lecture 3: bacterial differentiation

what are the 4 fundamental motivations for differentiating

1. physiological specialisation

2. stress survival

3. cell dispersal

4. symbiotic relationship

the vegetative phase (physiological specialisation: myxococcus)

(abundant food) the bacteria move together in large groups using social gliding motility, hunting microbes by secreting digestive enzymes

the starvation response (physiological specialisation: myxococcus)

when food runs out, individual bacteria sacrifice themselves for the survival of the colony. the population differentiates into 3 specialised cell types:

• autolysed cells (80%): undergoes apoptosis to provide nutrients for the remaining colony

• myxospores (15%): these form inside the fruiting bodies and stay dormant until food returns

• periphal rods (5%): these remain outside of the fruiting bodies, trying to find new food sources

germination (physiological specialisation: myxococcus)

once conditions improve, the spores within the fruiting body germinates and turn back into the vegetative swarms

stress survival: bacillus subtilis

can form highly resistant endospores that can survive extremely harsh conditions

what are the components of a spore

• the core

• original cell wall

• 2 membranes

• highly cross-linked protein coats

• cortex

• peptidoglycan

the cortex:

• very thick layer of modified peptidoglycan

• every 2nd N-acetylmuramic acid is converted to muramic delta lactan

• contains fewer peptide stems for a more flexible structure

• chemically distinct so it can be degraded without affecting the original cell wall

protein coat:

• multiple 2D layers forming laminate

• frequent disulfide crosslinking creates a sold sheet of proteins

core:

• contains 1 copy of the genome

• metabolically inert

• low water index to decrease the probability of enzymatic reactions

• dehydrated state maintained by dipicolinic acid, replacing water

• small acid soluble proteins (SASPs) coat the DNA and protect from UV and heat damage

regulation of sporulation

• phosphorylation of spoOA triggers a compartment specific sigma factor cascade

• spoOA + sigH + sigA → assymetric septation and expression of sigma F in the forespore compartment and pro-sigma E in the mother cell

• sigma F directs early stages of spore formation and engulfment required for sigma E maturation and expression of sigma E

• when engulfment of the forespore is complete, sigma E starts the cortex and coat and expression of pro-sigma K

• sigma G sequestered by SpoIII AB anti-sigma factor until relieved. When active, it leads to the maturation of the core activation of pro-sigma K

• sigma K finishes spore maturation and releases the spore

germinant

an environmental signal that occurs when conditions are suitable for vegetative cell survival and growth

what germinant receptors does B. subtilis have

GerA, GerB and GerK

regulation of germination

• once a receptor is triggered, Ca-DPA is pumped out and replaced with water

• enzymes called lytic hydrolases chew up the cortex, allowing the cell insde to expand

• the cell can start repairing its DNA/ proteins

• the newly formed vegetative cell hatches out of the remaining spore coat

how long does germination take

less than 90 minutes