Physiological Mechanisms of Resistance 177 



with respect to the ability of a resting cell suspension to form 

 pantothenate from its precursors, p-alanine and pantoic acid. 

 With intact cells the mutant showed less inhibition of this 

 reaction by D-serine than did the wild type. However, after 

 treatment with toluene to destroy permeability barriers the 

 two cell suspensions were equally susceptible to the inhibition. 

 Furthermore, in growing cultures, studies with radioactive 



Table I 



Inhibition by d-serine of pantothenate production by resting cell 

 SUSPENSIONS OF Esch. coU 



D-serine showed considerably less uptake by the mutant 

 strain than by the wild type (Table II). Finally, inhibition 

 of pantothenate synthesis required about 5 (xg./ml. with 

 growing sensitive cells, 25 [xg./ml. with growing resistant 

 cells, and 1000 (jig./ml. with toluenized cells. These results 

 provide strong evidence that a decrease in permeability to 

 D-serine is responsible for resistance in the mutant studied. 



Decreased permeability also appears to be responsible for 

 chloramphenicol resistance in Pseudomonas, since intact 

 resistant cells are less susceptible than the sensitive parental 



