Production of enterotoxin A in milk

Impact of vaccination during pregnancy and staphylococci concentration on the presence of Bacillus cereus in raw human milk

OBJECTIVE

This study aimed to determine whether vaccination during pregnancy, prematurity, and staphylococci concentration influenced the presence of B. cereus or staphylococcal enterotoxins (SEs) in raw human milk from healthy mothers.

STUDY DESIGN

Human milk samples were collected from 152 healthy women. B. cereus, S. aureus and coagulase-negative staphylococci (CNS) were enumerated using selective agar culture media. The detection of B. cereus spores and SEs were determined using ELISA.

RESULTS

CNS and B. cereus concentrations in milk from non-vaccinated mothers were higher than that from mothers vaccinated during pregnancy, but S. aureus did not differ. Prematurity did not affect B. cereus or staphylococci in human milk. S. aureus and CNS concentrations in human milk with the presence of B. cereus were higher than that with the absence of B. cereus. Viable B. cereus was present in 9.2% of raw human milk samples whereas SEs were not detected in any samples.

CONCLUSIONS

Vaccination during pregnancy and low concentration of staphylococci could reduce the risk of B. cereus in raw human milk. The screening of B. cereus in raw human milk must be performed before pasteurization to reduce the risk of B. cereus infection in preterm infants.

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.

The formation of Staphylococcus aureus enterotoxin in food environments and advances in risk assessment

The recent finding that the formation of staphylococcal enterotoxins in food is very different from that in cultures of pure Staphylococcus aureus sheds new light on, and brings into question, traditional microbial risk assessment methods based on planktonic liquid cultures. In fact, most bacteria in food appear to be associated with surfaces or tissues in various ways, and interaction with other bacteria through molecular signaling is prevalent. Nowadays it is well established that there are significant differences in the behavior of bacteria in the planktonic state and immobilized bacteria found in multicellular communities. Thus, in order to improve the production of high-quality, microbiologically safe food for human consumption, in situ data on enterotoxin formation in food environments are required to complement existing knowledge on the growth and survivability of S. aureus. This review focuses on enterotoxigenic S. aureus and describes recent findings related to enterotoxin formation in food environments, and ways in which risk assessment can take into account virulence behavior. An improved understanding of how environmental factors affect the expression of enterotoxins in foods will enable us to formulate new strategies for improved food safety.

Detection of staphylococcal enterotoxin A, B, and C in milk by an ELISA procedure

Enterotoxigenic reference strains of Staphylococcus aureus were cultivated in sterile whole and skim milk for 18 h at 37°G. Staphylococcal enterotoxin A, B, and C were detected directly in the milk by an enzyme linked immunosorbent assay (ELISA), sensitive down to 1 ng/ml. Enterotoxins in the range of 1 ng–20 µg/ml milk were detected without any concentration or extraction. Skim and whole milk were almost identical as medium for enterotoxin production.

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.

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

Different amounts of enterotoxin A-, B-, and C1-producing staphylococci were added to dry sausage prepared by normal processes, either alone or in conjunction with a starter culture (micrococci and lactobacilli). The sausage was examined after 0, 3, 7, 14, and 30 days for staphylococci, micrococci, and lactobacilli, and measurements were made of water activity, pH, enterotoxin, and thermostable nuclease. The results showed that in the absence of starter culture measurable amounts of enterotoxin A were formed in a 200-g sample of dry sausage in 3 days, the level of Staphylococcus aureus infection being over 10(6) cells/g. Enterotoxin B was not found, although the total number of staphylococci was over 10(8) cells/g. Enterotoxin C1 was observed when the Staphylococcus count was about 8 X 10(7) cells/g, but was no longer detectable after 7 days. The starter culture prevented the production of enterotoxin A in all cases investigated. By contrast, a very high-level inoculation of an enterotoxin C1-producing strain gave a positive result after 3 days of incubation even in the presence of a starter culture. Heat-stable nuclease was observed in all sausages to which enterotoxin-producing staphylococci were added. The cell count determined in a sample of sausage had no definite correlation with the thermonuclease activity of the sample.

Effect of selected lactic acid bacteria on growth of Staphylococcus aureus and production of enterotoxin

Representative strains of 15 species of lactic acid bacteria were examined for their ability to influence growth of Staphylococcus aureus and production of enterotoxin in associative culture. Among the organisms used as effectors the streptococci were most inhibitory, followed by Pediococcus cerevisiae. The lactobacilli and Leuconostoc citrovorum were not inhibitory to growth and only slightly inhibitory to production of enterotoxin. Enterotoxin was detected in all cultures in which the population of S. aureus reached 8 x 10(7) per ml. At lower S. aureus populations no enterotoxin was detected after incubation for 48 h. Mechanisms of inhibition of growth and enterotoxin production by S. aureus strain 243 grown in association with Streptococcus lactis A64 or P. cerevisiae 10791 in APT broth were investigated. Competition for vital nutrients, especially niacin and biotin, and probably production of hydrogen peroxide contribute to inhibition. Production of lactic acid appears to inhibit growth of S. aureus in the early but not the late stages of incubation.

Effect of pH and oxygen tension on staphylococcal growth and enterotoxin formation in fermented sausage

Commercial fermented 0sausages that contained significant numbers of viable coagulase-positive staphylococci were found to have the growth localized in the outermost areas of the sausage where oxygen tension was highest. Staphylococci were found to be more acid-tolerant aerobically than anaerobically. With chemical acidulation of sausage, growth could be controlled both aerobically and anaerobically with approximately 1.5% glucono delta lactone. Biological acidulation with a high inoculum of Pediococcus cerevisiae inhibited anaerobic staphylococcal growth but failed to suppress aerobic growth completely. A staphylococcal count of approximately 4 x 10(7) cells/g of sausage appeared to be necessary to produce detectable enterotoxin A within 24 hr in sausage. A minor difference existed in the relative rates of production of the different types of enterotoxin. Detectable enterotoxin A was produced in 24 hr in sausage held in atmospheres containing 10, 15, and 20% oxygen. In an atmosphere containing 5% oxygen, toxin was detected after 48 hr of incubation. No toxin was detected after 120 hr under anaerobic conditions. Most staphylococcal strains tested initiated growth and produced detectable enterotoxin aerobically at a pH of 5.1 in broth media. Anaerobically, however, most strains failed to produce detectable enterotoxin below pH 5.7.

Factors affecting the secretion of staphylococcal enterotoxin A

The biosynthesis of enterotoxin A by replicating and nonreplicating cells was investigated. Unlike enterotoxin B, a secondary metabolite, enterotoxin A secretion resembled that of a primary metabolite by being secreted during the exponential phase of growth. The amount of toxin produced per unit of growth was not influenced by NaCl, NaNO(2), or NaNO(3). Several surfactants increased toxin secretion. Toxin secretion by nonreplicating cells was inhibited by chloramphenicol and 2, 4-dinitrophenol but not by streptomycin or penicillin G. The optimal pH for enterotoxin A production was 6.5 to 7.0. The findings suggest a number of possible reasons for the higher incidence of food poisonings caused by enterotoxin A as compared to enterotoxin B.