M17 medium is a widely used culture medium for lactic streptococci. When it was first reported by Terzaghi & Sandine (1975), one of the reference media used for comparison was T5 medium (Thomas et al., 1974). T5 medium contains phosphate salts and shows buffering capacity equivalent to that of M17 medium, making it an excellent choice for lactic streptococci that produce large amounts of lactic acid. However, T5 medium was found to form a precipitate, and the authors concluded that the clear M17 medium was superior. Fifty years later, M17 medium has become the standard.
When I read the description that T5 medium produces a precipitate, I thought, “Hmm, is that so?”—and decided to prepare it myself. The resulting medium was perfectly clear, with no precipitate whatsoever. Then it clicked: perhaps they had used tap water rather than distilled water. I have heard that tap water in many countries outside Japan is hard water, which contains high concentrations of dissolved minerals. Phosphate ions tend to form precipitates with calcium ions, yet the composition of T5 medium includes no calcium —leading me to hypothesize that the water itself was hard.
So I prepared T5 medium using Contrex (total hardness: 1,468 mg/L; Ca: 468 mg/L; Mg: 74.5 mg/L), a mineral water well known for being extremely hard. And the result?
The medium became turbid with a white precipitate.
From this, I concluded that T5 medium prepared with distilled water is clear and practical. It remains possible that the phytone and yeast extract used in T5 medium available at the time contained higher levels of minerals than today’s highly purified reagents—meaning that even when distilled water was used back then, the medium may still have precipitated. In other words, the absence of precipitation I observed may simply reflect the improved purity of modern laboratory reagents.
In any case, the key takeaway is this: when prepared under modern laboratory conditions with distilled water, T5 medium is clear.
