Effects of penicillin on macromolecular synthesis and surface growth of a tolerant streptococcus as studied by computer reconstruction methods.

Strains of Streptococcus mutans are very susceptible to growth inhibition by benzylpenicillin, but are tolerant to lysis when exposed to even high concentrations of this drug. These properties enabled this study of S. mutans GS-5 surface growth and peptidoglycan, ribonucleic acid, protein, and deoxyribonucleic acid syntheses in the absence of osmotic stabilization. Inhibition of syntheses of peptidoglycan, ribonucleic acid, and protein was dose dependent. Synthesis of peptidoglycan was most susceptible. Substantial but less severe inhibitions of ribonucleic acid and protein syntheses rapidly followed decreased peptidoglycan synthesis, whereas inhibition of deoxyribonucleic acid synthesis was delayed and minimal. Computer-assisted reconstructions of surface growth zones and poles observed in electron micrographs of replicas were performed and indicated that at low concentrations of benzylpenicillin (0.03 micrograms/ml), growth sites reached abnormally large sizes and surface/volume ratios. The observed shifts in surface/volume ratio were attributed to an inhibition of the normal constrictive division mechanism. The poles of these cells also increased in size over those of the controls, but the relatively smaller change in surface/volume ratio confirmed the visual impression that the shape of the poles was much less altered than the shape of the growth sites. As the concentration of benzylpenicillin used was raised from 0.03 to 2 micrograms/ml, the ability of growth sites and poles to enlarge was restricted in a manner that most closely agreed with the extent of inhibition of peptidoglycan (rather than deoxyribonucleic acid, ribonucleic acid, or protein) synthesis. This correlation suggested that increases in cell size may be regulated by the supply of peptidoglycan precursors.