Introduction to microbiology Flashcards
why should medical students study microbiology?
The human body contains about 1014 cells.
• 10% of these are human
• Most of the rest are normal flora (“commensals”)
• Micro-organisms may cause disease if:
- Introduction of pathogen to susceptible host
- Displacement of normal flora from one part of body to another
what are phylogenetic trees?
pic
- Phylogenetic trees are built by genetic similarities
* Bacteria, archaea, eukaryote
give the definitions of eukaryotic species, prokaryotic species and viral species
- Eukaryotic species:
- A group of closely related organisms that breed among themselves
- Prokaryotic species:
- A population of cells with similar characteristics
- Clone: Population of cells derived from a single cell
- Strain: Genetically different cells within a clone
- Viral species:
- Population of viruses with similar characteristics that occupies a particular ecological niche
what are key functions of some bacteria?
- SOIL and Plant Fertilising
- Food digestion
- Commensalism
Further good?
- Bacteria have long been used by humans to create food products such as cheese, yoghurt, pickles, soy sauce and vinegar.
- We are also able to use bacteria to break down our sewage and to clean up oil spills.
LOOK AT PASTEUR GERM THEORY EXPERIMENTATION
Tools of the trade
• Compound microscopes :1700s Robert Hooke developed instruments with magnifications of 3-500x.
• Introduction of staining procedures: fine visualisation of microorganisms.
• By 1877. Robert Koch was using methylene blue to stain bacteria. Two years later Hans Christian Gram introduced his famous stain.
DESCRIBE THE THEROY BEHIND THE GRAM STAIN
Gram stain
There are two types of gram bacteria: Gram positive bacteria and gram-negative bacteria.
How does it work?
There are three steps involved with gram staining: staining with crystal violet, decolourisation and counterstaining.
1. Cells are stained with crystal violet. Iodine and potassium iodine are added to form a complex between the crystal violet and the iodine. The complex formed is much larger than both crystal violet and iodine on its own and is insoluble.
2. Acetone is then added to the sample. As it dehydrates the peptidoglycan layer, it begins to shrink and get tighter. The crystal violet- iodine complex is so big that is unable to move and is trapped in the gram-positive bacteria. When acetone is added to the gram-negative bacteria, the solvent dissolves the lipids, which combined with the thinner cell walls, washed out the crystal violet stain.
3. A counterstain, safranin dye, is added to the sample staining it red. Safranin dye is lighter than crystal violet, so it doesn’t show up if the crystal violet stain is present. If crystal violet is not present, for example in gram negative bacteria, it will stain red (Bruckner, 2016).
Summary Gram positive The cell wall of the gram-positive bacteria is a thick layer of peptidoglycan. The peptidoglycan traps crystal violet, masking the safranin dye – which is red. - Single thick peptidoglycan layer - Thick cell walls with more lipids - Stains purple with crystal violet
Gram negative:
The cell wall of the gram-negative bacteria consists of the outer membrane and a thin layer of peptidoglycan. Crystal violet is easily rinsed away, revealing the red safranin dye.
(lipopolysaccharide doesn’t allow the dye to infiltrate)
- Thinner peptidoglycan but complex lipid ‘outer membrane’
- Thin cell walls with more lipids
- Stains red with safranin dye
Give an overview on gram positive bacteria (Examples)
- Round cells are called cocci (sing. coccus) e.g. staphylococcus aureus
- rod-shaped cells are bacilli (bacillus) e.g. Clostridium spp
Give an overview on gram negative bacteria (Examples)
Cocci e.g. neisseria meningitidis
bacilli e.g. E.Coli
what is acid-fast stain?
- shows that not all living pathogenic organisms are going to be stained by gram
- AFB stain will be able to detect mycobacterium–> as they have a mycolic acid layer (waxy layer)
Very thick layer–> isn’t stained by gram! Thus, need ACB
- Apply primary stain of carbolfuschin (which is pink) for 30 seconds
- Heat fix cells to the slide using the flame
- Decolorise with acid alcohol for 15-20 seconds
- apply counterstain of methylene blue for 30 seconds then rinse excess stain
acid fast organism is stained pink and non acid fast organism is stained blue
what are the other ways to classify bacteria
classification by phenotype e.g. look at how the bacteria reacts around enzymes or antibiotics. Bacteria from the same family will often have very similar reaction to a particular enzyme or antibiotic. Through trial and error, large books have been compiled stating each type of bacteria and how it reacts to a certain stress. If a new bacterium is found, it can be placed under the same conditions to see which family of bacteria it could belong too.
classification by genotype:
- G + C ratio
- DNA hybridisation
- DNA fragment analysis
- whole genome sequencing
(look at week 1 pbl notes -LO4)
Classification by analytical methods:
- whole cell lipids
- cell wall
- whole cell protein (proteomics)
- spectroscopy
- western blottinh=g
describe mycoses-fungi
picture
- Contain spores
* Also have hyphae
describe parasites (picture)
- Almost animal like
- Hook and sucker in the mucosa
what are the living characteristics of viruses?
- They reproduce at a fantastic rate but only in living host cells.
- they can mutate thus can feel the evolutionary pressures and respond to them.
what are the non-living characteristics of viruses
- they are acellular–> they contain no cytoplasm or cellular organelles.
- they carry out no metabolism on their own and must replicate using the host cell’s metabolic machinery . In other words. viruses don’t grow and divide. Instead, new viral components are synthesised and assembled within the infected host cell.
- The vast majoirty of viruses possess either DNA or RNA but not both
Describe prions
- prions are protein-containing particles with no detectable nucleic acid.
- “slow” infectious diseases
- Highly resistant infectious agent
- no inflammation or immune response in affected
