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Chapter 3

Biological Sciences 55-237 Chapter 3: Microbio Lecture 3

Biological Sciences
Course Code
BIOL 2070

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Microbio Lecture 3
Certain bacteira can secrete slimy stuff to move along a surface
Surface of bacteria can be used for gliding motility
Some types of bacteria move by twitching motility
To twitch, they need to have a type of pilus that can be extended or retracted
End of pilus will attach to surface, and they retract it, which pulls them forward
When you get a group of bacteria that can twitch, you see this sort of stop motion
that occurs
Gram stain
Developed in 1882 by someone named Gram
Modified a bit since then
Allows us to differentiate most bacteria into broad categories of Gram positive and
Gram negative
There are differences between cell walls of Gram positive and Gram negative
Gram negative is pink
Gram positive is purple
When you first have your bacteria on the slide, they are clear
o Tough to see on light microscope
o That is why we use stains or different types of microscopes
First step is application of primary stain
o Crystal violet (dark purple)
o This initial die will get into cell wall of pretty much all bacteria (at least the
ones we can stain)
Second step does not mae a visual change
o Add idione because iodine binds to CV
o When that happen,s CV and I form a complex that is larger
o Now you have a cell wall with large complexes that have the die
o Sometimes called a mordant because it helps the cells to be stained
o Keeps the die in circumstances when it might otherwise be lost
Third step uses alcohol
o Important in terms of differentiation
o We should see a difference
o When you add alcohol, different things happen with Gram-positive and
Gram-negative cells
o Peptidoglycan traps CV-I complexes
o When you add alcohol, it dries up peptidoglycan, causing it to shrink, and
traps die even better
o Grampositive cells hold on tightly to purple colour
o Gram-negative cells have a structure not present in gram-positives (outer
membrane, which is considered to be part of the cell wall)
o Glycocalyx is considered outside of cell wall and does not factor into gram-
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