114 S. S. COHEN 



Why do the cells die? Two types of hypothesis may be offered: (1) Con- 

 tinuing cytoplasmic growth rings the nuclei in such a fashion that some stage 

 in the division process aborts, or (2) The nuclei attempt to divide in the 

 absence of an incomplete complement of DNA, producing incomplete and 

 dispersed fragments of the genetic complement. 



Recent data suggest that the latter may be more hkely, although this is 

 far from proved. Strains of 15^", carrying an additional requirement for 

 a single amino acid will die for the first hour, as does the parent, and then 

 stop, i.e., 90 % of the cells will die, although cytoplasmic growth is very 

 slight (Earner and Cohen, 1957). The lag observed in Fig. 20 may be shortened 

 by incorporation and decay of P^^ in the DNA of stram 15^", suggesting 

 that this lag describes the inactivation of multiple bacterial nuclei, of which 

 the average cells of this strain have at least two (Fuerst and Stent, 1956). 

 However, it may be noted that the DNA of "dead" cells does not become 

 acid-soluble; this substance remains sufficiently viscous to produce a self- 

 sharpening effect when extracts are examined in the analytical idtracentrifuge 

 (Cohen and Earner, unpublished data). Thus, if breaks have occurred in this 

 DNA, they have not led to an extensive depolymerization of this bacterial 

 substance. Furthermore, synchrony effected by addition of thymine at the 

 end of the presumed nuclear decay results m a doubhng of DNA (Earner and 

 Cohen, 1956). Thus, the presumably inactivated nuclei appear to be able to 

 make a normal complement of DNA. 



2. Mode of Action of Penicillin 



It has been kno^m for many years that the lethal action of penicillm on 

 microogranisms is a function of microbial growth. Many workers have 

 observed the production of spherical bodies and L forms in the presence of 

 the antibiotic. The production of similar forms durmg the production of 

 protoplasts by means of lysozyme suggested to Lederberg (1956) that the 

 bizarre cell types observed in the presence of penicillin were intermediates in 

 protoplast formation. When E. coli was grown in broth in the presence of 

 5 % sucrose and 0.1 % MgS04, the bacteria were quantitatively transformed 

 into spherical protoplasts, which lysed readily in distilled water. In the 

 absence of penicillin, about half the protoplasts will revert to rods and form 

 typical baciUary colonies; thus this system can provide an excellent experi- 

 mental system for the selective study of cell wall synthesis, as well as for tlie 

 production of uncorseted, perhaps more permeable, bacteria. 



It had been observed that staphylococci in the presence of penicillin 

 accumulated large amounts of various peptides (Park, 1951) which were 

 found to be associated with uridine nucleotides and amino sugars. The 

 isolation of the 3-0-lactyl ether of N-acetylhexosamine (Strange, 1956) and 

 of derived peptides from the cell walls of gram-positive bacteria (Strange and 



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