The article "Isolating lactic acid bacteria from milk" has been updated following queries why controls are required using M17 agar to selectively enumerate Streptococcus thermophilus in environments containing L. delbrueckii subsp. bulgaricus.  While many strains of L. delbrueckii subsp. bulgaricus are inhibited by the  β-glycerophosphate in M17, Shankar and Davies (1977) found that 23% of the strains of L. delbrueckii subsp. bulgaricus tested grew well on this medium. Hence the need for controls to confirm colonies as Sthermophilus.