338 ◾ Fundamental Food Microbiology
Salmonella enterica
Importance
Prior to the 1940s, Salmonella enterica serovar Typhi and Paratyphi were considered the major
causes of worldwide foodborne and waterborne diseases in humans caused by Salmonella.
However, with the pasteurization of milk and chlorination of water supplies, the spread of typhoid
and paratyphoid fever through food and water was greatly reduced, at least in developed countries.
As efficient techniques for the isolation and identification of other Salmonella serovars from foods
and environmental samples were developed, it became apparent that the worldwide incidence of
foodborne salmonellosis caused by other Salmonella is quite high. Since the 1950s, it has been recognized that foodborne salmonellosis has been the major cause of all foodborne diseases by pathogenic bacteria and viruses, both in number of incidents (sporadic and outbreaks) and number of
cases. Although, at present, scientific information about their habitats, mode of transmission in
foods, growth characteristics, and survival parameters are available, and methods to control their
contamination of foods have been developed, foodborne salmonellosis is still the leading cause of
foodborne bacterial diseases and is the second leading cause of all foodborne diseases (norovirus is
the first) in the United States and other developed countries. Salmonella is estimated to cause 1.2
million illnesses each year in the United States, with 23,000 hospitalizations and 450 deaths. This
is quite puzzling. The control measures seem to be working for several other foodborne pathogens,
at least in the United States, such as Clostridium perfringens, Bacillus cereus, Yer. enterocolitica,
Vib. parahaemolyticus, and probably Staphylococcus aureus. Even incidences from Sta. aureus and
Lis. monocytogenes seem to have declined since the 1990s. It is somewhat astonishing why it is not
working against Salmonella. Between 1969 and 1976, the average number of reported foodborne
salmonellosis outbreaks was about 37 per year in the United States. In contrast, between 1983
and 1987, the average number of outbreaks per year was over 68. In 2009, Salmonella was responsible for 113 outbreaks while in 2010, 121 outbreaks. Not only is the incidence not decreasing, it
continues to increase at a high rate. It is difficult to point out the exact cause(s) of this increase.
Maybe it is related to the large number of serotypes present in high frequency in the carrier state
in food animals, birds, pets, insects, and humans, and their ability to grow in foods, or it may be
the way the food animals and birds are raised, processed, and marketed, or it may be a result of
better surveillance systems by the regulatory agencies, or maybe our lifestyles and food habits give
these pathogens an edge.
Tauxe 1 indicated that the present increase in salmonellosis (including foodborne salmonellosis) in the United States could be related to four factors: the increase in number of
antimicrobial-resistant Salmonella isolates, the increase in individuals with immunodeficiency who are extremely susceptible to Salmonella, the increase in egg-associated Salmonella
enterica serovar Enteritidis contamination resulting from the increase in laying hens with
infected ovaries, and food production in centralized facilities that can lead to, if contamination occurs, extremely large and widespread outbreaks. It is necessary to understand the
importance of these factors in regard to the increase of salmonellosis and to develop corrective
measures to control the incidence.
There are about 2600 serovars (based on somatic, flagellar, and capsular antigen types) of
Salmonella and some that are capable of causing salmonellosis in humans. Along with fecal-oral
direct transmission, contaminated food and water can cause salmonellosis. Recently, a better system of nomenclature has been developed to group these serovars in a few species instead of considering each as a separate species. 2
Salmonella enterica
Importance
Prior to the 1940s, Salmonella enterica serovar Typhi and Paratyphi were considered the major
causes of worldwide foodborne and waterborne diseases in humans caused by Salmonella.
However, with the pasteurization of milk and chlorination of water supplies, the spread of typhoid
and paratyphoid fever through food and water was greatly reduced, at least in developed countries.
As efficient techniques for the isolation and identification of other Salmonella serovars from foods
and environmental samples were developed, it became apparent that the worldwide incidence of
foodborne salmonellosis caused by other Salmonella is quite high. Since the 1950s, it has been recognized that foodborne salmonellosis has been the major cause of all foodborne diseases by pathogenic bacteria and viruses, both in number of incidents (sporadic and outbreaks) and number of
cases. Although, at present, scientific information about their habitats, mode of transmission in
foods, growth characteristics, and survival parameters are available, and methods to control their
contamination of foods have been developed, foodborne salmonellosis is still the leading cause of
foodborne bacterial diseases and is the second leading cause of all foodborne diseases (norovirus is
the first) in the United States and other developed countries. Salmonella is estimated to cause 1.2
million illnesses each year in the United States, with 23,000 hospitalizations and 450 deaths. This
is quite puzzling. The control measures seem to be working for several other foodborne pathogens,
at least in the United States, such as Clostridium perfringens, Bacillus cereus, Yer. enterocolitica,
Vib. parahaemolyticus, and probably Staphylococcus aureus. Even incidences from Sta. aureus and
Lis. monocytogenes seem to have declined since the 1990s. It is somewhat astonishing why it is not
working against Salmonella. Between 1969 and 1976, the average number of reported foodborne
salmonellosis outbreaks was about 37 per year in the United States. In contrast, between 1983
and 1987, the average number of outbreaks per year was over 68. In 2009, Salmonella was responsible for 113 outbreaks while in 2010, 121 outbreaks. Not only is the incidence not decreasing, it
continues to increase at a high rate. It is difficult to point out the exact cause(s) of this increase.
Maybe it is related to the large number of serotypes present in high frequency in the carrier state
in food animals, birds, pets, insects, and humans, and their ability to grow in foods, or it may be
the way the food animals and birds are raised, processed, and marketed, or it may be a result of
better surveillance systems by the regulatory agencies, or maybe our lifestyles and food habits give
these pathogens an edge.
Tauxe 1 indicated that the present increase in salmonellosis (including foodborne salmonellosis) in the United States could be related to four factors: the increase in number of
antimicrobial-resistant Salmonella isolates, the increase in individuals with immunodeficiency who are extremely susceptible to Salmonella, the increase in egg-associated Salmonella
enterica serovar Enteritidis contamination resulting from the increase in laying hens with
infected ovaries, and food production in centralized facilities that can lead to, if contamination occurs, extremely large and widespread outbreaks. It is necessary to understand the
importance of these factors in regard to the increase of salmonellosis and to develop corrective
measures to control the incidence.
There are about 2600 serovars (based on somatic, flagellar, and capsular antigen types) of
Salmonella and some that are capable of causing salmonellosis in humans. Along with fecal-oral
direct transmission, contaminated food and water can cause salmonellosis. Recently, a better system of nomenclature has been developed to group these serovars in a few species instead of considering each as a separate species. 2