BMED2801 – Lecture 4 - Bacterial structure: Special features
Bacterial size – the importance of being small
• Bacterial cells are much smaller than eukaryotic cells
• Advantages of being small:
Large surface area / volume ratio
 rapid nutrient uptake & waste elimination
Large populations
 rapid evolution
Larger organisms need special systems to get oxygen + nutrients + wastes out of cells.
Due to small size, bacteria can use simple passive diffusion + active transport + selective transport –
focus energy on other things.
Prokaryote size – a few BIG exceptions
Some rare bacteria are much larger than eukaryote cells e.g.
- Epulopiscium(>0.1mm) The secret to an unusual bacterium's massive size
o Able to copy its genome tens of thousands of times. - arraying it in a kind of fabric just
under the cell membrane,
o Epulopiscium sp. may maintain its large size by keeping its DNA close to the outer
surface
o advantages: It is highly mobile and too big to eat for most protozoan predators that also
live in the surgeonfish's gut.
o Some think that the large size might correlate with its unique method of reproduction
and/or give it a selective advantage against protozoan predation.
- Thiomargarita (0.1mm-0.2mm) The nutrient-rich waters nurture the growth of phytoplankton — so
much so that the almost all the oxygen in the water gets used up. The bacteria living in the olivegreen mud therefore need to “breathe” something else.
o For Thiomargarita, chemicals known as nitrates in the water serve as their oxygen, while
hydrogen sulfide (sulfur oxidation as food source), produced by other bacteria in the
sediment, is their food.
Why are they so big?
Evasion of predators (protozoan)?, Unusual metabolism
Prokaryote shape – balls vs. strings
• Different morphologies have different surface area / volume ratios
coccus – the toughest – for survival – minimum surface area exposed to external environment –
resit descation or radiation etc.
filament – grow fastest + nutrient uptake or excrete effectively - greatest SA:V.
Prokaryote shape – be there and be square !
• Special environments or lifestyles lead to unusual shapes eg. the square Archaeon Haloquadratum
walsbyi
• Environment: Surface of hypersaline lakes (~ 4 M NaCl)
• Lifestyle: Halophilic (loves salt), Phototrophic (eats light)
• Shape & arrangement: Thin square cells in a floating mat
Complex life cycles (does not just undergo binary fission) – how shape changes over time
– the Actinomycetes
- filmentouse bacteria
• Mycelia (filaments) made during rapid growth: - lets it live in soil
- allows to grow in gaps between soil – this
gives it motility - moves via growing
between particles within the soil.
Good for nutrient uptake & for bridging gaps
between soil particles
• Exospores made during starvation / inhibitory conditions: Good for stress resistance and dispersal
- extend up into air- for dispersal via air when allowing it to move on when it has run out of food
• Some Actino’s instead alternate between rods & cocci, but similar rationale (rapid growth vs. resistance)
Complex life cycles – the Myxobacteria
• Alternate between single cells (myxospores or
vegetative cells) and organised colonies
(swarms or fruiting body)
• Abundant nutrients favour the growth of vegetative
cells
• Starvation triggers aggregation and ultimately spore
formation = fruiting body
-
at some point – stalk + spore organisation – a lot of chemicals secreted for communication for
organisation.
Internal membranes
For organisation
• The Planctomycetes use internal membranes to organise their cells into different compartments
• Some plancto’s have a membrane-bound nucleoid  break fundamental rule that bacteria don’t have
membrane bound nucleus
though to be example of convergent evolution.
For metabolism
• Plasma membrane infoldings are common in many bacteria and can become extensive and complex in
photosynthetic bacteria.
• Infoldings – may be aggregates of spherical vesicles, flattened vesicles or tubular membranes.
• They provide a larger mebran surdace for greater metabolic activity
• The cyanobacteria are photoautotrophs (light as energy source and CO2 as carbon source)
• Photosynthetic pigments (eg. chlorophyll) and electron transport proteins are membrane-bound
• How to maximise growth by photosynthesis?
 make more internal membranes !
Inclusion bodies and vacuoles
• Granules of organic or inorganic materials usu. 100-200 nm diameter, found in cytoplasmic matrix
• May be membrane-bound, but are NOT organelles
• Most often storage reserves (e.g. carbon compounds, inorganic substances, and energy) + reducing
osmotic pressure by tying up molecules in particulate form.
• some inclusion bodies lie free in cytoplasm – others enclose by a shell (singe layer + protein or
membranous structure containing protein + phospholipids)
• Useful for microscopic identification
For carbon and energy storage
• Supply of carbon in Nature is variable; microbes need a way to store excess C at ‘feast’ times for later
‘famine’
• Excess C converted to energy-rich storage materials eg:
* Glycogen = a polymer of glucose
* PHB (Polyhydroxybutyrate) = a polymer of butryate
* Lipids = long-chain fatty acids joined to glycerol
For storage of other nutrients
• Nitrogen: Some cyanobacteria contain cyanophycin inclusions: polymers of arginine and aspartate +
carboxysomes – store enzyme rubisco for CO2 fixation
• Phosphorous: Many bacteria store P as polyphosphate granules – a.k.a metachromatic granules
• Sulfur: inorganic sulfur is stored by (sulfur granules) sulfur-oxidising bacteria as a source of energy (SO
 SO42-)
For other cell functions
• Gas vacuoles: an organic inclusion body  found in some aquatic bacteria, provide buoyancy (ability to
float)  useful for photosynthesis  these are aggreagets of enournmous numver of small, hollow,
cycindical gas vesicles. – protein subunits assempple to form a rigid enclosed cylinder.
• Enyzme inclusions: eg. carboxysomes contain the photosynthetic enzyme RuBisCO
• Magnetosomes: inclusions made of magnetite (Fe3O4), allow magnetotactic bacteria to follow magnetic
fields (determining location)
Fimbriae and pili: attachment and sex
• Thin protein strands found on surface of G -ve bacteria
• Fimbriae: short/ fine hair like appendages  very thin (~ 5 nm diameter), very numerous (up to 1000 per
cell), used for attachment to surfaces
 they can aid in attachment to objects, for twitching motility + gliding motility by myxobacteria.
• Pili: larger, thicker (~10 nm diameter), less numerous (a few per cell), used for transferring DNA – ie. for
sex.
• Pili are often encoded by conjugative plasmids, which are self-transmissible between bacterial cells
Flagella
• Bacteria can be motile or non-motile. Most motile bacteria swim using flagella – threadlike protein
structures (~ 20 nm x 20 μm) extend out from cell wall
• Presence/absence of flagella, & their distribution pattern around the cell are useful for bacterial
identification e.g. monotichous bacteria – have one flagellum located at one end (polar flagellum) and
amphitrichous bacteria – single flagellum at each end.
• they are slender rigid structures – threadlike locomotor appendages extending outwards from plasma
membrane + cell wall.
3 parts
1) Flagellar filament – extends from cell
surface to tip  hollow rigid cylinder
constructed of flagellin protein subunits.
– ends with a capping protein.
2) Basal body – embedded in cell
- gram –ve – basal body has 4 rings
connected to central rod
- gram +ve – 2 basal body rings
3) Flagellar hook – short, curved segment
linking filament to basal body and acts a
flexible coupling.
Flagella: function and significance
• Flagellum filament is helical, rotates like a boat propeller to push bacterium forward
• Source of energy for the flagellum motor is the membrane proton gradient
• Advantage of flagella: allow chemotaxis = movement toward nutrients, or away from toxins  bacteria
are attracted by nutrients (sugars and amino acids) and repelled by harmful subs. and bact. waste
products + respond to environment cues e.g. temp, light, oxygeb, osmotic pressure, gravuty
• Disadvantages of flagella: - need protein to build + costs energy to run
Endospores
- special resistant domant structures – produced by gram +ve bacteria.
• Some Gram positive bacteria (eg. Bacillus, Clostridium, Sporosacrina) make endospores to survive
under stressful conditions
• Endospores are metabolically inactive, but can germinate to yield new vegetative cells if conditions are
favourable
• Endospores are extremely tough - survive harsh conditions - eg. heat, dessication, radiation, chemical
disinfectants
Endospores: long term survival !
• Endospores from a 25-million year old bee trapped in amber were germinated on agar  Bacillus
sphaericus
- spore – surrounded by thin, delicate covering = exosporium + spore coat underneath made up of
thick protein layers  makes it impermeable to many toxic molecules
- coat also thought to contain enzymes involved in germination
- cortex beneath spore coat  made up of peptidoglycan + ribosomes and a nucleoid – but
metabolically inactive
- calcium ions located in the core – aid in resistance to wet heat, oxidizing agents + dry heat
- specialize small, acid soluble DNA-binding proteins (SASPs) – saturate spore DNA and protect it
- cortex – may osmotically remove water from protoplasts – thereby protecting it from heat and
radiation damage
- + contain DNA repair enzymes
-  Spore resistance due to  calcium-dipicolinate, acid-soluble protein stablisation of DNA,
protoplast dehydration, spore coat resistance, DNA repair + greater stability of cell proteins
adapted to growth at high temps.
Endospores – clinical and other significance
• Presence, location in the cell, and effect on mother cell  ferq differes among species – making it
considerable value in identification
• Spore-forming bacteria are difficult to kill by standard methods  require autoclaving (steam under
pressure) – because of their resistance and that fact that sev. species of endospore-forming bacteria are
dangerous pathogens  they are of great practical importance in food, industrial and medical
microbiology,
• Many clinically-important bacteria are spore-formers: eg. Bacillus anthracis  anthrax
• Endospores are also of considerable theoretical interest  because bacteria manufacture these
intricate structures in an organised fashion over a period of a few hours, hence spore formation is well
suited for research on the construction of complex biological structures.
Glycocalyx, capsule, slime layer
• Glycocalyx is a layer of polysaccharides that is secreted by bacteria, and present on the outside of cell
envelope
• Capsule is a well-organised glycocalyx that is difficult to remove from the surface of cells
• Slime layer is a more diffuse glycocalyx that can be removed by washing the cells
Glycocalyx: functions and significance
• Used in attachment – esp. formation of biofilms
• Protection against dessication and other stresses
• Help bacteria to evade immune system – particularly, make it difficult for phagocytes to recognise
bacteria
Bacterial structure: summary
• Although small, prokaryotes are very complex entities
• Prokaryotes have specialised internal and external structures, each with specific functions
• Knowing the structure of prokaryotes helps us to understand their function
• By understanding the function of prokaryotes, we can better manage their behaviour – eg. in
infections