The combined affects of modified atmosphere, temperature, nisin and ALTA™ 2341 on the growth of Listeria monocytogenes

A cocktail of seven Listeria monocytogenes isolates of food, human and environmental origin was used to assess the antilisterial activity of the bacteriocins nisin and ALTA™ 2341 in combination with various atmospheres: air, 100% N2, 40% CO2:60% N2, or 100% CO2. Buffered tryptone soya broth (pH 6.0) was used as the growth medium and incubation was at 4°C (21 days) or 12°C (7 days), or when temperature fluctuated between these values for defined periods. It was observed that atmosphere alone influenced the growth rate of L. monocytogenes, with 100% CO2 exerting the greatest inhibition. A 5 log population increase was observed in all atmospheres after 7 days at 12°C. At 4°C a 4–5 log population increase was observed in air, 100% N2 and 40% CO2:60% N2 within 21 days. Growth was prevented by 100% CO2. In the presence of nisin (400 IU/ml), an increase in the lag phase was observed before growth (5 log population increase after 7 days) in all atmospheres at 12°C. This effect was enhanced at 4°C where a maximum 2 log population increase was observed in all atmospheres except 100% CO2, in which growth was prevented. Increasing the concentration of nisin to 1250 IU/ml prevented L. monocytogenes growth in all atmosphere combinations at 4 and 12°C. Two concentrations of ALTA™ 2341 were also tested. In the presence of 0.1% ALTA™ 2341 and at 12°C, a 3–5 log population increase was observed in all atmospheres with the exception of 100% CO2, which prevented L. monocytogenes growth. At 4°C, growth was observed in the combination of 0.1% ALTA™ 2341 and 100% N2 only (3 log population increase). Use of a higher concentration of ALTA™ 2341 (1.0%) resulted in a population decrease below the detection level within 24 h in all atmosphere/temperature combinations. Re-growth occurred in the presence of 1.0% ALTA™ 2341 in all atmospheres at 12°C, and in combination with air or 100% N2 at 4°C. When the effectiveness of either nisin or ALTA™ 2341 and atmosphere was tested against L. monocytogenes as temperature fluctuated for periods between 4 and 12°C, only the combination of 100% CO2 and 1.0% ALTA™ 2341 prevented growth. Cells surviving exposure to nisin or ALTA™ 2341 were recovered from 28 of the 32 combinations tested that contained bacteriocin. Nisin survivors remained sensitive to the bacteriocin. ALTA™ 2341 survivors had become resistant to the bacteriocin.