Nisin-resistant (Nisr) Listeria monocytogenes and Nisr Clostridium botulinum Are Not Resistant to Common Food Preservatives

Enhancing inactivation of Staphylococcus aureus in skim milk by combining high-intensity pulsed electric fields and nisin

High-intensity pulsed electric fields (HIPEF) can be used as a nonthermal preservation method that is believed to enhance the effect of nisin on microorganisms such as Staphylococcus aureus. The survival of S. aureus inoculated into skim milk and treated with nisin, with HIPEF, or with a combination of nisin-HIPEF was evaluated. Nisin dose, milk pH, and HIPEF treatment time were the controlled variables that were set up at 20 to 150 ppm, pH 5.0 to 6.8, and 240 to 2,400 micros, respectively. HIPEF strength and pulse width were kept constant at 35 kV/cm and 4 micros, respectively. No reduction in S. aureus concentration was observed in skim milk at its natural pH after treatment with nisin, but 1.1 log units were recovered after 90 min of treatment at pH 5.0 with 150 ppm nisin. A reduction in viable S. aureus counts of 0.3 and 1.0 log unit in skim milk treated with HIPEF at its natural pH was observed at 240 and 2,400 micros, respectively. The nisin-HIPEF treatment design was based on a response surface methodology. The combined effect of nisin and HIPEF was clearly synergistic. However, synergism depended on pH. A maximum microbial inactivation of 6.0 log units was observed at pH 6.8, 20 ppm nisin, and 2,400 micros of HIPEF treatment time, whereas a reduction of over 4.5 log units was achieved when pH, nisin concentration, and HIPEF treatment times were set at 5.0, 150 ppm, and 240 micros, respectively.

Consequences of the development of nisin-resistant Listeria monocytogenes in fermented dairy products

Wild Listeria isolates representing serovars found in artisanal cheeses commercialized in Asturias (northern Spain) were assessed for their susceptibility to several bacteriocins. Pediocin PA-1 was the most active bacteriocin followed by enterocin AS-48, nisin, and plantaricin C. However, some Listeria monocytogenes and Listeria innocua strains were already highly resistant to PA-1. Among the wild L. monocytogenes populations, the frequency of development of nisin resistance ranged from 10(-6) up to 10(-3), depending on the strain. Highly stable mutants with increased nisin resistance (two- to fourfold) were isolated and tested for potential cross-resistance to lysozyme, EDTA, and various NaCl concentrations and pH values. All mutants were cross-resistant to lysozyme but sensitive to EDTA. In contrast, no clear correlation could be established between nisin resistance and an altered susceptibility to NaCl or pH changes. Nisin-resistant variants were able to survive and even to multiply in milk fermented by a nisin-producing Lactococcus, but the growth of the wild-type strain was inhibited. The different phenotypes evaluated in this study are indicative of the unpredictability of the consequences of the development of nisin resistance in a dairy environment. This resistance should be considered when making a risk assessment of the long-term use of nisin to control L. monocytogenes.

Effective control of Listeria monocytogenes by combination of nisin formulated and slowly released into a broth system

In order to identify conditions for efficient food preservation by nisin, the sensitivity of Listeria monocytogenes to this preservative was studied under the following three model conditions: (1) the instantaneous addition of nisin into broth medium to simulate the formation of nisin in foods, (2) the slow delivery of nisin solution into broth medium using a pump to simulate the slow release of nisin from packaging materials to foods, (3) a combination of the two delivery methods. Based on the following results, we conclude that the antimicrobial effectiveness of nisin strongly depends on its mode of delivery. The instantaneous and slow methods for adding nisin inhibited L. monocytogenes, but over time of exposure, L. monocytogenes developed tolerance to nisin. Our data indicate that cells treated with instantaneously added nisin developed resistance to higher concentrations of nisin (200 IU/ml), compared to cells treated with slowly added nisin at the same total amount of the antimicrobial. Further studies indicated that nisin-tolerant cells recovered from treatments in which 200 IU/ml nisin was added instantaneously were likely to be mutants, which became resistant to the bacteriocin. In contrast, when 200 IU/ml of the antimicrobial was added slowly to the cells, only a temporary tolerance was developed; these cells became nisin-sensitive after passage through nisin-free medium. Due to the development of nisin-resistant cells, excessive amounts of nisin in the model system did not further inhibit L. monocytogenes. These results signify that excess nisin in foods does not necessarily improve the efficiency of controlling L. monocytogenes. Our data suggest that the combination of packaging material containing nisin used in conjunction with nisin-containing foods will provide the most effective means of preventing L. monocytogenes growth.

Arming the enemy: the evolution of resistance to self-proteins

A remarkable range of novel antibiotics is attracting increasing interest as a major new weapon in the campaign against bacterial infection. They are based on the toxic peptides that provide the innate immune system of animals, and it is claimed that bacteria will be unable to evolve resistance to them because they attack the 'Achilles' heel' of bacterial membrane structure. Both experimental evidence and theoretical arguments suggest that this claim is doubtful. If so, the introduction of these substances into general use may provoke the evolution of resistance to our own defence proteins and thus compromise our natural defences against infection.