ABSTRACT
The effect of starter culture and chemical acidulation on the growth and enterotoxigenesis of Staphylococcus aureus strain S-6 in Italian dry salami under commercial manufacturing conditions was studied. The experimental design included two levels of S. aureus (10 and 10/g), three levels of starter culture (0, 10, and 10/g), three levels of initial pH (pH(0)) (6.1, 5.5, and 4.8), two manufacturing plants, and three replications. S. aureus growth in the salami was affected significantly (P < 0.005) by pH(0), initial levels of S. aureus (staph(0)) and lactic acid bacteria (LAB(0)), day of fermentation, and by the interactions of pH(0) x day, pH(0) x LAB(0), LAB(0) x staph(0), pH(0) x staph(0), and pH(0) x location of fermentation. In general, the lower the pH(0) and the higher the LAB(0), the greater the inhibition of S. aureus. The LAB levels during the fermentation were affected significantly (P < 0.005) by pH(0), LAB(0), day of fermentation, location, LAB(0) x pH(0), and LAB(0) x day. Derived regression equations related level of S. aureus and LAB at any day of fermentation to a number of microbiological and chemical variables. Close similarity of observed and predicted levels of S. aureus and LAB growth demonstrated the usefulness of the experimental approach in evaluating the safety of a process. No detectable enterotoxin or thermonuclease was found at any stage of processing even when S. aureus reached levels of 10/g of salami.
ABSTRACT
Three strains of Staphylococcus aureus (S-6, 137 and 472) were inoculated, in duplicate, into Italian-style dry salami made with finished product as starter and processed under commercial manufacturing conditions. Five levels of S. aureus ranging from 2.2 × 102 - 1.8 × 107 cells/g were used. A fourth strain (264) was inoculated at a level of 105 cells/g. All strains of S. aureus grew at every level of inoculation, but the amount of growth was dependent on inoculum size. Strains S-6 and 472 increased in number by 1.2 - 2.9 logs (mean 2.14) at inoculum levels of 2.3 × 102 - 2.5 × 103 cells/g, and by 2.1 - 3.2 logs (mean 2.66) at inoculum levels of 3.7 × 104 - 6.6 × 105 cells/g. Strain 137 was very sensitive to salami environment and only increased by 0.47 - 1.86 logs (mean 1.23) even at the greatest inoculum level. Strain 264 increased in numbers by 1.5 logs in the presence of 5 × 105 inoculated lactobacilli/g and by 2.5 logs in the presence of 6 × 104 naturally occurring lactic acid bacteria. Staphylococci occurring naturally in salami mix were unable to grow to levels greater than 2 × 104 cells at any time during processing of experimental sausages. Thermonuclease was detected only in salamis inoculated with strains S-6 and 472 at initial levels of greater than 3.7 × 104 cells/g and only when growth reached levels greater than 107 cells/g. No enterotoxin was detected in any of the inoculated samples. Development of regression equations allowed description of the growth of inoculated S. aureus in the salami during manufacturing as affected by a number of variables.
ABSTRACT
In optimizing previously reported coagulase agar media to obtain a rapid, reliable, and inexpensive coagulase test agar, variations in plasmas, pH, buffer system, fibrinogen, and fibrinolytic inhibitor were investigated. The agar with the following composition was determined best for the demonstration of coagulase production by Staphylococcus aureus: 25 ml of 15% bovine fibrinogen (fraction I, type I, citrated, Sigma Chemical Co.), 25 ml of rehydrated rabbit plasma (coagulase plasma ethylenediaminetetraacetic acid, Difco), 10.0 mg of soybean trypsin inhibitor (Schwarz/Mann), and 450 ml of brain heart infusion agar (Difco). In additional studies involving 7 different temperatures and 11 heating times, the thermal destruction of microbial nucleases on plate count agar and coagulase test agar was investigated. Heating the plates for 2.5 h at 65 degrees C destroyed all heatlabile nucleases, but not thermonucleases of S. aureus. A tandem agar plate method for the identification of S. aureus was developed. Coagulase and thermonuclease activity of 50 colonies can be detected on a single agar plate. Suspect S. aureus colonies isolated on various selective media are transferred to coagulase test agar, the plates are incubated at 37 degrees C for 18 h, and the coagulase reaction is recorded. The plates are then heated at 65 degrees C for 2.5 h, overlaid with toluidine blue-metachromatic diffusion agar, and reincubated at 37 degrees C for 3 h, and the thermonuclease reaction is recorded. Studies based on 88 enterotoxigenic S. aureus strains and 133 and 48 suspect S. aureus strains isolated from fresh salami mixtures on mannitol salt and tellurite-polymyxin-egg yolk agars, respectively, demonstrated 100% agreement between the tandem agar plate method and standard coagulase and thermonuclease tests. Overall, the tandem agar plate method is a rapid and convenient approach contributing to the identification of S. aureus from foods.
