Drug Resistance in Bacteria 19 



distinct secondary colony will result. Heterogeneity of colony 

 form is sometimes manifested in ways other than papilla 

 formation — colonies with crinkled edges, "rough" colonies 

 and so on. In the course of our work we have often observed 

 in ageing colonies the development of "lenticular" areas of 

 changed internal colony texture, possibly connected with 

 local lysis and re-growth. The papilla is one of quite a series 

 of departures from regular monotonous colony development. 

 (6) Several lines of experimental evidence show that 

 papillae need not arise from mutants. 



(a) When drugs, such as phenol or thymol, are added to the 

 solid medium, papillae occur in numbers which do not increase 

 with the total number of cells grown (i.e. with the chance of 

 mutation) but simply with the age and diameter of the colonies. 

 The same total growth distributed among a large number of 

 smaller colonies may be associated with no papilla formation 

 at all, even though the chance of mutation is as great, and the 

 opportunity for mutants to develop is probably greater. 



(b) Re-spreading of inocula derived from the secondary 

 colonies sometimes give colonies still showing papillae — even 

 to the sixth re-spreading. In general, re-spreading may lead 

 to fresh papilla formation or not, according to circumstances. 

 Where it does not, the disappearance is usually attributed to 

 the selection of the mutants. This conclusion is, however, 

 ambiguous. When any form of adaptive response, whether 

 by mutation or otherwise, has once occurred (as it can do 

 during the slow growth of a primary colony on an initially 

 unsuitable medium) the times of colony formation in the 

 "re-spreading" experiment are less than in the original test. 

 Thus the period elapsing between the initiation of growth 

 and the final complete exhaustion of the medium is reduced. 

 By the time any parts of the colony have aged enough for 

 papilla formation to occur, all nutrients may have been 

 removed and no secondary growths can develop. In the first 

 plating the utilization is so slow that some parts of the colony 

 age sufficiently while there is still unexhausted material for 

 growth. 



