Inactivation of Escherichia coli and Shigella in acidic fruit and vegetable juices by peroxidase systems.

AIMS: To study the bactericidal properties of the lactoperoxidase (LPER)-thiocyanate and soybean peroxidase (SBP)-thiocyanate systems at low pH, their efficiency for inactivation of Escherichia coli and Shigella in acidic fruit and vegetable juices, their effect on colour stability of the juices and interaction with ascorbic acid. METHODS AND RESULTS: Three-strain cocktails of E. coli and Shigella spp. in selected juices were supplemented with the LPER or SBP system. Within 24 h at 20 degrees C, the LPER system inactivated both cocktails by > or = 5 log10 units in apple, 2-5 log10 units in orange and < or = 1 log10 unit in tomato juices. In the presence of SBP, browning was significant in apple juice and white grape juice, slight in pink grape juice and absent in orange or tomato juice. Ascorbic acid protected E. coli and Shigella against inactivation by the LPER system, and peroxidase systems significantly reduced the ascorbic acid content of juices. CONCLUSIONS: Our results suggest a different specificity of LPER and SBP for SCN-, phenolic substrates of browning and ascorbic acid in acidic juices. The LPER system appeared a more appropriate candidate than the SBP system for biopreservation of juices. SIGNIFICANCE AND IMPACT OF THE STUDY: This work may open perspectives towards the development of LPER or other peroxidases as biopreservatives in acidic foods.

Antimicrobial effect of spices and herbs on Shigella sonnei and Shigella flexneri.

Of 17 spices and herbs tested at 1% (wt/vol) in Mueller-Hinton (MH) agar, only cloves, thyme, oregano, allspice, basil, rosemary, and marjoram showed antimicrobial effects on Shigella. The MICs of thyme, oregano, basil, and rosemary (as determined by the agar dilution method) ranged from 0.5 to 1% (wt/vol) depending on the Shigella strain used. With the use of various combinations of temperatures (12, 22, and 37 degrees C), pHs (5.0, 5.5, and 6.0), and NaCl concentrations (1, 2, 3, and 4%, wt/vol) and the inclusion or exclusion of thyme or basil at 1% (wt/vol) in an MH agar model system, it was established that basil or thyme can contribute to combination processing as a growth-inhibitory factor for Shigella spp. In the presence of basil and thyme, Shigella flexneri did not develop CFU during the 7-day incubation period for, respectively, 14 and 16 of the 18 tested combinations, while growth was noted in the corresponding temperature-pH-NaCl concentration combinations without basil or thyme. A growth-inhibitory effect on Shigella sonnei was also noted. The results of an orientation study involving the addition of basil and thyme to spaghetti sauce prior to autoclaving and S. sonnei inoculation indicated that basil and thyme contributed to the reduction of S. sonnei after 16 days at 12 degrees C but not at 4 degrees C.

Acid tolerance of Shigella sonnei and Shigella flexneri.

AIMS: Determination of the behaviour of Shigella sonnei and Sh. flexneri under acid conditions. METHODS AND RESULTS: The growth and survival of Shigella spp. (9 isolates) in acidified Brain Heart Infusion (BHI) (pH 5.0-3.25 with pH intervals of 0.25) was determined after 6, 24 and 30 h incubation at 37 degrees C. Subsequently, survival of shigellae was studied in apple juice and tomato juice stored at 7 degrees C and 22 degrees C for up to 14 days and in strawberries and a fresh fruit salad, kept at 4 degrees C for 4 and 48 h. CONCLUSIONS: The minimum pH for growth in acidified BHI for Sh. flexneri and Sh. sonnei was, respectively, pH 4.75 and pH 4.50. Survival in fruit juices and fresh fruits depended upon their pH, the type of strain and the incubation temperature. Shigella spp. Survived for up to 14 days in tomato juice and apple juice stored at 7 degrees C. The shortest survival time (2-8 d) was observed in apple juice at 22 degrees C. Sh. sonnei but not Sh. flexneri was recovered after 48 h from strawberries and fruit salad kept at 4 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: Acid foods, especially if kept at refrigeration temperatures, support survival of Shigella spp. and may cause Shigella food poisoning.

Inhibitory effects of spices and herbs towards Shigella sonnei and S. flexneri.

The use of spices and herbs, their essential oils or their active compounds as means of control of pathogens constitutes an alternative to chemical additives In the present study the antibacterial activities of cloves, thyme, oregano, rosemary and basil on Shigella have been established. Although in a model system in agar media addition of 1% basil could contribute to the 'hurdle' principle and delay outgrowth of Shigella sp. this was not confirmed in real food conditions: the presence of 1% basil did not affect growth of Shigella in potato puree at 22 degrees C or survival at 7 degrees C in spaghetti sauce. Thyme and essential oils and thymol and carvacrol showed inhibition of Shigella sp. in the agar well diffusion method (MIC 0.1-1.0%) and they have potential to be used as a desinfectant in the washing water e.g. in the process line of minimal processed vegetables. However, more studies combining sensoric properties with microbial analysis are needed to investigated the possible use of these compounds.

Evaluation of culture media for enrichment and isolation of Shigella sonnei and S. flexneri.

The performance of Gram-negative (GN) broth with (10 microg/ml) and without novobiocin, Shigella broth (SB) with 0.5 and 3.0 microg/ml novobiocin, all incubated at 37 degrees C (SB with 3.0 microg/ml novobiocin also at 42 degrees C) and buffered brilliant green bile glucose (EE) broth with 1.0 microg/ml novobiocin incubated at 37 and 42 degrees C were evaluated for resuscitation and growth of Shigella sonnei and S. flexneri (eight strains; unstressed, chill-stressed and acid-stressed) and non-shigellae (11 strains). GN broth with or without novobiocin supported significantly less growth of Shigella sp. No significant differences in growth of shigellae were obtained between the other culture media. Performance depended more on the Shigella strain used. None of the tested media were significantly superior for suppressing the competitive flora. Electivity and selectivity of MacConkey agar (MAC), tergitol-7 agar (T7), desoxycholate citrate agar (DCA), xylose lysine desoxycholate agar (XLD), Salmonella Shigella agar and Hektoen enteric agar (HEA) were determined by ecometric testing. HEA confirmed to be a high selective medium for both non-shigellae and stressed Shigella sp. Klebsiella sp., Enterobacter sp., Citrobacter sp.. Salmonella sp. and the Escherichia strains can mask the presence of shigellae. In vitro competition experiments and experiments with artificially contaminated foods showed higher resistance of S. sonnei than S. flexneri towards the stress imposed by the food matrix and its indigenous flora. Reliable detection, however, of shigellae in foods with the current enrichment and isolation media was not achieved.