Microbiology Flashcards
How can medicines become contaminated?
- During manufacture: personnel, environment, raw materials (especially water), packaging
- During use by the patient
2 super kingdoms
Prokaryotes and eukaryotes
3 domains
bacteria, archaea, eukaryotes
Difference between prokaryotes and eukaryotes
- Prokaryotes are simple in their cellular organisation. Eukaryotes are much m
- ore complex.
The defining difference is the encapsulation of chromosomal DNA in the nucleus in eukaryotes, but there are many other differences too, particularly the presence of membrane-bound organelles such as mitochondria.
- ore complex.
Viruses
- Viruses are infectious particles made of genetic material (DNA or RNA) wrapped in a protein coat - they can only reproduce by hijacking the machinery of living cells and are not considered truly alive on their own.
Prions
Prions are even simpler than viruses, consisting only of misfolded proteins that can convert normal proteins to the same misfolded state. The most well-known prion disease is BSE (bovine spongiform encephalopathy) or “mad cow disease”, which can be transmitted to humans as variant Creutzfeldt-Jakob disease (vCJD)
Another example of a prion disease
scrapie, a prion disease that affects sheep and goats, which has been known to shepherds for centuries and helped scientists understand how these unique pathogens work.
Alzheimer’s, Parkinson’s, and Huntington’s
some researchers to investigate whether they might share some mechanisms with prion diseases.
What is visible and what isn’t?
- Single bacterial cells are not visible to the naked eye, but can be viewed under a light microscope if stained appropriately.
- Colonies of bacteria are visible to the naked eye. Each colony is formed of tens of billions of bacterial cells. Experienced microbiologists can use the appearance of a bacterial colony to help identify the organism.
Bacterial shape
2 common shapes: round and elongated
Round - cocci
(can grow as single cells, or as groups of bacteria together in clusters of chains. The particular way a bacteria grows can be further used to help identify it)
Elongated - rods
There is quite a lot of variation in shape between different bacteria which are rods. For example, some bacteria have a typical rod shape (see top image), some appear to be more rounded and appear more similar to a cocci (middle image), and some are curved rods (lower image). There is huge diversity amongst bacteria and thus other shapes are also known though are less common, such as spiral and filaments. The bacteria we will consider in these lectures will be predominantly cocci and rods.
Prokaryotic cell wall
- Confer cell shape
- Prevent the cell from bursting by providing rigidity
Cell wall is also the site of action of many important classes of antibiotics such as the penicillin or glycopeptides
- Prevent the cell from bursting by providing rigidity
Gram positive bacteria
- Take up Gram stain and appear purple under the microscope.
- Have a thick layer of peptidoglycan – a complex polymer which is heavily crosslinked.
Gram negative bacteria
- Do not retain Gram stain and appear red/pink under the microscope.
- Have a thin layer of peptidoglycan which is not as tightly crosslinked as that in Gram positive bacteria.
- Possess an additional outer membrane outside of the peptidoglycan layer which is not present in Gram positive bacteria.
Lipopolysaccharide (LPS) and the Gram-negative outer membrane (OM)
- Lipopolysaccharide (LPS) is a very important feature of the outer membrane in Gram negative bacteria. It is a complex molecule which is only found in the outer leaflet of the Gram negative outer membrane.
- The presence of LPS means that the outer membrane does not behave like a typical lipid membrane which we studied in PHAY0003.
- LPS is a potent regulator of the immune response, causing fever and sometimes causing septic shock.
- LPS is a huge problem in medicine sterilisation because it is extremely heat stable and therefore very difficult to remove during sterilisation processes.
Acid fast bacilli
- Gram-positive organisms with additional long chain ‘waxy’ mycolic acids in cell wall
- E.g. TB and leprosy
- Acid-fast bacteria are Gram positive, and therefore possess a thick layer of peptidoglycan in their cell wall. However, unlike other Gram positives, acidfast species possess a large amount of mycolic acids (a lipid with very long tails, C55) embedded in the outside surface of the cell wall. This makes the cell wall of acid-fast bacteria very hydrophobic and makes it very difficult for hydrophilic compounds to enter the cell.
Bacterial growth - how
- Binary fission
- Divide as fast as environmental conditions will allow (nutrients and temp)
Growth is exponential
- Divide as fast as environmental conditions will allow (nutrients and temp)
Bacterial growth curve
Lag phase
Log phase
Stationary phase
Death phase
Lag phase
Preparing to divide but have not yet undergone any division, so cell number remains the same
Log phase / exponential phase
Bacteria double at regular intervals and no grows exponentially - continues for as long as environmental conditions allow
Stationary phase
Cell death and cell growth are evenly matched, so there is no overall growth in the number of cells
Death phase
Number of alive cells begins to decrease
Why do cells enter the stationary phase?
Bacterial cells typically stop growing exponentially either because they run out of nutrients or because they become surrounded by toxic waste products which prevent their growth.
Many genes are upregulated at the end of log phase which help bacteria to survive when they are experiencing these harsh conditions. This is important because some of these genetic changes alter the bacterial cell properties, affecting how resistant they are to antibiotics and to the human body’s immune response.
