Foodborne Gastroenteritis Caused by Salmonella and Shigella
623
Food Products
Salmonellae have been found in commercially prepared and packaged foods with 17 of 247 products
examined being positive.
1 Among the contaminated foods were cake mixes, cookie doughs, dinner
rolls, and cornbread mixes. These organisms have been found in coconut meal, salad dressing, mayonnaise, milk, and many other foods. In a study of health foods, none of plant origin yielded salmonellae,
but from two of three lots of beef liver powder from the same manufacturer were isolated S. Minnesota,
S. Anatum, and S. Derby.
54 See Chapters 4–7 and 9.
Growth and Destruction of Salmonellae
These organisms are typical of other Gram-negative bacteria in that they are able to grow on a
large number of culture media and produce visible colonies well within 24 hours at about 37
◦ C.
They are generally unable to ferment lactose, sucrose, or salicin, although glucose and certain other
monosaccharides are fermented, with the production of gas. Although they normally utilize amino
acids as N sources, in the case of S. Typhimurium, nitrate, nitrite, and NH 3 will serve as sole sources of
nitrogen.
43 Although lactose fermentation is not usual for these organisms, some serovars can utilize
this sugar.
The pH for optimum growth is around neutrality, with values above 9.0 and below 4.0 being
bactericidal. A minimum growth pH of 4.05 has been recorded for some (with HCl and citric acids),
but depending on the acid used to lower the pH, the minimum may be as high as 5.5.
15 The effect
of acid used to lower the pH on minimum growth is presented in Table 26–2. Aeration was found to
favor growth at the lower pH values. The parameters of pH, water activity (a w ), nutrient content, and
temperature are all interrelated for salmonellae, as they are for most other bacteria.
56 For best growth,
the salmonellae require a pH between 6.6 and 8.2. The lowest temperatures at which growth has
been reported are 5.3
◦ C for S. Heidelberg and 6.2
◦ C for S. Typhimurium.
38 Temperatures of around
45
◦ C have been reported by several investigators to be the upper limit for growth. Regarding available
moisture, growth inhibition has been reported for a w values below 0.94 in media with neutral pH, with
higher a w values being required as the pH is decreased toward growth minima.
Unlike the staphylococci, the salmonellae are unable to tolerate high salt concentrations. Brine
above 9% is reported to be bactericidal. Nitrite is effective, with the effect being greatest at the lower
pH values. This suggests that the inhibitory effect of this compound is referable to the undissociated
HNO 2 molecule. The survival of Salmonella spp. in mayonnaise was studied by Lerche,
34 who found
that they were destroyed in this product if the pH was below 4.0. It was found that several days may
be required for destruction if the level of contamination is high, but within 24 hours for low numbers
of cells. S. Thompson and S. Typhimurium were found to be more resistant to acid destruction than
S. Senftenberg.
With respect to heat destruction, all salmonellae are readily destroyed at milk pasteurization temperatures. Thermal D values for the destruction of S. Senftenberg 775W under various conditions are
given in Chapter 17. Shrimpton et al.
50 reported that S. Senftenberg 775W required 2.5 minutes for a
10
4 –10
5 reduction in numbers at 54.4
◦ C in liquid whole egg. This strain is the most heat resistant of all
salmonellae serovars. This treatment of liquid whole egg has been shown to produce a Salmonella-free
product and destroy egg α-amylase (see Chapter 17 for the heat pasteurization of egg white). It has
been suggested
7 that the α-amylase test may be used as a means of determining the adequacy of heat
pasteurization of liquid egg (compare with the pasteurization of milk and the enzyme phosphatase).
In a study on the heat resistance of S. Senftenberg 775W, Ng et al.
42 found this strain to be more heat
623
Food Products
Salmonellae have been found in commercially prepared and packaged foods with 17 of 247 products
examined being positive.
1 Among the contaminated foods were cake mixes, cookie doughs, dinner
rolls, and cornbread mixes. These organisms have been found in coconut meal, salad dressing, mayonnaise, milk, and many other foods. In a study of health foods, none of plant origin yielded salmonellae,
but from two of three lots of beef liver powder from the same manufacturer were isolated S. Minnesota,
S. Anatum, and S. Derby.
54 See Chapters 4–7 and 9.
Growth and Destruction of Salmonellae
These organisms are typical of other Gram-negative bacteria in that they are able to grow on a
large number of culture media and produce visible colonies well within 24 hours at about 37
◦ C.
They are generally unable to ferment lactose, sucrose, or salicin, although glucose and certain other
monosaccharides are fermented, with the production of gas. Although they normally utilize amino
acids as N sources, in the case of S. Typhimurium, nitrate, nitrite, and NH 3 will serve as sole sources of
nitrogen.
43 Although lactose fermentation is not usual for these organisms, some serovars can utilize
this sugar.
The pH for optimum growth is around neutrality, with values above 9.0 and below 4.0 being
bactericidal. A minimum growth pH of 4.05 has been recorded for some (with HCl and citric acids),
but depending on the acid used to lower the pH, the minimum may be as high as 5.5.
15 The effect
of acid used to lower the pH on minimum growth is presented in Table 26–2. Aeration was found to
favor growth at the lower pH values. The parameters of pH, water activity (a w ), nutrient content, and
temperature are all interrelated for salmonellae, as they are for most other bacteria.
56 For best growth,
the salmonellae require a pH between 6.6 and 8.2. The lowest temperatures at which growth has
been reported are 5.3
◦ C for S. Heidelberg and 6.2
◦ C for S. Typhimurium.
38 Temperatures of around
45
◦ C have been reported by several investigators to be the upper limit for growth. Regarding available
moisture, growth inhibition has been reported for a w values below 0.94 in media with neutral pH, with
higher a w values being required as the pH is decreased toward growth minima.
Unlike the staphylococci, the salmonellae are unable to tolerate high salt concentrations. Brine
above 9% is reported to be bactericidal. Nitrite is effective, with the effect being greatest at the lower
pH values. This suggests that the inhibitory effect of this compound is referable to the undissociated
HNO 2 molecule. The survival of Salmonella spp. in mayonnaise was studied by Lerche,
34 who found
that they were destroyed in this product if the pH was below 4.0. It was found that several days may
be required for destruction if the level of contamination is high, but within 24 hours for low numbers
of cells. S. Thompson and S. Typhimurium were found to be more resistant to acid destruction than
S. Senftenberg.
With respect to heat destruction, all salmonellae are readily destroyed at milk pasteurization temperatures. Thermal D values for the destruction of S. Senftenberg 775W under various conditions are
given in Chapter 17. Shrimpton et al.
50 reported that S. Senftenberg 775W required 2.5 minutes for a
10
4 –10
5 reduction in numbers at 54.4
◦ C in liquid whole egg. This strain is the most heat resistant of all
salmonellae serovars. This treatment of liquid whole egg has been shown to produce a Salmonella-free
product and destroy egg α-amylase (see Chapter 17 for the heat pasteurization of egg white). It has
been suggested
7 that the α-amylase test may be used as a means of determining the adequacy of heat
pasteurization of liquid egg (compare with the pasteurization of milk and the enzyme phosphatase).
In a study on the heat resistance of S. Senftenberg 775W, Ng et al.
42 found this strain to be more heat