An investigation of the effects of four variables on the growth of Salmonella typhimurium using two types of gradient gel plates.

The effect of four variables (pH, temperature, sodium chloride concentration and sodium nitrite concentration) on the growth of Salmonella typhimurium CRA663 was investigated using a two-dimensional gradient gel technique. Two methods were used. In the first method the gradients comprised NaCl and pH and in the second method a temperature gradient incubator was used to produce a temperature-pH gradient. Using image analysis, the growth on the plates was depicted as three-dimensional wire frame graphs. At neutral pH and in conditions of low salt, growth was observed over the temperature gradient range of 14-41 degrees C. The optimum growth range was reduced to 21-29 degrees C in conditions of acid pH and/or increased NaCl concentration. The growth on the temperature-pH gradient plates had an irregular surface appearance suggesting that changes in growth rate were occurring at different points of temperature and pH. The effects of increased salt concentration together with acidic pH increased the inhibitory effect of the sodium nitrite. The gradient gel plate technique may be a means of rapidly screening the effect of multiple variables on the growth on microorganisms that may be found in food.