Bacteria: Structure , Growth, Taxonomy Flashcards Preview

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Explain what bacteria are and a few features

- Unicellular free living microorganisms (in soil, water, hot springs)
- Commensals- one organism derives food or benefits from another microorganism without hurting it
- Many are host-dependent


What kingdom were bacteria originally placed in and how has this changed?

- Placed in Protista (meaning single- celled organisms) to be distinguished from animals and plants
- Yet different to single- celled eukaryotes so placed into kingdom monera (meaning single/ solitary) and protists in separate kingdom
- This changed to prokaryotes (unpaired chromosome and no nucleus) and eukaryotes (paired chromosome and nuclear structure)
- Prokaryotes split into archaebacteria and eubacteria (differences in rRNA)
- Eukaryotes split into protozoa and algae/ slime mould


Which bacteria are medically relevant?



Outline features of typical bacterial cell

Genome- single circular DNA molecules
- Well organised/ structured organelle- associated with proteins, circular and high level of packing despite no nuclear membrane (NOT just floating around in cytoplasm)
Ribosomes- site of protein synthesis
- different sedimentation coefficient (terminal velocity) and smaller than eukaryotes
Cell membrane- Lipid bilayer- hydrophobic
- Proteins for uptake and secretion of small diffusible molecules
Cell wall- mycoplasmas are the exception- only bacteria that don't have a cell wall
Outer membrane- another hydrophobic lipid bilayer- differentiates gram +ve and gram -ve bacteria (except for subtle differences in cell wall)
Appendages- e,g, pili from pilling protein- important in adhesion to epithelial cells
- Gonococcus- adhere using pili to urogenital tract
E.g.2 Flagella- move to more advantageous environment for nutrients- anchored to the cell membrane as need energy (ATP) to be motile (move) from ETC


Compare following features of prokaryote VS eukaryote protist
Nucleic acids
Structured nucleus
Cell membrane
Rigid cell wall
Mesosomes- what is this?

Nucleic acids- P, E
Structured nucleus- E
Mitochondria- E
Prokaryotes get energy from ETC
Ribosomes- P, E
80 VS 70 S a
Cell membrane- P, E
Rigid cell wall- P
Mesosomes- P
(artefact or invagination of inner membrane important for cell division- only visible under EM- fixing and dehydration could lead to artefact)
ER/gogli- E


Outline taxanomic ranks

Phylum/ Division (same body plan e.g. backbone),
Class (same general traits e.g. no. legs),
Order (additional info e.g. herbivores or carnivores),
Family (closely related genera),
Genus (closely related species),
Species (some variation, essentially same)
Remember: King Prawn Curry Or Fat Greasy Sausages?

No of taxonomic units increase from Kingdom--> species - but no. of organisms occupying taxonomic units decrease until we get to species level where 1 organism occupying this entity

Medical microbiology: Can also have subspecies/ isolates- same bacteria but may have different genes e.g. 1 producing toxin another not


Bacterial naming/ nomenclature- how does this work

Genus + species


Why is bacterial taxonomy important (5)

1) handling information- facilitates storage and retrieval of info through databases
2) Learning- facilitates learning and understanding of large amounts of complex info on diverse org
3) Commmunication- communication about bacteria more accurate
4) Identification- identification of unknown bacteria possible
5) Evolution- provides possible guide to evolution of bacteria allowing epidemiological data to be understood (e.g. in outbreak- where start/ come form)


Classification of bacteria by Phenotypic characteristics- what needs to be considered

- Morphology- macroscopic- on agar plate what do individual cells look like?
microscopic- under microscope?
- Biotyping- biochemical tests e.g. sugar fermentation (different bacteria ferment different sugars) and indole test (characteristic of E . coli as indole produced from deamination of aa tryptophan to tryptophanase and kovacs reagent goes bright red in presence of indole)
- Serotyping- differences in antigenic determinants on outer cell membrane- e.g, different types of salmonella
- Antibiogram properties- Different resistances and susceptibilities to different antibiotics
- Pyocin/phage typing:
- Pyocins are bacteriocins- antibiotic proteins produced by bacteria that have activity against other bacteria
- Bacteriophages- viruses that bind to bacterial cells and take over their cell metabolism
Different resistances to these pyocins and phages useful in typing different bacteria


Classification of bacteria by genotypic characteristics- what needs to be considered?

- G+C ratios: Different organisms have different ratios of G+C/A+T
- Campylobacter- food poisoning causes gastroenteritis (inflammation of stomach and intestine)- low G+C ratio (30%)
- Pseudomonas aeruginosa - high G+C/A+T (60%)
- cant use this as an identifying factor past genus level

- DNA hybridisation: Isolating genome, breaking 2 strands and probing with single-stranded small probes specific to certain species of bacteria-
- if probe binds indicative of type of bacteria dealing with

- Nucleic acid sequence analysis- e.g. MLST- multi local sequence typing- used to different subspecies of bacterium
- Looking at housekeeping genes- needed for growth and sustainability of organism
- Mutations within these genes within isolates (subspecies!) of the same species
- Use type mutations to put them in different regions of database

- Chromosomal DNA fragment Analysis: Extracting DNA, enzymically breaking this (cutting with restriction enzymes) and running it on agarose gel (polymer) with electrophoresis- checking for differences in banding pattern

- Ribotyping: looking at ribosomal RNA- within these genes are hypervariable regions which are different
- Differences within these can identify bacteria across different species and genus divides
- Clostridium difficile (infection of large intestine- colon- rare) has very good ribotyping system to recognise different isolates


Within macroscopic morphology what is it important to look for?

- Texture (smooth or rough)
- Appearance- glistening or dull
- Pigmentation- non-pigmented (white, cream, tan, colourless) or pigmented (purple, red, yellow)
- Optical density (opaque/ translucent- indicative against dark background)


Explain appearances of some colony morphology

- E. coli- translucent whereas Staph. aureus opaque when grown on agar
- On blood agar can have a-haemolysis (incomplete breakdown of RBC- hence green colour) or b-haemolysis (complete breakdown of RBC)
e.g. Str. pnuemoniae (a-haemolysis) and resistant to optochin (antibiotic) VS Str. pyogenes (b haemolysis) and resistant to bacitracin


Explain microscopic morphology (morphology of single cells)

- Rod/bacilli
- Club (thicker rod)
- Coccus (round)
- Curved (vibrio)
- Spirilllum
- Spirochaete

- Individual
- Grouped

- difficult to differentiate (microbiologist)

- Staining characteristics
- Gram stain- identify true bacteria
- Acid fast stain- mycobacteria have different cell wall so cant use gram stain, use acid fast stain


What is gram staining used for

- First line of diagnosis- shows shape and size and good for empirical antibiotic treatment
- Differentiates based on cell wall
- Gram +ve= thick cell wall
- Gram -ve= thin cell wall


Explain steps in gram staining

1) Fix sample to slide so not removed when adding all solutions
2) Add crystal violet- dye which stains all bacteria purple colour
3) Gram's iodine- acts as mordant- fixes dye into cell wall
4) Add decolouriser- type of alcohol or acetone- removes crystal violet from thin gram-ve wall but cant be removed from thicker gram +ve cell wall so remain purple
5) Add counter stain- so gram -ve take up red safranin red stain
As gram +ve not lost original crystal violet purple stain- retain this colour


Cell envelope characteristics for gram +ve and -ve
Cell wall
Outer membrane
LPS (endotoxin)
Teichoic acid

Cell wall +ve thicker
Outer membrane -ve
LPS (endotoxin) -ve
Teichoic acid +ve
Sporulation some strains +ve produce endospores to survive in adverse conditions
Capsule sometimes +ve, sometimes -ve
Lysozyme sensitive in +ve
Penecillin more susceptible in +ve as they have cell wall exposed (no outer membrane)

capsule/ LPS (-ve)/ teichoic acid (+ve) act as virulence factors


What are virulence factors

Virulence Factors- factors produced by bacteria, viruses, fungi and protozoa that add to their effectiveness and allow them to:
- Colonisation of niche in host
- Immunoevasion- evasion of host's immune system
- Immunosuppression- inhibition of host's immune system
- Exit and entry to cells (if pathogen is an intracellular one)
Obtain nutrition from host


How do capsules act as a virulence factor

Capsules- mainly gelatinous material
- May contain polysaccharides or polypeptides
- Surrounds organism
- Lack antigenic determinants so act as a virulence factor as protect bacterial cell from immune system
Present in gram +ve/-ve


Explain spores in greater detail

- allow survival of gram +ve in adverse conditions
- Contain complete copy of DNA and some proteins
- Produced inside cell (endospores)
- Spores are oval/ spherical
-Terminal/ central bulging or non-bulging
Tetnus bacteria- characteristic drum like appearance


Explore other appendages (pili and flagella) in further detail

- cell mortality
- 1(arranged at polar side) or perichorus 20 (random arrangement)
- coiled in structure
- Anchored in bacterial membrane (for ATP driven motor membrane potential)
- Made up of protein flagellin
- Chemotaxis

Fimbriae (pili)
- Smaller and many +100
- peritrichous arrangement
- Not coiled ]
- Protein (pilin)
- Adherence via adhesin
Eg. E.coli in Urinary Tract Infections- cystitis (swelling of bladder caused by UTI
E,g. N. Gonorrhoeae- can cause infection of genitals, throat and eyes
- Sex pili- larger transferring genetic information between donor and recipient- important in microbial resistance (genetic info can be exchanged between different species)


How do peritrichorus flagella move

Rails all orientate towards poles for movement