Physiological Mechanisms of Resistance 173 



about cell permeability was long dominated by a physico- 

 chemical approach to the kinetics and thermodynamics of 

 a passive membrane, with the cell viewed as a sort of cello- 

 phane bag filled with enzymes. It is therefore hardly surpris- 

 ing that studies based on such a naive model failed to lead to 

 much enlightenment. Indeed, it would be difficult to explain 

 with such a model how quantitative decreases in permeability 

 could lead to corresponding decreases in uptake. For with a 

 non-metabolizable drug, decreased permeability should lead 

 to a decreased rate of approach to equilibrium, but not to a 

 change in the distribution at equilibrium. 



In the past few years, however, our picture of the perme- 

 ability properties of bacteria has altered drastically. The 

 cellophane bag now possesses a variety of permeation systems, 

 each stereospecific for a structurally related group of sub- 

 strates; and the number of units of each kind per cell not 

 only affects the rate of equilibration between intracellular and 

 extracellular substrate, but also affects the value of the ratio 

 reached at equilibrium. 



This development has arisen not only from direct studies of 

 the intracellular concentration of various substances, but also 

 from studies of the phenomenon of "crypticity" — i.e. the 

 fact that certain enzymic activities can be demonstrated only 

 after disruption of the cells. It has long been suspected that 

 a permeability barrier prevented the added substrate from 

 reaching the enzyme in such cells ; but it could also be argued 

 that the enzyme might be present in the cell in an inactive or 

 latent form which became activated by the process of extrac- 

 tion. The question could be resolved if it could be proved that 

 the enzyme was active in the intact cell. 



This demonstration has now been accomplished in two cases 

 by the use of auxotrophic mutants to demonstrate that the 

 enzyme in question was essential for biosynthetic purposes, 

 and hence must be present in active form. Thus it has been 

 established by nutritional, isotopic, and enzymic methods that 

 5-dehydroquinic acid (DHQ), 5-dehydrosliikimic acid (DHS), 

 and shikimic acid (SA) are successive intermediates in the 



