162 



CONTRIBUTIONS TO OUR KNOWLEDGE OF ROIL 

 FERTILITY, xv. 



The Action of certain Micro-organisms upon the numbers 

 OF Bacteria in the Soil. 



By R. Greig-Smith, D.Sc, Macleay Bacteriologist to the 



Society. 



In making counts of soil-bacteria, it is not uncommon to find 

 colonies of Jiac. mycoides or of races of Bac, vnlgatus spreading 

 over the surface of the nutritive agar. Ver}- often it will be 

 noted that, while the majorit}^ of the colonies are covered by the 

 spreading growths, there are a few that are untouched. The 

 mycoides-colony may approach to within two, five, or ten milli- 

 metres, and then spread out and surround the colony, leaving a 

 ring of clear agar medium. It is evident that there is some 

 product secreted by these colonies which is obnoxious to the 

 spreading colony, whether it be Bac. mycoides, Bac. ridgatus, or 

 to spreading moulds such as PeniciUium or Asperyillus. 



An examination of the colonies producing this toxic effect 

 showed that the majority consisted of Actinomj^ces or Strepto- 

 thrix, as they have been called. Some of these darkened the 

 medium and were apparently Act. chromogenus. Certain of 

 these colonies were selected, and spotted upon fresh plates, in 

 the centre of which bacteria with spreading colonies were 

 planted. The white forms were found to be very toxic, while 

 the dark forms were feebly toxic to the spreading Bac. vulgatus 



The apparent toxicity of these Actinomyces- forms is interest- 

 ing when one is considering the nature of the limiting factor in 

 soils. This is considered to be protozoal by R,ussell and others, 

 but as the infection of sterilised soils with protozoa alone, does 

 not introduce the limiting factor, as has been shown by Russell, 

 by Goodey, and by the writer, there are doubts regarding the 

 validity of Russell's contention. Whatever the cause of the 



