FIG. 1.

Formation of stable atypical small colonies by GAS serotype M2 and M49. GAS serotypes M2 and M49 were incubated at 37°C in 5% CO₂ for a number of days in THY broth without addition of fresh medium. At various days after exponential growth, aliquots were withdrawn, plated on THY agar, and incubated at 37°C in 5% CO₂. The column indicated by “strain on THY” is representative of the colony dimorphism that was obtained 1 day after exponential growth and continued throughout stationary-phase incubation. To check for formation of stable atypical small isolates, representative colonies from the resulting THY plate were transferred to MH-SBA and incubated under identical conditions. The passage number is the number of times the small-colony isolates from the THY plates were sequentially passaged on MH-SBA. The results are representative of those obtained with at least three independently derived stabilized small colonies.