Factors influencing the production of staphylococcal enterotoxin A in milk

The effect of protective cultures on Staphylococcus aureus growth and enterotoxin production

Staphylococcus aureus is the causative agent of staphylococcal food poisoning and is a common contaminant in milk. Despite efforts to control S. aureus, recalls and outbreaks continue to occur, highlighting the need for additional interventions. This study determined the potential for protective cultures (PC) that are commercially available to producers to control S. aureus growth in raw milk and attenuate virulence by impeding staphylococcal enterotoxin (SE) production in raw milk and laboratory medium. Cultures of Hafnia alvei and Lactococcus lactis effectively inhibited S. aureus growth in raw milk to counts ~5 log CFU/mL lower than control when cocultured following a cheesemaking time and temperature profile; two cultures of Lactobacillus plantarum inhibited growth to ~1.5 log CFU/mL less than control. Cocultures of S. aureus with Lc. lactis, H. alvei and Lb. plantarum in raw milk reduced SE levels by 24.9%, 62.4%, and 76%, respectively. Lc. lactis also decreased SE production in raw milk in the absence of PC-mediated growth inhibition. Significant reductions in SE production in the absence of pathogen growth inhibition were also achieved in laboratory medium. Together, these results demonstrate the potential for PCs to inhibit S. aureus growth and impede SE production in the absence of growth inhibition.

Use of a water-based probiotic to treat common gut pathogens

This work reports the anti-pathogenic effect of a commercially available water-based probiotic suspension, Symprove™, against three commonly encountered infectious organisms; Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA) and Shigella sonnei. An isothermal calorimetric assay was used to the monitor growth of the species individually and in binary combinations, while colony plate counting was used to enumerate viable cell numbers. It was observed that all pathogenic species were faster growing than the probiotic bacteria in Symprove™ after inoculation into growth medium yet in all instances bacterial enumeration at the end of the experiments revealed a significant reduction in the pathogen population compared with the controls. A control population between 10⁸ and 10⁹ CFU/ml was obtained for E. coli and S. sonnei whilst approximately 10⁶ CFU/ml was obtained for MRSA. Upon co-incubation for 48 h, no viable counts were obtained for E. coli; a 4-log reduction was obtained for S. sonnei whilst MRSA numbers were down to less than 10 cells/ml. The results show that Symprove™ has antipathogenic activity against E. coli, S. sonnei and MRSA.

Degradation of staphylococcal enterotoxin A by a Pseudomonas aeruginosa isolate from raw milk

Recently, we found that staphylococcal enterotoxin A (SEA)-producing Staphylococcus aureus strains produced SEA in raw milk with microbial contaminants at high temperatures like 40 °C only. Moreover, the concentration of SEA produced in raw milk gradually decreased after the peak. The reason(s) for SEA degradation in raw milk was studied in this study. Degradation of SEA spiked in raw milk was observed at 40 °C, but not at 25 °C. A Pseudomonas aeruginosa isolate from raw milk degraded SEA spiked in broth at 40 °C. A sample partially purified with a chromatographic method from culture supernatant of the isolate degraded SEA. Two main proteolytic bands were observed in the sample by zymographic analysis with casein. These results suggested that the SEA in raw milk might be degraded by a protease(s) produced by the P. aeruginosa isolate. This finding might be the first report on SEA degradation by a proteolytic enzyme(s) derived from Pseudomonas bacteria to our knowledge.

Production of Staphylococcus aureus enterotoxin a in raw milk at high temperatures

Improper processing and storage of raw milk contaminated with Staphylococcus aureus at elevated temperatures can result in the production of staphylococcal enterotoxins, especially type A (SEA), which is most frequently associated with food poisoning outbreaks such as the large one in Osaka, Japan, in 2000. In this study, the characteristics of S. aureus growth and SEA production at various high temperatures in raw milk samples were studied using two raw milk samples naturally containing low and high levels of natural microflora. The optimal temperatures found for SEA production in the two milk types were as high as 40 and 44°C (range, 36 to 48°C), and SEA production was dependent on the initial dose of S. aureus. These high temperatures were close to that of the outbreak in Japan. Thus, it was concluded that temperature was critical for SEA production in raw milk. It was also observed that natural microflora in the milk samples considerably suppressed SEA production but not staphylococcal growth. On the other hand, the amount of toxin in most milk samples decreased after peaking during storage.

Survival of Staphylococcus aureus and Salmonella enteritidis on salted sardines (Sardina pilchardus) during ripening

The ripening period for salted sardines ranges from 4 to 6 months, depending on the season. Sometimes producing industries need to distribute the product earlier owing to market needs, and when this happens the product's safety needs to be assured. The purpose of this work was to study the survival of Staphylococcus aureus and Salmonella Enteritidis on salted sardines during a ripening period of 115 days. Salted sardines were inoculated with pure cultures of S. aureus and Salmonella Enteritidis (10(5) CFU/g of fish on day 0). After 5 days of ripening, the water activity value for the sardines decreased from 0.93 to 0.69. The survival of both pathogens and that of total viable cells were evaluated during the ripening process. Total viable counts decreased by 2 log units over the 115-day ripening period. Salmonella Enteritidis and S. aureus survived for 60 and 90 days, respectively. Therefore, the use of a 90-day ripening period could be effective in assuring the safety of the final product.

Effect of six organic acids on staphylococcal growth and enterotoxin production

Four Staphylococcus aureus strains were incubated at 37 degrees C for 24 h in broth progressively acidified with lactic, citric, ascorbic, acetic, pyruvic and propionic acids, and their survival rate and enterotoxin producing ability was studied. Acids were chosen based on their frequent use by the food industry. Periodically, samples were withdrawn to determine counts, pH and the presence of enterotoxins A, B, C, and D. For a given acid, the effect on growth and enterotoxin synthesis was different. The most inhibitory acid for the growth of strains FRI-100 and FRI-472 was pyruvic acid, for strain FRI-137 was lactic acid, all six acids were equally effective on strain S6. Lactic acid was very inhibitory to enterotoxin synthesis, but the effect on this parameter of acetic and citric acids was almost nil. Enterotoxins were seen to be inactivated at acid pH values; enterotoxin B was the most resistant to inactivation.

Production of staphylococcal enterotoxins C1 and C2 and thermonuclease throughout the growth cycle

Synthesis of enterotoxins C1 and C2 and thermonuclease throughout the growth cycle was investigated with Staphylococcus aureus type strains FRI137 and FRI361 and S. aureus isolates M5 (C1) and L2 (C2) of animal origin. Both enterotoxins were produced during the exponential growth phase or at the beginning of the stationary phase. The minimal incubation time (7 to 12 h) and the lowest population (10(7) to 2 x 10(9) CFU/ml) associated with detectable enterotoxin (1 to 6.5 ng/ml) were related to the total amount of toxin produced after 24 h. Thermonuclease was detected in all samples whenever enterotoxins were detected. Furthermore, strain FRI137 produced thermonuclease earlier and at lower cell populations than it did enterotoxin C1. Patterns of enterotoxin and thermonuclease synthesis did not correlate. The concentration of toxins increased throughout the growth cycle, while the concentration of thermonuclease remained constant during the last hours of the growth cycle.

Growth and production of enterotoxin A by Staphylococcus aureus in cream

Behavior of Staphylococcus aureus strains 100-A, 196-E, 254, 473, 505, and 521 in sweet (18 to 80% milk fat) and neutralized sour cream was studied. Cream was inoculated to contain approximately 10(3) to 10(4) S. aureus/ml, depending on milk fat content, and was incubated at 4, 22, or 37 degrees C. Determinations were made of aerobic plate count, S. aureus count, and pH. When growth in cream exceeded 10(7) S. aureus/ml, enterotoxin analysis was done. Sweet and neutralized sour cream supported growth of all strains of S. aureus tested. Strains 100-A, 196-E, 473, 505, and 521 grew sufficiently to produce enterotoxin in sweet cream of 18 or 32% milk fat held at 37 degrees C for 18 h or at 22 degrees C for 52 h. Populations of strains 100-A, 196-E, 505, and 521 exceeded 10(6) cells/ml in sweet cream of 36% milk fat held for 18 h at 37 degrees C. Strains 100-A and 521 grew to more than 10(6) cells/ml in sweet cream of 40% milk fat held for 18 h at 37 degrees C. No strain of S. aureus grew to levels associated with detectable enterotoxin production at 4 degrees C within 14 d in any cream. Incubation temperature, milk fat content of cream, and variation among strains influenced the ability of S. aureus to grow and produce enterotoxin.

Growth of Staphylococcus aureus and synthesis of enterotoxins in home-made yoghurt

Staphylococcus aureus strains FRI-100, S6, FRI-137 and FRI 472 were inoculated into milk to study growth and enterotoxin production in home-made yogurts. The yogurt used as starter was progressively weakened by successive inoculations (up to four) in milk to prepare other yogurts in order to study the ability of yogurt microflora to inhibit staphylococci. After elaboration, yogurts were stored at 4 degrees C, 22 degrees C, and 37 degrees C for a maximum of 21 days. Periodically, staphylococcal counts, pH and the production of enterotoxins A, B, C, and D were determined. Enterotoxins were only detected in the last batch. It was concluded that the inhibitory effect of the starter culture is not only due to the decrease of pH, but also to other factors.

Behaviour of Staphylococcus aureus strains, producers of enterotoxins C1 or C2, during the manufacture and storage of Burgos cheese

Burgos cheese was manufactured from pasteurized ewes' milk inoculated with Staphylococcus aureus strains FRI 137 and FRI 361, at levels of ca 10(3) and 10(5) cfu/ml and stored at 4 degrees, 10 degrees and 15 degrees C and at room temperature (10 degrees-15 degrees C). Populations of Staph. aureus and mesophilic aerobes, pH, and production of thermonuclease and enterotoxins C1 and C2 were investigated. Aerobic counts increased during cheese-making and storage. With both test strains, important growth was observed only during the storage period, the larger levels corresponding to the higher temperatures. Although Staph. aureus strains attained populations of over 10(8) cfu/g, no enterotoxin was detected. Strain FRI 361 reached 10(7) cfu/g without production of a detectable amount of thermonuclease. With strain FRI 137, the minimal population associated with enzyme activity was influenced by the inoculum size. Staphylococcus aureus counts are better indicators of staphylococcal growth in Burgos cheese than the thermonuclease test.

Production of Italian Dry Salami: Effect of Starter Culture and Chemical Acidulation on Staphylococcal Growth in Salami Under Commercial Manufacturing Conditions

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 4 and 10 5 /g), three levels of starter culture (0, 10 5 , and 10 6 /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 × day, pH 0 × LAB 0 , LAB 0 × staph 0 , pH 0 × staph 0 , and pH 0 × 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 × pH 0 , and LAB 0 × 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 7 /g of salami.

Staphylococcal enterotoxin production in the presence of non-enterotoxigenic staphylococci

Enterotoxigenic Staphylococcus aureus strains were grown with a non-enterotoxigenic strain in laboratory medium, in milk, and in ham. Differences in pigmentation were used to differentiate the enterotoxigenic strains from the non-enterotoxigenic ones. Enterotoxin was detectable in milk when the colony counts of the non-enterotoxigenic strain were 15 to 20 times greater than those of the enterotoxigenic ones and in ham when the ratio was 60 to 77:1. Enterotoxin was detectable in milk when the enterotoxigenic strains reached counts of 10(7) colony-forming units per ml and in ham when the counts reached 10(8) colony-forming units per ml. It may be necessary in some food poisoning outbreaks to examine many isolates (up to 50 or 60) for enterotoxin production to be able to detect the enterotoxigenic staphylococci.

Staphylococcal enterotoxin and thermonuclease production during induced bovine mastitis and the clinical reaction of enterotoxin in udders

Enterotoxin A- and C-producing strains of Staphylococcus aureus and partially and extensively purified enterotoxin A were inoculated into the udder quarters of cows. In the course of experimentally induced mastitis caused by the inoculated S. aureus strain, enterotoxin C but not A was detected in the infected udder. Enterotoxin C was observed in mastitic milk samples at very low S. aureus population levels (10(2) to 10(3) colony-forming units per ml). The results suggest that either the synthesis of enterotoxin C is stimulated in vitro or that growth of S. aureus cells in udders was, in fact, higher than the colony-forming unit values indicated. Thermonuclease was shown to be excreted into mastitic milk at a slower rate than was enterotoxin. An inoculation of 1 microgram of enterotoxin A in autogenic milk returned to the udder caused clinical reactions (swelling, palpation sensitivity, and increase in the level of somatic cells) within 6 h.