HEAT-STABLE NUCLEASE FOR ASSESSMENT OF STAPHYLOCOCCAL GROWTH AND LIKELY PRESENCE OF ENTEROTOXINS IN FOODS

Helichrysum italicum extract interferes with the production of enterotoxins by Staphylococcus aureus

AIMS

The purpose of this work was to evaluate the effect of Helichrysum italicum extract on enterotoxin (A-D) production by Staphylococcus aureus strains.

METHODS AND RESULTS

The production of enterotoxins A-D in the presence or absence of H.italicum diethyl ether extract was estimated in microtiter plates using a reversed passive latex agglutination (SET-RPLA) kit (Oxoid, Basingstoke, UK). The results indicate that, in culture medium, inhibition of staphylococcal growth and enterotoxins appeared with 250-125 microg ml(-1) of the extract. Lower concentrations of the extract (62.5-31.25 microg ml(-1)) did not affect the final viable count of Staph. aureus but reduced the production of enterotoxins B and C.

CONCLUSIONS

H. italicum interferes with growth and production of enterotoxins by Staph. aureus.

SIGNIFICANCE AND IMPACT OF THE STUDY

There is considerable interest in the use of natural compounds as alternative methods to control undesirable pathogenic micro-organisms.

An automated method for the detection of staphylococcal heat stable deoxyribonuclease in dairy products

An automated sandwich immunoassay with specific polyclonal antibodies for the detection of Staphylococcus aureus thermostable nuclease (DNase) is described. To evaluate this assay, different quantities of purified S. aureus nuclease were added to dairy products. Additionally, staphylococcal counts and nuclease activity of milk samples inoculated with S. aureus were determined. Different extraction procedures were performed and compared. The results indicated that the automated test was a reliable method for detecting DNase activity in milk products. The procedure was completed in 2 h and detected 1 ng of DNase ml-1. Detection of the DNase was especially useful in cheeses and could be used to confirm positive enterotoxin results.

Staphylococcal growth and enterotoxins (A-D) and thermonuclease synthesis in the presence of dehydrated garlic

The inhibition of Staphylococcus aureus growth and enterotoxin and thermonuclease production by various concentrations of garlic (Allium sativum) was studied in BHI broth. The growth of Staph. aureus was inhibited by dehydrated garlic at levels of 1.5% (w/v) and over. Enterotoxins A, B and C1 were only detectable in broth containing < 1% of garlic while enterotoxin D was produced at a level of 2%. Garlic also inhibited thermonuclease (TNAse) production, complete inhibition being observed at levels > or = 1.5%. TNAse was not always detected when enterotoxin was present.

Simple solutions to false-positive staphylococcal enterotoxin assays with seafood tested with an enzyme-linked immunosorbent assay kit (TECRA)

The TECRA kit, a commercial staphylococcal enterotoxin visual immunoassay kit, is an enzyme-linked immunosorbent assay system which utilizes polyvalent antisera against staphylococcal enterotoxin types A to E. The test is simple and rapid to perform (4 h) and has therefore been widely used for screening purposes. In this study, the TECRA kit produced a number of false-positive reactions with seafood; 25% of 218 samples of seven types of seafood gave false-positive results, particularly shellfish such as mussels (85%), clams (32%), oysters (23%), winkles (20%), and squid (13%). Some nonshellfish samples also gave false-positive results with the TECRA kit (smelt [20%] and trout [10%]). The substance contributing to the false-positive results differed from true staphylococcal enterotoxins in that it was: (i) heat labile, being completely inactivated by heating for 3 min at 70 degrees C, compared with 5% inactivation of true staphylococcal enterotoxins by the same heat treatment, (ii) in a selective reaction with normal rabbit or calf serum (nonspecific reactions were completely abolished by these sera, whereas staphylococcal enterotoxins were not affected), and (iii) incapable of binding to a copper-chelate Sepharose gel (all of the substance remained in the unbound wash fraction, whereas staphylococcal enterotoxins were quantitatively bound to the gel). The false-positive reactions occurring with seafood were not associated with substances produced by microorganisms, since the bacterial isolates from the samples did not give positive results with the TECRA kit.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Production of enterotoxins and thermonuclease by Staphylococcus aureus in cooked egg-noodles

Production of enterotoxin A (SEA), enterotoxin C (SEC), and thermonuclease (TNase) by Staphylococcus aureus was determined during growth in cooked egg-noodles at different temperatures (15-37 degrees C). Both SEA and SEC and TNase were detected when greater than or equal to 4.0 x 10(7) colony forming units (cfu)/g were present. The contents of SEA, SEC, and TNase in egg-noodles mainly increased at the end of the exponential growth phase. In contrast with SEA and SEC the production of TNase always continued till the end of each experiment. Recovery rates of SEA and TNase in cooked noodles were dependent on their amounts. High amounts (64 ng SEA/g; 1 unit TNase/g) were recovered at a rate of 93% (SEA) and 54% (TNase) respectively, whereas low concentrations (1 ng SEA/g; 0.004 units TNase/g) were recovered at a rate of only 45% (SEA) and 1.1% (TNase). TNase usually is produced at all conditions which allow growth of S. aureus. Evidence of TNase was proposed for screening for staphylococcal enterotoxins (SE) in foods. But sometimes foods contain no TNase but SE. For this reason the ELISA-test which is simple and sensitive should be used for determination of SE-production in foods.

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.