Myxobacteria, life cycle, genetic basis of differentiation

Neha Sharma
Punjab Agricultural University

1. MYXOBACTERIA, LIFE CYCLE, GENETIC BASIS OF DIFFERENTIATION Presented By: Neha Sharma Punjab Agricultural University

2. INTRODUCTION Transitions from unicellular to multicellular life- myxobacteria , the most sophisticated transition into multicellularity . One of the largest genome (i.e., 9-10 Mbp long) among the prokaryotes. Sorangium cellulosum   has the largest bacterial genome about 13.0 Mbps . Many of the rich organic odours that permeate the forest air are generated by myxobacteria . This group includes Myxococcus xanthus , Stigmatella aurantiaca , Nannocystis exedens , Corallococcus coralloides , Chondromyces apiculatus , and Archangium sp.

3. Special features Myxobacteria - only gliding bacteria , having the ability to form a multicellular structure (fruiting body) which are resistant to desiccation. are of various colours , shapes and sizes. In nature, have capacity to degrade whole organisms and complex macromolecules ( chemoorganotrophs ). Aerobic, Gram-negative, inhabit topsoil, mainly in the pH range from 5-8, spore-formers, elongated rods, nonflagellated . Functions- Group independent gliding, group-dependent gliding, starvation-induced developmental cycle.

4. Taxonomy

5. Why these??

6. LIFE CYCLE Nutrient sufficient conditions - Vegetative cycle Nutrient starvation conditions- Stationary Developmental cycle CONDITIONS- When available nutrients decline When increase in high cell density When solid surface available

7. VEGETATIVE CYCLE As single cells or small groups of cells growth, cell division occurs during this phase High cell density due to cell-to-cell interactions Generation times ~ 3.5 hours. Form multicellular swarms Show gliding type of pattern Produce a broad spectrum of antibiotics, to prey on other bacteria for nutrient source (in nature) During predation, they tend to couple (wolf-pack effect) excrete a wide assortment of hydrolytic enzymes, allowing them to grow on a variety of macromolecules Have A- and S- motility

8. DEVELOPMENTAL CYCLE Cell-signaling mechanisms, cell-to-cell interactions, cell aggregation As a result, cell aggregation ( aggregation centres ) with extensive loss of viable cells due to lysis ( developmental autolysis ). The surviving cells go on to form myxospores and fruiting bodies. Favourable conditions- metabolically active, vegetative cells formed. Starvation conditions- multicellular fruiting bodies

9. Genetics of Myxococcus development

10. 1. AGGREGATION With nutrient limitation, solid surface and at a high cell density- developmental cycle initiated, leads to aggregation. INTRACELLULAR SIGNAL- As primary source is N source, depletion of any amino acid triggers stringent response. Signal- (p) ppGpp accumulation ( relA dependent) 2 additional factors- socE & csg A socE - depletes in starved cells by stringent response, leads to the induction of developmental cycle, - ve regulator of developmental cycle csgA – is C-signal, activated 5-folds, with the inhibition of socE product. it is needed for growth arrest and directional motility

11. ii. INTERCELLULAR SIGNAL- A-SIGNAL: bacteria secreted proteases, produced mixture of amino acids and peptides. 3 genes ( asgA , B, C ) required for generation of A-signal. B-SIGNAL: function not identified yet, bsgA gene. C-SIGNAL: coded by csgA gene, + vely regulated by ppGpp . D-SIGNAL: encodes by dsgA , homologue of translation intiation factor of E.coli . E-SIGNAL: esgA , esgB gene products, play a role in generating the E-signal from fatty acids in the environment. All these signals require high cell densities for cell-to-cell interactions. 1. AGGREGATION

12. 2. SIGNAL TRANSDUCTION SYSTEM

13. Myxobacteria actively grow over a solid surface to receive nutrients (swarming), to the edge of the swarm, where there are few cells and is less competition for nutrients. In center - cells compete for nutrient, oxygen, i.e. why, form flat spreading swarms on agar and able to eliminate their wastes readily. Swarms of M. xanthus are large compared with colonies, relatively thin, and resemble an egg fried. They use both polar type IV pili (S motility) and polar slime secretion (A-motility) for their swarming movements. 3. CELL MOVEMENT- PILI

14. Polar engines- Permanent parts of the cell At the poles of each cell, 100s of nozzles secretes polysaccharide slime These nozzles reveal their structural and functional similarity to the group 1 & 4 capsular slime secreting pores of E. coli Slime secretion has been specifically associated with A motility, and not with S motility A-motility

15. 2. Focal Adhesions- Are transient parts of the cell They are periodically distributed along the line of cell contact with the substrate (agar) They first appear in the front half of the cell; then migrate toward the lagging pole as the cell moves forward. Others reported that AglZ is a cytoplasmic protein with a regulatory function rather than a cell surface protein with motor function A-motility

16. S-motility

17. Locomotion patterns by two engines Hodgkin found roughly equal number of mutants that lacked either engine A or engine S, and distinguished them by their different swarm patterns. A - S + cells- clusters in stubby peninsulas and in rafts of 20–50 cells, leading to name this pattern S, for social. A + S - cells- individual cells & clusters A - S - cells - unable to swarm or form fruiting bodies.

18. Elasticotaxis A + S + cells swarm outwards rapidly from the edge of a colony, moving equally in all direction when swarming on stretched or compressed agar, the swarm disc becomes asymmetrically elongated, this effect is called elasticotaxis . A engine is responsible for this asymmetry because elasticotaxis does not occur in an A– mutant It has also been shown that elasticotaxis is diminished in a weak A– mutant, in proportion to the decrease in the swarming rate

19. Elasticotaxis helps myxobacteria locate bacterial colonies on which to feed. When a Myxococcus swarm passes near the prey, a branch of the swarm is extended directly towards it Myxococcus also extends a swarm branch directly towards a colony-sized plastic bead that contains no organic material and found that an A– mutant failed to attack a colony or a bead, whereas an S– mutant retained the ability to attack Most likely, a colony of prey bacteria (or a bead) produces elastic stress in the gelatinous substratum on which the colony (or bead) rests, with stress lines radial to the colony Myxococcus cells orient and move in the direction of these stress lines, so heading directly towards the prey colony

20. Filamentous structures, 50 μm long, composed of polysaccharides and integral fibril proteins (IFPs) Important for cell-to-cell cohesion & characteristic social behavior associated with the developmental cycle. By mutational study- strains lacking fibrils, dsp mutants, were defective in several social functions . Fibrils are proposed to function in the normal cell-to-cell positioning required for efficient intercellular signaling. Part of this function appears to lie in their ability to allow individual cells to perceive adjacent cells and analyze cell density. 4. CELL-TO-CELL COHESION; BY FIBRILS

21. Unanswered questions?? Related engine synthesis and its relation to cell division? What is the sugar sequence of the slime polysaccharide and are they identical as they proposed? Is Tgl the target of any effector ? Does A- signalling directly activates the operon or it activates via any cascade module? Many more…….?