Effect of starter culture on staphylococcal enterotoxin and thermonuclease production in dry sausage

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.

Staphylococcal growth and enterotoxin production in the presence of meat cultures (non LAB)

The effect of meat cultures (non lactic acid bacteria) on the growth and production of enterotoxins and thermonuclease by Staphylococcus aureus was studied. Micrococcus varians did not affect growth nor the synthesis of metabolites. Levels of enterotoxins A, B and D produced by the respective S. aureus strains were reduced by S. xylosus, S. saprophyticus and S. carnosus. The two latter species prevented production of enterotoxin C(1) and S. xylosus markedly reduced the amount produced. The three coagulase-negative staphylococci showed little inhibitory effect on the growth of S. aureus. Penicillium nalgiovense did not show inhibitory activity against the four S. aureus strains. Debaryomyces hansenii slightly inhibited growth of the enterotoxin A-producing strain, but reduced enterotoxin synthesis at 30 °C. Thermonuclease was detected whenever enterotoxins were detected though the influence of the effector organism was dependent on the test strain.

Staphylococcus aureus virulence expression is impaired by Lactococcus lactis in mixed cultures

Staphylococcus aureus is responsible for numerous food poisonings due to the production of enterotoxins by strains contaminating foodstuffs, especially dairy products. Several parameters, including interaction with antagonistic flora such as Lactococcus lactis, a lactic acid bacterium widely used in the dairy industry, can modulate S. aureus proliferation and virulence expression. We developed a dedicated S. aureus microarray to investigate the effect of L. lactis on staphylococcal gene expression in mixed cultures. This microarray was used to establish the transcriptomic profile of S. aureus in mixed cultures with L. lactis in a chemically defined medium held at a constant pH (6.6). Under these conditions, L. lactis hardly affected S. aureus growth. The expression of most genes involved in the cellular machinery, carbohydrate and nitrogen metabolism, and stress responses was only slightly modulated: a short time lag in mixed compared to pure cultures was observed. Interestingly, the induction of several virulence factors and regulators, including the agr locus, sarA, and some enterotoxins, was strongly affected. This work clearly underlines the complexity of L. lactis antagonistic potential for S. aureus and yields promising leads for investigations into nonantibiotic biocontrol of this major pathogen.

Model for the survival of Staphylococcus aureus in nongrowth environments

A model was developed to estimate the survival times of Staphylococcus aureus in nongrowth environments. A four strain mixture of S. aureus was inoculated into BHI broth that had a lactate buffer with various combinations of pH (3-7) and lactate (0-1%), NaCl (0.5-20%) and NaNO2 (0-200 ppm) and stored at different temperatures (4-42 degrees C). At appropriate times the survivors were enumerated by sampling and spreading on TSA plates. The survival curves were modeled with two forms of a logistic equation and the D values were determined. Polynomial regression equations were then calculated to predict the effect of the environmental factors on the D values. Survival times were increased with higher pH values, lower temperatures, and lower nitrite and lactate concentrations. Added salt increased survival times until the salt concentrations exceeded that of most foods.

Effect of three commercial starters on growth of Staphylococcus aureus and enterotoxins (A-D) and thermonuclease production in broth

The growth of four enterotoxigenic Staphylococcus aureus strains was partially inhibited by three commercial starters used in the meat sausage industry when grown in APT broth at 30 degrees C statically. Starter SP318 (a mixture of selected strains of Lactobacillus sake, Pediococcus pentosaceus and Staphylococcus xylosus) showed the most inhibitory activity. Staphylococcal enterotoxins (A, B, C1 and D) synthesis was totally inhibited by the growth of the three starters, whereas staphylococcal thermonuclease production was partially inhibited in mixed cultures.

Growth of Staphylococcus aureus and synthesis of enterotoxin during ripening of experimental Manchego-type cheese

To study the possible presence of staphylococcal enterotoxins in Manchego-type cheese, milk was inoculated with the enterotoxigenic Staphylococcus aureus collection strains FRI-100, S6, FRI-137, and FRI-472 to a final concentration of 10,000 to 25,000 cfu/ml. Cheese was prepared following the industrial specifications and ripened for 60 d. Batches were prepared with 1 and .1% lactic acid culture and labeled with the abbreviated name of the strain and the concentration of lactic acid culture. Mean staphylococcal counts in .1% lactic bacteria cheeses were usually more than 1 log higher than the corresponding 1% ones. Staphylococcal counts declined markedly after d 35 to 42, and, by the end of ripening, they had disappeared from some cheeses. Enterotoxins were present in five of the cheeses, three prepared with .1% and two with 1% lactic bacteria. Enterotoxins detected were A and D, the enterotoxins most commonly associated with human intoxication. The maximum level of enterotoxin A detected in cheese with strain FRI-100 and with the .1% culture was 222 ng/100 g of cheese; in cheese FRI-100 with 1%, 111 ng/100 g; in cheese S6 with .1%, 769 ng/100 g; and in cheese S6 with 1%, 33 ng/100 g. Maximum level of enterotoxin D detected in cheese FRI-472 with .1% was 38 ng/100 g.

Staphylococcus aureus and Escherichia coli in nham (Thai-style fermented pork sausage)

The fate of Staphylococcus aureus and Escherichia coli was determined when they were introduced into ground pork made into nham (Thai-style fermented pork sausage) with or without 0.75 or 1.5% added starter culture. Without starter culture, the numbers of E. coli remained little changed but there was slow multiplication of S. aureus. With 0.75% starter culture, S. aureus was no longer detectable after 48 h and E. coli numbers decreased by 1 log after 96 h. No viable S. aureus or E. coli were recovered after 36 h and 96 h, respectively, when 1.5% starter culture was added. The addition of a starter culture is recommended when making nham.

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.

Microbiological safety of traditional and starter-mediated processes for the manufacture of Italian dry sausage

Microbiological changes occurring during the commercial manufacture of Italian dry sausages (Genoa and salametti) were studied in two urban Canadian centres over a 5 month period. A comparison was made between 6 plants which used bacterial starter cultures and 4 plants where more traditional processes (without starters) were used. A total of 600 samples of raw, fermented and finished products were tested for the presence of coliforms, salmonellae, staphylococci, streptococci, the rate of pH reduction and final water activity (aW). Numbers of total bacteria peaked earlier and were significantly higher in sausages at the fermentation stage produced with starter cultures than in those traditionally manufactured. This corresponded with a more rapid drop in pH of the starter-inoculated products. Staphylococci and streptococci were significantly higher in starter-fermented Genoa sausages at the fermentation stage, but no significant differences were seen in the microbiological content or aW of mature finished sausages manufactured by the two different techniques. Of 128 randomly chosen isolates of coagulase-positive staphylococci, 34.4% were enterotoxin producers and 80% of these produced type A toxin. Enterotoxigenic staphylococci were found in 2 different samples of finished salametti and one sample of finished Genoa made with starter cultures and in one sample of finished Genoa made without added culture. Total numbers of staphylococci in these samples were not greater than 500/g. No correlation between the method of manufacture and presence of enterotoxigenic staphylococci could be made. Five subsamples from one lot of raw Genoa were the only samples positive for Salmonella during this study. Results indicated that low temperature traditional fermentations can yield products which are as safe as those produced by the higher temperature starter-controlled process. One of the most important elements in the traditional process was believed to be the selection and use of raw materials of the highest possible quality.

Effect of growth of a commercial starter culture on growth of Staphylococcus aureus and thermonuclease and enterotoxins (C1 and C2) production in broth cultures

Staphylococcus aureus strains FRI 137 (enterotoxin C1 producer) and FRI 361 and L 2 (enterotoxin C2 producers) were grown alone and in the presence of a mixed commercial starter culture (Streptococcus lactis, Streptococcus cremoris and Streptococcus lactis subsp. diacetylactis). Lactic acid bacteria had a slight inhibitory effect on S. aureus population and only during the late stages of growth. In contrast, enterotoxin synthesis was strongly inhibited, inhibition at 18 h being 89% (FRI 137), 80% (FRI 361) and 69% (L 2). Enterotoxin C1 was produced and accumulated during all phases of growth in pure and mixed culture, but associative growth resulted in reduction of enterotoxin C2 after 24-36 h. In mixed culture, high producers of thermonuclease (FRI 137 and L 2) showed an early decrease in enzyme activity followed by an increase, but it never reached levels attained in pure culture. Thermonuclease was detected whenever enterotoxin was detected, but production curves did not parallel each other.

Growth and enterotoxin A production by Staphylococcus aureus S6 in Manchego type cheese

Milk (from cow, goat and sheep) was inoculated with Staphylococcus aureus strain S6, which is generally considered to be a strong enterotoxin B producer and a weak enterotoxin A producer. It was then used to make Manchego type cheese as prepared industrially. Two concentrations of starter culture (1% and 0.1%) were tested. Staphylococcal growth was good in both but better in the more dilute culture. Staphylococcal enterotoxin B was not detected at any stage of the ripening process of any cheese tested. However enterotoxin A was detected in both starter concentrations, reaching as high as 769 ng/100 g of cheese in the 0.1% starter batches.

Rapid detection of staphylococcal thermonuclease on casings of naturally contaminated fermented sausages

Staphylococcal food poisoning associated with fermented sausages has been a recurring problem. By testing for thermonuclease by direct application of sausage casing disks on the surface of thermonuclease assay agar plates, possible Staphylococcus aureus growth in fermented sausages could be detected simply and rapidly. Koupal-Deibel deoxyribonucleic acid agar was somewhat superior to toluidine blue deoxyribonucleic acid agar for thermonuclease assay of fermented sausage casings. The sensitivity of the thermonuclease casing test was comparable to that of the extraction procedure, and the thermonuclease casing test results were in complete agreement with the thermonuclease assay results by the extraction procedure. The thermonuclease casing test offers government and industry laboratories a useful screening tool which could significantly reduce the problem of staphylococcal enterotoxins in fermented sausages.

Rapid qualitative method for detecting staphylococcal nuclease in foods

A rapid method for the detection of heat-stable staphylococcal nuclease in foods is described. The procedure consists of an acid precipitation, boiling, and centrifugation followed by enzyme detection in an agar plate containing deoxyribonucleic acid. To test the efficacy of the procedure, purified Staphylococcus aureus nuclease was added to various foods and recovery experiments were performed. Additionally, foods were inoculated and incubated with S. aureus, and the staphylococcal counts were compared with nuclease activity. The results indicate that the procedure possesses merit for a rapid method that can be incorporated into quality control programs. The procedure requires approximately 2.5 h, and it will detect nuclease levels as low as 10 ng/g of food.

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.

The identification of staphylococci in clinical and food microbiology laboratories

A comprehensive review of the methods which have been utilized for the identification of staphylococci is presented. Biochemical characteristics which have assisted in the primary isolation of staphylococci, such as pigmentation, hemolytic activity, the egg yolk phenomenon, and deoxyribonuclease and coagulase production, are also analyzed. The potential applicability of advanced techniques to identify staphylococci, such as the detection of enterotoxin production, base ratio analysis, cell wall analysis, phage typing, and serology, is discussed. The following procedures are recommended for routine use: Idnetification of Staphylococcus sp. (clinical laboratories): microscopic observation, catalase activity, coagulase production, lysostaphin sensitivity, and (optional) facultative growth in thioglycolate medium. Identification of Staphylococcus aureus (food laboratories): microscopic observation, catalase activity, coagulase production, thermonuclease production, and (optional) lysostaphin sensitivity.