Studies on the growth and survival of Staphylococcus aureus in corned beef

Study on the Growth and Enterotoxin Production by Staphylococcus aureus in Canned Meat before Retorting

Possible contamination by Staphylococcus aureus of the production environment and of the meat of a canned meat production factory was analysed. A total of 108 samples were taken from nine critical control points, 13 of them were positive for S. aureus. None of the isolates produced enterotoxins. To determine how much time can elapse between can seaming and sterilisation in the autoclave without any risk of enterotoxin production by S. aureus, the growth and enterotoxin production of three enterotoxin A producing strains of S. aureus (one ATCC strain and two field strains) in canned meat before sterilisation was investigated at three different temperatures (37, 20 and 10 °C). Two types of meat were used, one with and one without sodium nitrite. In the canned products, the spiked bacteria spread throughout the meat and reached high levels. Enterotoxin production was shown to start 10 hours after incubation at 37 °C and after 48 h after incubation at 20 °C; the production of enterotoxin was always detected in the transition between the exponential and the stationary growth phase. At 10 °C, the enterotoxin was never detected. The statistical analysis of the data showed that the difference between the two different types of meat was not statistically significant (p value > 0.05). Since it is well known that following heat treatment, staphylococcal enterotoxins, although still active (in in vivo assays), can be undetectable (loss of serological recognition) depending on the food matrix and pH, it is quite difficult to foresee the impact of heat treatment on enterotoxin activity. Therefore, although the bacteria are eliminated, the toxins may remain and cause food poisoning. The significance of the results of this study towards implementing good manufacturing practices and hazard analysis critical control points in a canned meat factory are discussed with reference to the management of pre-retorting steps after seaming.

Roles of Staphylococcus aureus Mnh1 and Mnh2 Antiporters in Salt Tolerance, Alkali Tolerance, and Pathogenesis

Staphylococcus aureus has three types of cation/proton antiporters. The type 3 family includes two multisubunit Na⁺/H⁺ (Mnh) antiporters, Mnh1 and Mnh2. These antiporters are clusters of seven hydrophobic membrane-bound protein subunits. Mnh antiporters play important roles in maintaining cytoplasmic pH in prokaryotes, enabling their survival under extreme environmental stress. In this study, we investigated the physiological roles and catalytic properties of Mnh1 and Mnh2 in S. aureus. Both Mnh1 and Mnh2 were cloned separately into a pGEM3Z+ vector in the antiporter-deficient KNabc Escherichia coli strain. The catalytic properties of the antiporters were measured in everted (inside out) vesicles. The Mnh1 antiporter exhibited a significant exchange of Na⁺/H⁺ cations at pH 7.5. Mnh2 showed a significant exchange of both Na⁺/H⁺ and K⁺/H⁺ cations, especially at pH 8.5. Under elevated salt conditions, deletion of the mnhA1 gene resulted in a significant reduction in the growth rate of S. aureus in the range of pH 7.5 to 9. Deletion of mnhA2 had similar effects but mainly in the range of pH 8.5 to 9.5. Double deletion of mnhA1 and mnhA2 led to a severe reduction in the S. aureus growth rate mainly at pH values above 8.5. The effects of functional losses of both antiporters in S. aureus were also assessed via their support of virulence in a mouse in vivo infection model. Deletion of the mnhA1 gene led to a major loss of S. aureus virulence in mice, while deletion of mnh2 led to no change in virulence.

IMPORTANCE This study focuses on the catalytic properties and physiological roles of Mnh1 and Mnh2 cation/proton antiporters in S. aureus and their contributions under different stress conditions. The Mnh1 antiporter was found to have catalytic activity for Na⁺/H⁺ antiport, and it plays a significant role in maintaining halotolerance at pH 7.5 while the Mnh2 antiporter has catalytic antiporter activities for Na⁺/H⁺ and K⁺/H⁺ that have roles in both osmotolerance and halotolerance in S. aureus. Study of S. aureus with a single deletion of either mnhA1 or mnhA2 was assessed in an infection model of mice. The result shows that mnhA1, but not mnhA2, plays a major role in S. aureus virulence.

Staphylococcal food poisoning in the United Kingdom, 1969-90

Between 1969 and 1990 strains of Staphylococcus aureus from 359 outbreaks and sporadic cases of staphylococcal food poisoning in the United Kingdom were examined in the PHLS Food Hygiene Laboratory for the production of enterotoxin. In a number of instances the incriminated foods were also examined for the presence of enterotoxin. Strains from 79% of incidents produced enterotoxin A alone or together with another enterotoxin. The level of S. aureus present in the foods ranged from no viable S. aureus detected to 1.5 x 10(10) c.f.u./g with a median of 3.0 x 10(7) c.f.u./g. Enterotoxin was detected in foods in the absence of viable S. aureus in only two outbreaks and in both cheese was the implicated food. Meat, poultry or their products were the vehicle in 75% of incidents with ham and chicken most frequently implicated. Other foods included fish and shellfish (7%) and milk and milk products (8%). Most contamination took place in the home followed by restaurants and shops. Seventy-one percent of the incident strains were lysed by phages of group III or I/III.