Thermo Scientific™ M17 Agar Oxoid™

Lactic streptococci are nutritionally fastidious and require complex media for optimal growth^2,3. Their homofermentative acid-producing nature require that the medium is well buffered so that the culture pH is maintained above 5.7 during active growth. This maintenance of the pH is important as lower pH can results in injury and reduced recovery of lactic streptococci.

M17 Agar contains di-sodium glycerophosphate which has sufficient buffering capacity to maintain the pH above 5.7 of actively growing cultures even after 24 hours at 30°C. This buffering agent also allows the addition of calcium without a precipitation complex being formed. The calcium-containing medium is used for the assay of bacteriophages of lactic streptococci¹

Shankar and Davies⁴ reported that M17 Agar was suitable for the isolation and enumeration of Streptococcus thermophilus from yogurt as the high concentration of di-sodium-glycerophosphate resulted in suppression of Lactobacillus bulgaricus. M17 Agar has been recommended^5,6 by the International Dairy Federation for the selective enumeration of Streptococcus thermophilus from yogurt.

M17 Agar is also suitable for growing and maintaining starter cultures for cheese and yogurt manufacture as it has little deleterious effect on their subsequent acid-producing ability in milk at either 30°C or 22°C ¹.

One further useful property of this agar is its ability to detect streptococcal mutants which are unable to ferment lactose¹. These mutant Lac- strains form much smaller colonies than the parent lactose fermenting strain.

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