Drug Resistance in Bacteria 13 



assumed, the usefulness of the basic assumption about pre- 

 existing mutants loses most of its point. 



Adaptation of Bacterial Cultures during the Lag Phase 

 in Media containing New Substrates or Antibacterial 

 Agents 



There is a long lag when cultures of Bad. lactis aerogenes 

 are introduced for the first time into media containing 

 D-arabinose as the sole carbon source. Baskett and Hinshel- 

 wood (1951) showed that if samples are withdrawn at intervals 

 during this lag phase and are plated on D-arabinose agar the 

 time taken by the colonies to reach the A+ size progressively 

 diminishes as the time of sojourn in the liquid medium 

 increases. Similar results have been reported for Bad. coli 

 mutabile and lactose (Dean and Hinshelwood, 19546). Since 

 the majority of the cells in the culture took part in the 

 adaptive response and since the response preceded the growth 

 of the culture these results were interpreted as showing that 

 the substrate induces the adaptation. An explanation based on 

 the selection of spontaneously-arising pre-adapted cells is not 

 compatible with the experimental findings. 



More recently Dean (unpublished) has investigated this 

 topic in greater detail. He used Bad. coli mutabile with 

 lactose, Escherichia coli Kl2 with D-arabinose and with dul- 

 citol and Bad. ladis aerogenes with D-arabinose. His experi- 

 ments confirmed the earlier interpretation but in addition he 

 found that the response of the cells to the environment was 

 of three types. Usually there was a considerable increase in 

 cell mass towards the end of the lag phase and this preceded 

 any division. Less frequently, however, division preceded 

 any swelling of the cells. The third type of behaviour was 

 characterized by swelling and division taking place almost 

 simultaneously. 



The experiments in which the first two types of behaviour 

 were observed showed the progressive reduction in plate lag 

 reported earlier. Those in which the third type of behaviour 



