Zero to Genetic Engineering Hero - Chapter 3 - Growing E. coli Cells 72
You may recall the reasons why lab strains of K12 E. coli
are safer to use: i) they cannot effectively share genetic
information and; ii) they do not have a phage infection.
Here is another reason: K12 E. coli are missing some
sugars in their LPS called the ‘O antigen’. The O anti-
gen is usually part of the sugar ring string on the LPS,
serving as a rst line of defense for the cell against the
environment. Not having the O antigen means that
the rst defense layer of the microfactories is severely
disrupted and makes the E. coli highly susceptible to
dying from antibiotics, chemicals, acids, surfactants,
and dehydration. It is as though the factory fence is
only 1 foot tall with very large mesh holes. It offers
some protection but not great protection. For this
reason, K12 E. coli are not good at surviving outside the
petri dish and are great to use in experiments.
The Outer Wall (B)
Have a look at your tour map (Figure 3-16). We have
passed through the fence/capsule layer, and now we’re
moving on to the next stop - the outer factory wall.
The outer factory wall has one primary function - to
be a strong barrier between the inside of the factory
and the outside environment. The factory wall should
protect the interior from rain, snow, hail, and even a
tornado. The walls of the factory are made of solid red
brick that is reinforced with iron.
The outer factory wall in our E. coli is the outer cell
membrane (Figure 3-19). The function of the cell
membrane is not only to protect the cell from the outside
environment, but it also acts as a container for all activ-
ity inside the cell. Without the cell membrane, all of the
cell insides would spill out. There would be no cell!
As a general rule, the membranes of E. coli cells are
made up of lipids (Figure 3-20). Lipids are essential
for keeping cell structure. Lipids are made up of CHO,
CHON, CHONS, or CHONP, and in general, have the
common characteristic of having a “tail group” and a
“head group” (Figure 3-20), just like the surfactants
you used for cell lysis in Chapter 1.
The “head group” segment of the membrane lipid is
hydrophilic (water-loving). This means that this is the
part of the lipid that interacts with the water environ-
ment inside or outside of the cell. The head group is
hydrophilic because it is charged, making it interact
with water, which is also charged. The structure and
composition of the “head group” can vary greatly. For
example, in Figure 3-22, a negatively charged phos-
phate molecule is present. In other lipids, the phos-
phate is replaced by a sulfate or other chemical group.
The “tail group” segment of the membrane lipid is
hydrophobic (water avoiding) (Figure 3-20). This
means that this part of the lipid prefers not to make
contact with water. It is hydrophobic because it is
uncharged and does not like interacting with charged
molecules or atoms. Because of this, it prefers to inter
-
act with the tail groups of other lipids (Figure 3-20 and
3-21). This difference in binding preferences leads to
a structure called a lipid bilayer, where the water-lov-
ing head groups face outwards into the watery envi-
ronment, and the water-avoiding tails face inwards
toward other water-avoiding tails to form a protec-
tive barrier (Figure 3-20 and 3-21). When millions of
lipids are produced by a cell, the layer creates a robust
barrier that separates the inside of the cell from the
outside environment (Figure 3-16).
During the rst point of the tour, we talked about LPS –
the chain of sugars that make up the slimy part of the
capsule layer. However, those sugars are bound to a
lipid anchor called “Lipid A”. Lipid A has a hydrophobic
tail that ts snuggly with the other “tail groups” inside
the outer lipid bilayer at the surface of the cell. This
hydrophobic tail is the anchor that holds the
entire LPS
molecule in the membrane.
Figure 3-19. Brick barrier compared to a lipid bilayer barrier.
Figure 3-20. In membranes, the head groups are charged
molecules that interact well with water and other charged
molecules. The tail groups are non-charged, and they do not
like to interact with charged groups but like interacting with
other non-charged groups. For this reason, many biological
membranes are made of lipids that form of a lipid bilayer.
Head Group
hydrophillic
hydrophobic
Tail Group
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