Complex media derived from plant and animal materials normally contain a considerable concentration of inorganic phosphate.

Complex media derived from plant and animal materials normally contain a considerable concentration of inorganic phosphate.

changes when external control of the pH is not being used.

In specific processes the concentration of certain minerals may be very critical. Some secondary metabolic processes have a lower tolerance range to inorganic phosphate than vegetative growth. This phosphate should be sufficiently low as to be assimilated by the end of trophophase. In 1950, Garner et al. suggested that an important function of calcium salts in fermention media was to precipitate excess inorganic phosphates, and suggested that the calcium indirectly improved the yield of streptomycin. The inorganic phosphate concentration also influences production of bacitracins, citric acid (surface culture), ergot, monomycin, novobiocin, oxytetracycline, polyenes, ristomycin, ri-famycin Y, streptomycin, vancomycin and viomycin (Sensi and Thieman, 1967; Demain, 1968; Liu et al, 1970; Mertz and Doolin, 1973; Weinberg, 1974). However, pyrrolnitrin (Arima et al., 1965), bicyclomycin (Miyoshi et al., 1972), thiopeptin (Miyairi et al, 1970) and methylenomycin (Hobbs et al., 1992) are produced in a medium containing a high concentration of phosphate. Two monomycin antibiotics are selectively produced by Streptomyces jamaicensis when the phosphate is 0.1 mM or 0.4 mM (Hall and Hassall, 1970). Phosphate regulation has also been discussed by Weinberg (1974), Aharonowitz and Demain (1977), Martin and Demain (1980), Iwai and Omura (1982) and Demain and Piret (1991).

In a recent review of antibiotic biosynthesis, Liras et al. (1990) recognized target enzymes which were (a) repressed by phosphate, (b) inhibited by phosphate, or (c) repression of an enzyme occurs but phosphate repression is not clearly proved. A phosphate control sequence has also been isolated and characterized from the phosphate regulated promoter that controls biosynthesis of candicidin.

Weinberg (1970) has reviewed the nine trace ele metal phosphates. Gaunt et al. (1984) demonstrated that when the medium of Mandels and Weber (1969) was autoclaved, a white precipitate of metal ions formed, containing all the iron and most of the calcium, manganese and zinc present in the medium.

The problem of insoluble metal phosphate(s) may be eliminated by incorporating low concentrations of chelating agents such as ethylene diamine tetraacetic acid (EDTA), citric acid, polyphosphates, etc., into the medium. These chelating agents preferentially form complexes with the metal ions in a medium. The metal ions then may be gradually utilized by the microorganism (Hughes and Poole, 1991). Gaunt et al. (1984) were able to show that the precipitate was eliminated from Mandel and Weber's medium by the addition of EDTA at 25 mg dm~3. It is important to check that a chelating agent does not cause inhibition of growth of the micro-organism which is being cultured.

In many media, particularly those commonly used in large scale processes, there may not be a need to add a chelating agent as complex ingredients such as yeast extracts or proteose peptones will complex with metal ions and ensure gradual release of them during growth (Ramamoorthy and Kushner, 1975).

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