344 ◾ Fundamental Food Microbiology
isolated from the leftover baked ziti and from the unused eggs from the carton. Sal. enterica ser.
Enteritidis was also isolated in laying hens at the poultry farm that supplied the eggs.
The poultry farm should have tested the birds and culled those carrying the pathogen as a
regulatory requirement. However, at home, proper methods should have been taken in the preparation of the dish to avoid the incidence. It is likely, as only one egg was used for a serving of 24+
people, that Sal. enterica ser. Enteritidis was present initially in the food in small numbers, and
multiplied before or during (slow) refrigeration. Also, if the dish was taken out of the refrigerator a
long time prior to baking, Salmonella cells could have multiplied. During the 30 minutes of baking, the content was not thoroughly and evenly heated (the center was cold), so some Sal. enterica
ser. Enteritidis cells survived and, when ingested, caused salmonellosis. The dish could have been
prepared thin and baked immediately after mixing for a longer time for the food to attain a high
and uniform temperature and then refrigerated quickly. It could have been reheated to a high and
uniform temperature prior to serving to prevent this salmonellosis outbreak.
Listeria monocytogenes
Importance
Human listeriosis has been recognized for a long time. However, the presence of Lis. monocytogenes in many foods of animal and plant origin and illnesses resulting from the consumption of
contaminated foods were recognized rather recently in the 1980s. Human listeriosis is considered
by some to be an opportunistic rare disease. Individuals with normal health may not develop the
symptoms or show a very mild enteric form of the disease. However, it is highly fatal (30%–40%)
to fetuses, newborns, infants, the elderly, pregnant women, and immunocompromised people,
such as those with cancer (receiving chemotherapy), renal diseases, heart diseases, and AIDS.
Individuals receiving organ transplants and treated with immunosuppressive drugs are also susceptible to listeriosis. In addition, its ability to grow in many foods at refrigerated temperatures
helps the organism to reach from a low initial level to an infective-dose level during storage of
refrigerated foods, which include those that originally harbored the pathogen and those that were
post-heat contaminated. The increase in consumption of many types of ready-to-eat foods that are
stored for fairly long periods of time, and the fact that many of these foods are consumed without
properly reheating or by microwave heating, has given an edge for this pathogen to cause the disease. Many technological developments used for the production of these ready-to-eat foods may
have steps that can contaminate food with the pathogens in low levels, which then can reach a
higher level during subsequent refrigerated storage prior to consumption. Any temperature abuse,
even for a short time, can accelerate the growth rate.
It is quite clear that many of the above conditions have given an advantage to Lis. monocytogenes to become a newly emerging foodborne pathogen in many countries. However, an understanding of the type of foods that are mostly involved in listeriosis, the food processing steps that
can contaminate ready-to-eat foods with this pathogen, and the special groups of people that are
most susceptible to the disease helped the regulatory agencies develop procedures to reduce the
number of foodborne listeriosis cases. These have been achieved through changes in the processing
steps and testing of the foods so that contaminated ready-to-eat foods do not reach the consumers
and by educating the susceptible consumer groups about food choices, eating habits, and sanitary
practices in food preparation. As a result, in developed countries, the number of listeriosis cases
has dropped; in the United States since 1991, the number of human listeriosis cases has fallen
isolated from the leftover baked ziti and from the unused eggs from the carton. Sal. enterica ser.
Enteritidis was also isolated in laying hens at the poultry farm that supplied the eggs.
The poultry farm should have tested the birds and culled those carrying the pathogen as a
regulatory requirement. However, at home, proper methods should have been taken in the preparation of the dish to avoid the incidence. It is likely, as only one egg was used for a serving of 24+
people, that Sal. enterica ser. Enteritidis was present initially in the food in small numbers, and
multiplied before or during (slow) refrigeration. Also, if the dish was taken out of the refrigerator a
long time prior to baking, Salmonella cells could have multiplied. During the 30 minutes of baking, the content was not thoroughly and evenly heated (the center was cold), so some Sal. enterica
ser. Enteritidis cells survived and, when ingested, caused salmonellosis. The dish could have been
prepared thin and baked immediately after mixing for a longer time for the food to attain a high
and uniform temperature and then refrigerated quickly. It could have been reheated to a high and
uniform temperature prior to serving to prevent this salmonellosis outbreak.
Listeria monocytogenes
Importance
Human listeriosis has been recognized for a long time. However, the presence of Lis. monocytogenes in many foods of animal and plant origin and illnesses resulting from the consumption of
contaminated foods were recognized rather recently in the 1980s. Human listeriosis is considered
by some to be an opportunistic rare disease. Individuals with normal health may not develop the
symptoms or show a very mild enteric form of the disease. However, it is highly fatal (30%–40%)
to fetuses, newborns, infants, the elderly, pregnant women, and immunocompromised people,
such as those with cancer (receiving chemotherapy), renal diseases, heart diseases, and AIDS.
Individuals receiving organ transplants and treated with immunosuppressive drugs are also susceptible to listeriosis. In addition, its ability to grow in many foods at refrigerated temperatures
helps the organism to reach from a low initial level to an infective-dose level during storage of
refrigerated foods, which include those that originally harbored the pathogen and those that were
post-heat contaminated. The increase in consumption of many types of ready-to-eat foods that are
stored for fairly long periods of time, and the fact that many of these foods are consumed without
properly reheating or by microwave heating, has given an edge for this pathogen to cause the disease. Many technological developments used for the production of these ready-to-eat foods may
have steps that can contaminate food with the pathogens in low levels, which then can reach a
higher level during subsequent refrigerated storage prior to consumption. Any temperature abuse,
even for a short time, can accelerate the growth rate.
It is quite clear that many of the above conditions have given an advantage to Lis. monocytogenes to become a newly emerging foodborne pathogen in many countries. However, an understanding of the type of foods that are mostly involved in listeriosis, the food processing steps that
can contaminate ready-to-eat foods with this pathogen, and the special groups of people that are
most susceptible to the disease helped the regulatory agencies develop procedures to reduce the
number of foodborne listeriosis cases. These have been achieved through changes in the processing
steps and testing of the foods so that contaminated ready-to-eat foods do not reach the consumers
and by educating the susceptible consumer groups about food choices, eating habits, and sanitary
practices in food preparation. As a result, in developed countries, the number of listeriosis cases
has dropped; in the United States since 1991, the number of human listeriosis cases has fallen