LIFE-CYCLES IN BACTERIA 129 



H: SUMMARY 



The type of life-cycle which is seen in its most perfect form in myxobactcria 

 is also common to most other bacterial groups. 



The resting stage, a group of microcystic cells, is transplanted upon a fresh 

 medium, and germinates to produce the vegetative culture or swarm, which 

 is the reproductive condition. When the substrate is exhausted the vegetative 

 cells undergo a sexual process to produce the resting stage, which remains in 

 that condition until again transplanted, or until the food supj ly is renewed. 



The resting stage may be a resistant spore, or may not be markedly 

 resistant, except to inanition, hi the case of myxobactcria the microcysts are 

 contained in elaborate fruiting bodies. 



Sessile bacteria, the mycelium-forming streptomyces, filamentous chlamy- 

 dobacteria and stalked caulobacteria can only be distributed by the agency 

 of free, reproductive units. Streptomyces produce aerial spores in large 

 numbers, and the aquatic chlamydobacteria and caulobacteria produce motile 

 swarm cells, which swim away and found new colonies. 



Many bacteria may produce very small gonidia from which typical 

 bacteria are regenerated, but little is known of their nature, or the circumstances 

 under which they are formed. Such gonidia are found in their most perfect 

 form in the hfe-cycles of Rliizohiiini, Azotobactcr and certain spiral bacteria. 



BACTERIAL GONIDIA 



The production of gonidia is seen in its most perfect form in the root-nodule symbiotic 

 bacterium Rhizobium. Small, spherical gonidia are released by the rupture of the cell wall in 

 specialised, large, septate mother-cells. The gonidia have single or occasionally double 

 fiagella and show appearances suggestive of conjugation. They rapidly grow up into small 

 baciUi. 



(1,2) Production of gonidia. (Acid-Giemsa, •, 3000). 



(3, 4) The same. (Tannic-acid-violet). 



(5) Electron micrograph of gonidium. (Gold-shadowed, ,< 20,000). 



(6, 7) Electron micrographs of gonidia showing appearances suggestive of conjugation. 



It will be noticed that both members of a pair have well-developed fiagella, which would not 



normally appear in cases of division, where one daughter cell would have short fiagella or none. 



(X 16,000). 



( (8) Small, motile bacterium, replacing gonidia in some strains of Rhizobium. (x 16,000). 



