Introduction to Microbiology
Microbiology is a branch of Biology that deals with the study of the organisms that are too small to be seen by the naked eye.
Microbiology includes the study of:
Why study Microbiology?
- To gain knowledge of the method of transfer of microorganisms from one person to another to prevent the occurence and spread of disease
- To understand the importance of immunization processes and techniques to prevent diseases
- Useful in understanding the underlying principles of health procedures for maintenance of health and prevention of disease
- It makes medical and surgical procedures less dangerous because of the proper applications of the principles of disinfection and sterilization
Microorganisms are classified as either Cellular or Acellular
Differences between Prokaryotes and Eukaryotes:
2. Nucleoid (no true nucleus; (+) of nuclear material)
3. Not bound by nuclear membrane
4. Circular chromosome made up of DNA and histone-like protein, except B. borgdorferi and Streptomyces
5. Binary fission (ave. 18 mins) —> mode of reproduction
6. Cell wall is made up of peptidoglycan except Mycoplasma and Ureaplasma
7. Site of energy production is cytoplasmic membrane
8. CHON synthesis occurs in the ribosomes
9. Ribosomal size - 70s (Svedberg unit)
1. Protozoan, Fungi, Algae
2. With true nucleus
3. Bound by a nuclear membrane
4. One or more paired linear chromosomes made up of DNA and histone
5. Mitosis (ave. 18-44 hours) —-> mode of reproduction
6. Animals and protozoan lack cell wall; Fungi and algae cells are made up of chitin
7. Mitochondria is the powerhouse of the cell
8. CHON synthesis occurs in Endoplasmic Reticulum
9. Ribosomal size - 80s (Svedberg unit)
There are two types; Prokaryotes and Eukaryotes.
- Usually single celled
- Lack distinct nucleus
- Lack membrane bound organelles
- Circular chromosomes
- Plasmids (circular DNA)
- Cell wall made of nurein
- Contain ribosomes
- Kingdom: Monera
- nucleus present
- membrane bound organelles
- Vacuoles present
- Cell wall (except in animals)
- Kingdoms: Animals, plants, Fungi, Protists
Plant (not all have chloroplast):
Prokaryotic Cells, Cholera and Oral Rehydration Solutions
Prokaryotic cells (eg. bacteria) are relatively simple and small, compared to eukaryotic cells (animal cells, ect), which are complex and larger. Prokaryote literally means ‘before the nucleus’- they have no separate nucleus and show little organisation.
- unicellular bacteria
- no nucleus or membrane-bound organelles!!!
- nucleoid region: DNA floating in cytoplasm, no true nucleus or nuclear membrane
- cytoplasm: inside portion of the cell
- cytosol: fluid within the cell
- generally considered first form of life - were most likely anaerobic
- cell membrane: regulates transport; selective permeability
- cell wall: protective layer external to cell membrane
- does not contain phospholipids or transport proteins
- ribosomes: protein synthesis location
- capsule: outside of cell wall; made of carbohydrate
- pathogenic (causes disease)
Gene Expression in Prokaryotes. (AKA, the lac and trp operons)
So we all know a little bit about how genes are expressed in animal cells, but what about bacteria? Despite the fact that they are unicellular organisms, they still need to control which proteins are produced, and when they are produced. Like how we discussed earlier that it would be useless if your toes starting producing pepsinogen, bacteria need to conserve energy and it would be of no use to them whatsoever to start producing enzymes to digest lactose, when there is no lactose to digest, or there’s plenty of glucose to go round everyone!
The first example of gene expression in bacteria we’ll look a is the lac operon. Now just to start us off you should know that bacteria will only begin using lactose as an energy source when there is no glucose present (kind of like how we as humans, don’t start using fat as an energy source unless there are no carbohydrates to use), and so the enzymes which digest lactose only need to be produced in certain environmental conditions, so why bother wasting energy producing proteins which aren’t needed!
There are three gene sequences which code for the enzymes which digest lactose (β-galactosidase, β-galactoside permease and thiogalactoside transacetylase, but lets not get to caught up with that, it’s not too important to know the names right now) and they are located next to each other, and are referred to as lacZ, lacY, and lacA. When these genes are transcribed by mRNA, a single mRNA strand is produced, and this strand is referred to as polycistronic, as it codes for more than one protein. When lactose is not present, the lac repressor protein binds to the operator region (the start point for mRNA transcription) and this prevents RNA polymerase from binding to the DNA, and so prevents the genes being transcribed, and therefore the proteins being produced. However when lactose is present, lactose binds to the repressor protein, changing it’s conformation in such a way the does not allow it to bind to the operator region, and so the genes are transcribed and the proteins are produced!
The lac operon is said to be an inducible operon, as gene transcription is induced by the presence of lactose.
The trp operon works in a very similar way, except it is referred to as a repressible operon, because gene expression is repressed by the presence of high levels of the amino acid tryptophan. It works under the same principles as negative feedback in eukaryotes. “What, there’s loads of that stuff been produced!? Better stop for a bit until we get rid of some of it!”.
When low levels of tryptophan are present, the conformation of the repressor protein does not allow it to bind to the operator region, therefore the genes which code for tryptophan are transcribed as RNA polymerase can bind to the DNA. When high levels of tryptophan are present, the organism not longer needs to produce it, and two molecules of tryptophan bind to the repressor protein, and change it’s conformation in such a way which means that it can now bind to the DNA and prevent transcription.
Smart little things bacteria, eh?