Control of microbial surface-growth by density

IN synchronous cultures of bacteria, the rate of cell elongation ¹ or of envelope synthesis² seems to increase abruptly at a particular age. These and other findings suggest^3-5 that bacterial envelope is formed at a constant rate, possibly by enzymes organised in zones, and that some event in the cell cycle leads to a discrete increase in the amount of enzymes or number of zones concerned. (The actual event involved has been variously identified with initiation of chromosome replication⁴, with termination^1,5 and with the attainment of a critical cell length⁶.) Growth zones are known to occur in the filamentous fungi⁷. The rate at which these zones are able to add new envelope is limited, and since mass increases exponentially⁸, hyphal density eventually begins to rise; when it reaches a critical value, new zones are formed. Thus density (or indeed the concentration of any product synthesised exponentially⁹) could control the rate of surface growth, and we suggest that such is the case in bacteria.