This 1969 study examined the growth kinetics of plaque-forming streptococci, particularly in the presence of sucrose. The researchers used two dextran-forming caries-conducive strains (SL-I and E-49) and two non-dextran-forming strains (4M4 and JR8LG). Intact plaques of strain SL-I grew linearly when cultured on glass rods, with growth rates of approximately 6.8 μg dry weight per square millimeter per day, based on both dry weight and DNA measurements. In broth culture, dextran-forming strains exhibited linear growth with sucrose but exponential growth with other sugars like glucose and fructose. Non-dextran-forming strains showed exponential growth regardless of sugar type. When dextranase was added to sucrose-containing cultures, growth became rapid and exponential. The data demonstrate that extracellular dextran synthesized by these organisms in response to sucrose restricts bacterial growth, likely by creating a diffusion barrier around cellular clumps rather than individual cells. This finding contradicts earlier assumptions about rapid plaque growth in the oral environment and suggests in vivo plaque growth is slower than previously estimated.

Key findings

• Intact plaques of dextran-forming streptococci grew linearly rather than exponentially, requiring approximately 1.9 days for dry mass to double
• Dextran-forming strains (SL-I and E-49) exhibited linear growth with sucrose but exponential growth with other sugars; non-dextran-forming strains grew exponentially with all sugars tested
• Extracellular dextran formation specifically restricts growth of dextran-forming streptococci, evidenced by rapid exponential growth when dextranase was added to break down the dextran barrier
• Growth restriction by dextran appears to operate at the microcolony level, creating a diffusion barrier around cell clumps rather than individual cells