically cohesive, and share a recent common
For Bacteria and Archaea, genetic relatedness is usually assessed by looking at the
16S rRNA genes and DNA hybridization experiments
Species are expected to have at least 97% similarity in 16S rRNA sequences and
over 70% DNA hybridization
G + C content of the genome is also expected to be similar for genes in the same
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In E. coli, a number of different strains were analyzed for different properties
o G + C ranges from 48-52 percent
o More than 15 bases different in 16S or 18S rRNA sequences
o We do get at least 70% DNA hybridization
Looking at animals, in humans, the G + C content is 42%
o Also happens to be 42% for all of the tested primates
In humans, we see no discernible variability in 18S rRNA sequences
o This is also true for DNA hybridization
If you compare human sequence to the chimpanzee, you get 99% similarity
Same value for G + C content for humans and chimpanzees
We have to go to something like a mouse to see some differences in the rRNA
To get to the point where we are like E. coli in terms of DNA/DNA hybridization, you
would have to compare a human to a lemur
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E. coli
One of the best studied organisms
Many different strains
Different strains have a lot of differences
Looked at K-12 strain (lab strain) and a couple of strains that can cause disease
O157:H7 is one that can cause a dangerous diarrhea
There are strains of E. coli that can cause UTIs
Looking at the genome and the protein coding sequences, they found only about
39% that were found in all three
Although there was some overlap between two strains, each strain had some unique
A lot more variability when compared to something like animals
Bacterial and archaeal species are much broader than those of plants, animals, and
many other organisms
o Would not consider them equivalent
Within a bacterial species
Because these groups are so broad, we do sometimes narrow things down
Subspecies: Division within species based on consistent phenotypic variations or
genetically determined clusters of strains
o E.g., Bacillus thuringiensis subspecies israeliensis
o May be abbreviated as subsp. or ssp.
Rank: strains (within species or subspecies) sharing special properties
o Biovar/biotype- special biochemical/physiological properties
E.g., Rhizobium leguminosarum biovar (bv.) viceae
o Pathovar/pathotype- pathogenic properties
o Serovar/serotype- antigentic properties
o Phagovar/phage type- ability to be lysed by certain phage
o Morphovar/morphotype- special morphological features
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