The efficacy of phenolic compounds and lactoferrin against listeria monocytogenes in a model milk system

Listeria monocytogenes has been recognized as a human and animal pathogen for over 60 years. Found throughout the environment, this gram-positive bacilli has been isolated from a wide variety of materials, such as soil, water, vegetation, and also sewage (Botzler et al., 1974; Watkins and Sleath, 1981). Even though listeriosis only affects a small portion of the population, the illness still has a high mortality rate, especially among newborns, elderly, and those persons who are immunosuppressed. Symptoms of listeriosis include septicemia, meningitis, meningo-encephalitis, endocarditis, an infectious mononucleosis-like syndrome, and abortions (Gray and Killinger, 1966).

The first confirmed food-borne listeriosis outbreak was in 1981, when 41 cases of listeriosis were linked to consumption of contaminated coleslaw in Nova Scotia, Canada (Schlech et al., 1983). It is now recognized that all raw food products which come in contact with soil or fecal matter have the potential of carrying L. monocytogenes. Several outbreaks have been linked to contaminated dairy products (Fleming et al., 1985; Anonymous, 1985). The most recent isolated listeriosis case was from consumption of microwaved turkey franks (CDC, 1989). The control of L. monocytogenes contamination in food processing plants is difficult because of its ability to survive throughout the plant.

In dairy processing plants, the two major sources of Listeria are raw milk and fecal material. As the awareness of L. monocytogenes in the dairy processing plant has grown, the major concern is how to control its presence (FDA, 1986). Although Listeria is controlled effectively through pasteurization (Bradshaw et al., 1985; Sunning et al., 1988; WHO, 1988), post-processing contamination arises when pasteurized product is mixed with raw milk or is in contact with contaminated equipment (FDA, 1986). The ability of L. monocytogenes to grow readily in milk at refrigeration temperatures augments this problem (Donnelly and Briggs, 1986; Rosenow and Marth, 1987).

Stricter sanitation practices and utilization of the Hazard Analysis Critical Control Point (HACCP) program are used to help prevent contamination problems. The control measures of the HACCP program are fundamental and could be complimented by secondary controls such as antimicrobial agents.

This study will examine the efficacy of two types of antimicrobial systems against L. monocytogenes in a model milk system. The two antimicrobial systems include; a synthetic inhibitor, phenolic compounds, and a natural inhibitor, lactoferrin. The phenolic antimicrobial agents are active over a broad pH range (Busta and Foegeding, 1983) and thus have the potential of being effective under relatively neutral pH mediums of non-cultured dairy products.

The use of a natural antimicrobial system is attractive because of current consumer demands for "all-natural" products (Beuchat and Golden, 1989). Lactoferrin is an iron-binding protein present in milk which has been recognized for its role in preventing enteric infection in newborn infants (Lonnerdal, 1985) and its suggestive role in resistance of the bovine mammary gland to mastitic infections during involution (Breau and Oliver, 1986; Oliver and Bushe, 1987). The increased growth response of L. monocytogenes to iron supplemented growth medium (Sword, 1966; Trivett and Myer, 1971; Cowart and Foster, 1985) gives indication that lactoferrin could act as an effective natural antimicrobial system against L. monocytogenes.