Natural antimicrobials, from plant, animal or microbial sources, have good potential for use as preservative systems to improve food safety, extend shelf life and enhance the overall quality of food products, while promoting the image of "healthier" foods. The overall objective of this research was to evaluate the antimicrobial efficacy of phosvitin alone or combined with carvacrol or nisin against four human enteric foodborne pathogens. Growth inhibition of Salmonella enterica, Listeria monocytogenes, Escherichia coli O157:H7 or Staphylococcus aureus by the antimicrobials used singly or in combination in brain heart infusion (BHI) broth (35 yC, 24 hours) was evaluated using a Bioscreen C turbidometer (OD 600nm). Subsequently, selected concentrations of the antimicrobials were evaluated for their effectiveness in controlling growth of the pathogens and background microflora in onion mushroom soup at 12 yC and 35 yC. Additional experiments involving UV spectroscopy and transmission electron microscopy (TEM) were performed to investigate the antibacterial mechanism of action of carvacrol in E. coli O157:H7 and L. monocytogenes Scott A. In BHI broth the minimum inhibitory concentration (MIC) of phosvitin and carvacrol was 80 mg/ml and 0.14 mg/ml, respectively, for both S. enterica and L. monocytogenes. The MIC of phosvitin and carvacrol was greater than 100 mg/ml and 0.12 mg/ml, respectively (for S. aureus) and 80 mg/ml and 0.12 mg/ml, respectively, for E. coli. In onion mushroom soup, the combination of phosvitin (60 mg/ml) and carvacrol (0.40 mg/ml) exerted the greatest cidal effect throughout storage against all foodborne pathogens tested. Irrespective of storage temperature (12 yC or 35 yC), phosvitin combined with nisin did not offer enhanced antibacterial effect above that provided by phosvitin used alone. For all organisms, leakage of A260 absorbing material from the bacterial cells increased significantly with increased carvacrol concentration and exposure time. There was a very strong correlation between the initial rate of release of A260 material and death rate of the pathogens (r = 0.998). At 0.4 mg/ml carvacrol, E. coli O157:H7 cells appeared collapsed and showed signs of lysis when observed by TEM. At 0.4 and 0.6 mg/ml carvacrol, L. monocytogenes cells showed no apparent physical damage (distortion of cell shape) despite substantial loss of viability; however, exudation of intracellular material was detected by UV spectroscopy and TEM photographs of negatively stained cells.

Based on results of the present studies it is concluded that: i) use of phosvitin and carvacrol in combination has good potential for controlling growth of foodborne pathogenic bacteria in onion mushroom soup and ensuring the microbial safety of this potentially hazardous food product, ii) phosvitin/nisin combinations are far less effective for inhibiting growth of pathogens in soup, iii) death of bacterial cells exposed to carvacrol involves damage to the cytoplasmic membrane.