The Effects of Myoglobin, Nitrosylmyoglobin, and Free Iron on the Growth of Clostridium botulinum in Cured Meat

Although nitrite is a known inhibitor of Clostridium botulinum in cured meats, the mechanism of inhibition is not understood. The observation has been made that iron is required for growth of C. botulinum and that the role of nitrite may be to alter the pathway of iron uptake by these organisms. Since the color change in cured meats is due to the binding of nitrite to the heme group of meat pigments, it was hypothesized that nitrite may also be tying up an essential iron source, heme. This experiment was an investigation of the possibility that myoglobin added to a meat system would stimulate growth and toxin production by C. botulinum much more than myoglobin that had been nitrosylated before inclusion in the product. Treatments were included to compare the effects of a heme iron source, myoglobin, with that of an ionic source, ferric chloride. To help understand the role of free iron in botulinal growth, several treatments contained a metal ion chlator, ethylenediaminetetraacetic acid (EDTA). Nitrite caused a definite delay of growth, as evidenced by gas bubbles, when compared with a non-nitrite system. Addition of ferric chloride resulted in an increase in the rate of of appearance of swollen samples, although growth was enhanced even more when myoglobin was added. When nitrosylated myoglobin was included, growth was inhibited more than in the treatment with nitrite alone. EDTA inhibited growth of C. botulinum but a conclusion should not be made with respect to the chelation of iron since EDTA chelates many other metals. Residual nitrite levels had declined to below 10 ppm by the time swelling occurred. Although swelling did not occur until nitrite had declined in the products, the absence of nitrite alone did not allow growth and toxin production. Since nitrosylated myoglobin and EDTA inhibited botulinal growth even after residual nitrite had declined, it is possible that the inhibitory action of nitrite is creating a nutritional deficiency for C. botulinum