Bacterial classification Flashcards
Explain the steps for gram stain. Why is each step carried out?
- Add Crystal Violet dye to a heat fixed smear.
- Add iodine - this acts as a mordant & fixes the stain.
- Add ethanol - this decolourises gram NEGATIVE bacteria.
- Add safranin - this counterstains the gram negative back
What bacteria stain gram positive and why?
stain violet due to thick layer of peptidoglycan in cell walls & no lipid membrane - retains the crystal violet.
Stains blue or purple
E.g.
- Cocci- staphylococcus aureus, streptococcus pyogenes
- Bacilli- clostridium spp
What bacteria stain gram negative & why?
Do not retain crystal violet dye as they have thin peptidoglycan layer & have outer lipid membrane
Is pink
E.g.
- Cocci- neisseria meningitidis
- Bacilli- e.coli
Describe acid-fast stain & why it’s used?
acid-fast bacilli won’t stain by gram stain
- resist decolorisation by acids
- due to thick, impermeable wax on surface- cell wall made of lipids & mycolic acid
So heat used to dissolve mycolic layer
- Acid-fast stain is used
- stains red (for TB) & blue for other
How can bacteria be classified?
Shape
Cell wall composition
Gaseous requirements- aerobic, anaerobic?
Nutritional requirements
Shape of bacteria?
Round cells- cocci
Rod shaped- bacilli
Spiral
Vibrios- comma shaped
Spirochaetes- corkscrew shaped
What are the different arrangements that can be used to classify bacteria?
Staphyl- clusters
Strept- chains
Tetrads- group of 4
Diplo- pairs
What type of cell is bacteria? Form of DNA in bacteria?
Prokaryotes:
- no membrane bound organelles
- single celled
DNA in form of:
- Nucleoid: single circular chromosomes
- Plasmids: extracellular circular DNA Molecule
How can cell wall composition be used to classify bacteria?
Gram positive bacteria- thick layer of peptideoglycan
Gram negative bacteria- thin layer of peptidoglycan & layer of lipopolysaccharide
How can gaseous requirements be used to classify bacteria?
Aerobic- requires O2
Anaerobic- doesn’t require O2
facultative anaerobes- grow in presence or absence of oxygen
Microaerophile- bacteria that only require minimum levels of 0O- optimum conditions are low O2
How can nutrition requirements be used to classify bacteria?
- Autotrophs
- Heterotrophs
NOTE: all pathogenic bacteria are heterotrophs
Name 6 important representatives
- Escherichia coli
- Helicobacter pylori
- Salmonella typhi
- Mycobacteria
- Staphylococcus aureus
- Vibrio cholera
Classification, reservoir, transmission, common infections of E.coli?
Classification: gram negative, rod-shaped, facultative anaerobic bacterium
Reservoirs/sites of colonisation/transmission: GI Tract/animals/ fecal-oral
Infections: diarrhea, urinary tract, food poisoning, sepsis
Classification, reservoir, transmission, common infections of Heliobacter pylori?
Classification: gram-negative, helical-shaped, microaerophilic
Reservoirs/Sites of Colonisation/transmission: Stomach/endogenous/fecal-oral
Infections: peptic ulcer disease, gastric ulcer
Classification, reservoir, transmission, common infections of Staphylococcus aureus?
Classification: Gram-positive, spherical, facultative anaerobe
Reservoirs/sites of colonisation/transmission: skin, nares/ endogenous/direct contact, aerosol
Infection: soft tissue, bone joint, endocarditis, food poisoning
