Interactions of nisin with glutathione in a model protein system and meat.

Loss of nisin activity in meat has been ascribed, in part, to the formation of a nisin-glutathione adduct. Activity is lost more quickly in raw meat than in cooked meat, and this has been taken as evidence that the reaction is enzyme mediated. Formation of the nisin-glutathione adduct has been confirmed but is shown not to be enzyme mediated. Retention of activity in cooked meat is shown to be due to the loss of free sulfhydryl groups during cooking as a result of the reaction of glutathione with proteins and not a result of the inactivation of endogenous enzymes. Microbial enzymes do not appear to play a role, as similar losses are seen in raw and cooked meat extracts, both of which contained undetectable levels of microorganisms.

Inhibition of Staphylococcus aureus by oleuropein is mediated by hydrogen peroxide.

The inhibition of Staphylococcus aureus by oleuropein is shown to be largely due to hydrogen peroxide production by oleuropein. The reaction is initiated by noninhibitory levels of hydrogen peroxide present as a result of tryptone oxidation in the underlying medium. Inhibition is abolished by catalase and anaerobic incubation conditions, and the effect of tryptone can be replicated by exogenous H2O2. S. aureus strains with reduced catalase activity show greater sensitivity to oleuropein. A mechanism for hydrogen peroxide production is proposed. Inhibition is not entirely due to H2O2, since some organisms with similar sensitivity to H2O2 as S. aureus were resistant to oleuropein, suggesting that there may be a cooperative effect between H2O2 and oleuropein itself.

Detection and enumeration of Campylobacter jejuni and Campylobacter coli by indirect impedimetry with an oxygen scavenging system.

Six strains of Campylobacter jejuni and Campylobacter coli were shown to grow in a variety of media, but, with one exception, they were unable to produce sufficient change in the electrical properties of the medium to allow their detection by impedance monitoring. With the use of an indirect method based on absorption of evolved carbon dioxide and a medium containing the oxygen scavenger Oxyrase, all strains were detectable, and correlations between time to detection and the logarithm of the inoculum level were excellent. The level of interstrain variation was sufficiently low that all data could be consolidated into a single calibration curve (r = 0.987).

Effects of Acidification, Bacterial Fermentation, and Temperature on the Survival of Rotavirus in a Model Weaning Food.

The effect of acidification, bacterial fermentation, and temperature on the survival of SA-11 rotavirus in model infant weaning foods was investigated. The influence of added organic acids and of bacterial fermentation on rotavirus survival was explicable solely by the pH achieved. The rotavirus was stable at temperatures representative of tropical ambient and at pH values typical of lactic fermented foods (3.8-4.1). Starch gelatinization temperatures were sufficient to inactivate rotavirus rapidly at neutral pH. Thus, cooking would kill virus although recontamination would remain a concern. A similar lethality to that at neutral pH could be observed in acidified media at lower temperatures. Although effective against bacterial pathogens, lactic fermentation of weaning foods confers little protection against rotavirus unless combined with a mild heat treatment such as might be used prior to serving to enhance palatability.

Chlorination of indicator bacteria and viruses in primary sewage effluent.

Wastewater disinfection is used in many countries for reducing fecal coliform levels in effluents. Disinfection is therefore frequently used to improve recreational bathing waters which do not comply with microbiological standards. It is unknown whether human enteric viruses (which are responsible for waterborne disease) are simultaneously inactivated alongside fecal coliforms. This laboratory study focused on the chlorination of primary treated effluent with three doses (8, 16, and 30 mg/liter) of free chlorine as sodium hypochlorite. Seeding experiments showed that inactivation (>5 log(10) units) of Escherichia coli and Enterococcus faecalis was rapid and complete but that there was poor inactivation (0.2 to 1.0 log(10) unit) of F(+)-specific RNA (FRNA) bacteriophage (MS2) (a potential virus indicator) at all three doses. However, seeded poliovirus was significantly more susceptible (2.8 log(10) units) to inactivation by chlorine than was the FRNA bacteriophage. To ensure that these results were not artifacts of the seeding process, comparisons were made between inactivation rates of laboratory-seeded organisms in sterilized sewage and inactivation rates of organisms occurring naturally in sewage. Multifactorial analysis of variance showed that there was no significant difference (P > 0.05) between the inactivation rates for seeded and naturally occurring FRNA bacteriophage. However, laboratory-grown poliovirus was inactivated much more rapidly than were naturally occurring, indigenous enteroviruses (P < 0.001). This may reflect differences in the way indigenous virus is presented to the disinfectant. Inactivation rates for indigenous enteroviruses were quite similar to those seen for FRNA bacteriophage at lower doses of chlorine. These results have significance for the effectiveness of chlorination as a sewage treatment process, particularly where virus contamination is of concern, and suggest that FRNA bacteriophage would be an appropriate indicator of such viral inactivation under field conditions.

Bacteriological Safety of a Fermented Weaning Food Containing L-lactate and Nisin.

Using a rice-based model weaning food, the effect of Lactococcus lactis on the growth and survival of a range of enteric pathogens has been investigated. The starter organism used produces the bacteriocin nisin and the physiological L-lactate isomer, thus avoiding the risk of D-lactate acidosis when consumed by infants. L. lactis was a less effective antagonist than stronger acid producers such as the DL lactate producer, Lactobacillus plantarum , and only produced a potentially useful inhibition of pathogens when present in a large numerical superiority (>105:1). Prefermentation of the weaning food with L. lactis for 24 h produced a product with a pH of 3.7-3.8 containing ≈ 0.25% lactate (>96% L-lactate). The prefermented product was bactericidal for pathogens introduced subsequently. Despite the production of 100-150 international units nisin per g during fermentation, the inhibition of pathogens could be ascribed to acid production alone.