United States Department of Agriculture Food Safety and Inspection Service

Ready to Eat Food  Sampling Programs, 1998 to 2001

The Florida Dept of Ag tests for four types of Listeria in sprouts other than Listeria monocytogenes.  They do this because their presence is a good indication that the environment is prime for Listeria monocytogenes.  

Listeria monocytogenes is only one bacterium in the Listeria family of bacteria, but it's the dangerous one. It is ubiquitous (omnipresent) and can come from soil, and therefore seed.  It can also be airborne and is most commonly found in the environment, particularly in leaves. 

Listeria monocytogenes is a tenacious bacterium that not only survives freezing and high temperatures, it can continue to grow under refrigeration all the way up to 119^o F (48^o C).  It is relatively unaffected by chlorine and resists high salt levels, nitrite, and acid. It can grow in vacuum packaged products.

Despite it tenacity to live, it is slow growing and a poor competitor with other bacteria for available food.  It does well in a "clean environment", relatively free from other bacteria, or under a refrigerated environment in which most other bacteria is laying dormant.  This is why outbreaks of listeriosis are associated with ready-to-eat foods such as hot dogs, luncheon meats, cold cuts, fermented or dry sausage, and other deli-style meat.  If these products are contaminated with L. monocytogenes after being pasteurized, but before they are packaged, the Listeria has no competitors, is kept refrigerated at a temperature in which it can grow and most other bacteria slowed, and may incubate in the package for months before it is consumed.

Listeria monocytogenes has been recognized as a human pathogen for 60 years, but food was not thought to be a vehicle of transmission until recently. Listeriosis includes such flu-like symptoms as headache, fever and diarrhea. In the most serious cases, the bacteria can strike the nervous system, causing meningitis and encephalitis.

Nearly one out of four people seriously infected with Listeria monocytogenes die. In the United States, an estimated 2,000 persons become seriously ill with listeriosis each year. Of these, approximately 500 die.

The incubation period is from a few days to six weeks. Pregnant women, newborns, and adults with weakened immune systems are primarily at risk. About one-third of Listeria monocytogenes cases occur during pregnancy.

Some studies suggest that 1-10% of humans may be intestinal carriers of L. monocytogenes. It has been found in at least 37 mammalian species, both domestic and feral, as well as at least 17 species of birds and possibly some species of fish and shellfish.

A grower would be more likely to find L. monocytogenes from environmental swabs (particularly in drains) than from seed.  If it is found in an environmental swab, it may not be a reason for a recall, but is a very serious situation that needs dealt with immediately.  The sprouts, as well as the environment, would need continually tested for Listeria monocytogenes until all traces of it had been eliminated and tests come up clean for a period of time afterward.  

Effective sanitation includes the following steps:

a) dry cleaning, b) pre-rinsing equipment, c) foaming and scrubbing, d) rinsing, e) visual inspection of equipment, f) cleaning walls and floors, g) sanitizing, and h) drying (drying is important to reduce the opportunity for listeria to grow on floors - this organism needs moisture to grow! Floors should be kept drained of standing water and as dry as possible).

Sanitizers that have proven most effective against Lm are quaternary ammonia compounds (quats), chlorine solutions and newer products containing peracetic acid. Rotating sanitizers periodically is generally a good practice as it will provide more effectiveness against Lm and other bacteria. Rotating sanitizers for various applications, including boot dip stations for reentry into ready-to-eat areas is generally recommended. Alternating between alkaline-based detergents and acid-based detergents daily also helps to avoid "soapstone" or hard-water buildups and biofilms. Alternating detergents also helps change the pH regularly to prevent adaptation of bacteria to a particular environment. (Care must be taken to NOT use chlorine and acid-based detergents simultaneously due to potential chemical hazards to employees). Sprout growers should work with sanitation professionals to develop a plan best suited for a particular operation.

Care must be given when cleaning rooms used for storing equipment and products so as not to splash water from the floor onto the product, thus possibly contaminating it with bacteria.