Professor Dave here, I wanna tell you about bacteria.
Earlier we learned about prokaryotic cells and their components, and the simplest forms
of life on earth are single-celled prokaryotic organisms like bacteria.
Bacteria are everywhere.
They’re in the air, they’re in the water, and they’re inside you.
If you grab a handful of dirt, there are more bacteria in there than all the humans that
have ever lived.
And we didn’t even know they existed until the 19th century.
But once microscopy came about, the world of prokaryotes revealed itself.
As we examined these critters, we saw that they were much smaller and simpler than animal cells.
In terms of structure, animal cells have only a plasma membrane, while bacteria have a plasma
membrane and a cell wall, which is typically made of peptidoglycan.
This is a polymer of sugars cross-linked by short polypeptides, and this material encloses
and protects the bacterium.
Bacterial cell walls have some differences that can be differentiated with a technique
called Gram staining.
This is where a bacterial sample is stained with various dyes and then rinsed with alcohol,
and the color that remains tells us about the structure of the cell wall.
Gram-positive bacteria have cell walls with lots of peptidoglycan, so they retain more
of the violet stain and are thus easy to identify under a microscope.
Gram-negative bacteria have cell walls with less peptidoglycan, and they have another
outer membrane full of lipopolysaccharides.
These do not retain the stain very well, so they show up with more of a pink color.
These structures maintain cell shape and determine what comes in or out of the cell, and it is
the cell wall that is the target of certain antibiotics, called beta-lactams, in killing
certain pathogens.
These substances inhibit peptidoglycan cross-linking, which destroys the bacterial cell wall, so
beta-lactams kill bacterial cells without harming human cells, since these don’t have
peptidoglycan.
Other types of antibiotics target other structures, by inhibiting translation or metabolism in
bacterial cells.
Many bacteria have protruding appendages such as fimbriae, which are like large sticky hairs
that help them fasten to some substrate, as well as pili, which are larger appendages
that facilitate the transfer of DNA from one bacterium to another.
Some bacteria have flagella, which are like tails that spin around to help the cell move.
When bacteria move, this is called taxis, and when this movement is in response to a
chemical signal, it is called chemotaxis.
Chemotaxis can be positive, towards some kind of nutrient, or it can be negative, away from
something toxic or dangerous.
The result appears as though the bacterium is swimming with some kind of sentience or
awareness, but in reality it is just a series of chemical reactions that results in the
rotation of a flagellum.
The motor interacts with a proton gradient such that it and the hook turn, and the attached
filament rotates in a way that propels the cell in a particular direction.
Bacteria come in all kinds of shapes, but the most common are spherical ones called
cocci.
One of these is a coccus, two stuck together are diplococci, a chain of them makes streptococci,
and a bunch will be called staphylococci.
Rod-shaped bacteria are called bacilli, and these can exist as a lone bacillus, or a chain
of streptobacilli.
Then there are the spiral-shaped bacteria, like spirilla or spirochetes.
Regardless of the shape, they all house a certain amount of DNA in the nucleoid, an
amount that is typically much less than a eukaryotic cell.
In addition the chromosomes are usually circular rather than linear.
Beyond the singular bacterial chromosome, there are sometimes smaller circular DNA molecules
called plasmids.
These carry just a few genes, and are replicated independently of the chromosome.
There are also methods by which bacteria exchange genetic information amongst one another.
In transformation, a cell can take in a bit of DNA from its surroundings, which may then
become incorporated in its genome.
During transduction, genes can be carried by a virus from one cell to another, which
again is incorporated into the chromosome.
In conjugation, plasmids can be transmitted through pili directly from one bacterium to
another.
So there are a number of techniques at a bacterium’s disposal in terms of exchanging genetic material.
Bacteria reproduce through binary fission, which is sort like mitosis in eukaryotic cells
in that the result is two identical daughter cells with all the genetic information.
However, the process itself is much simpler.
Replication begins at a particular point on the chromosome called the origin of replication.
While this takes place, the cell elongates.
Once there are two copies of the chromosomes, the cell pinches closed in the middle, resulting
in two new daughter cells.
These two become four, then eight, and so on in exponential fashion, with each division
usually taking just a couple hours or less.
If there were no factors limiting this process, their numbers would grow to staggering levels.
But there are indeed limits to any bacterial population since each organism needs a certain
amount of nutrients.
As we know from biochemistry, sometimes errors occur during DNA replication, and this is
one cause of genetic differentiation.
Given that the probability of a mutation occurring in a particular gene is around one in ten
million, and that billions of new bacterial cells can be generated each day, several thousand
bacteria in such a culture will have a mutation in a particular gene.
This will impact the products of gene expression, which may increase or hinder the cell’s
ability to survive.
We will later see how this simple fact accounts for the incredible diversity of life on earth.
But let’s not get ahead of ourselves, first let’s continue looking at single-celled
organisms.