548 Modern Food Microbiology
INCIDENCE IN FOODS
In general, staphylococci may be expected to exist, at least in low numbers, in any or all food products
that are of animal origin or in those that are handled directly by humans, unless heat-processing steps
are applied to effect their destruction. They have been found in a large number of commercial foods
by many investigators (see Chapters 4, 5, and 9; Tables 4–3, 4–14, 5–6, and 9–1).
NUTRITIONAL REQUIREMENTS FOR GROWTH
Staphylococci are typical of other Gram-positive bacteria in having a requirement for certain organic
compounds in their nutrition. Amino acids are required as nitrogen sources, and thiamine and nicotinic
acid are required among the B vitamins. When grown anaerobically, they appear to require uracil. In
one minimal medium for aerobic growth and enterotoxin production, monosodium glutamate serves
as C, N, and energy sources. This medium contains only three amino acids (arginine, cystine, and
phenylalanine) and four vitamins (pantothenate, biotin, niacin, and thiamine), in addition to inorganic
salts
73 Arginine appears to be essential for enterotoxin B production.
116
TEMPERATURE GROWTH RANGE
Although it is a mesophile, some strains of S. aureus can grow as low as 6.7
◦ C.
5 The latter investigators found three food-poisoning strains that grew in custard at 114
◦ F (45.6
◦ C) but decreased at
116–120
◦ F (46.7–48.9
◦ C), with time of incubation. They grew in chicken `
a la king at 112
◦ F (44.4
◦ C)
but failed to grow in ham salad at the same temperature. In general, growth occurs over the range 7–
47.8
◦ C, and enterotoxins are produced between 10
◦ C and 46
◦ C, with the optimum between 40
◦ C and
45
◦ C.
98 These minimum and maximum temperatures of growth and toxin production assume optimal
conditions relative to the other parameters, and the ways in which they interact to raise minimum
growth or lower maximum growth temperatures are noted below.
EFFECT OF SALTS AND OTHER CHEMICALS
Although S. aureus grows well in culture media without NaCl, it can grow well in 7–10% concentrations, and some strains can grow in 20%. The maximum concentrations that permit growth depend
on other parameters such as temperature, pH, water activity (a w ), and oxidation–reduction potential
(Eh) (see below).
S. aureus has a high degree of tolerance to compounds such as tellurite, mercuric chloride, neomycin,
polymyxin, and sodium azide, all of which have been used as selective agents in culture media.
S. aureus can be differentiated from other staphylococcal species by its greater resistance to acriflavine.
In the case of borate, S. aureus is sensitive, whereas S. epidermidis is resistant.
59 With novobiocin,
S. saprophyticus is resistant, whereas S. aureus and S. epidermidis are not. The capacity to tolerate
high levels of NaCl and certain other compounds is shared by Micrococcus and Kocuria, which are
widely distributed in nature and occur in foods generally in greater numbers than staphylococci, thus
making the recovery of the latter more difficult. The effect of other chemicals on S. aureus is presented
in Chapter 13.
INCIDENCE IN FOODS
In general, staphylococci may be expected to exist, at least in low numbers, in any or all food products
that are of animal origin or in those that are handled directly by humans, unless heat-processing steps
are applied to effect their destruction. They have been found in a large number of commercial foods
by many investigators (see Chapters 4, 5, and 9; Tables 4–3, 4–14, 5–6, and 9–1).
NUTRITIONAL REQUIREMENTS FOR GROWTH
Staphylococci are typical of other Gram-positive bacteria in having a requirement for certain organic
compounds in their nutrition. Amino acids are required as nitrogen sources, and thiamine and nicotinic
acid are required among the B vitamins. When grown anaerobically, they appear to require uracil. In
one minimal medium for aerobic growth and enterotoxin production, monosodium glutamate serves
as C, N, and energy sources. This medium contains only three amino acids (arginine, cystine, and
phenylalanine) and four vitamins (pantothenate, biotin, niacin, and thiamine), in addition to inorganic
salts
73 Arginine appears to be essential for enterotoxin B production.
116
TEMPERATURE GROWTH RANGE
Although it is a mesophile, some strains of S. aureus can grow as low as 6.7
◦ C.
5 The latter investigators found three food-poisoning strains that grew in custard at 114
◦ F (45.6
◦ C) but decreased at
116–120
◦ F (46.7–48.9
◦ C), with time of incubation. They grew in chicken `
a la king at 112
◦ F (44.4
◦ C)
but failed to grow in ham salad at the same temperature. In general, growth occurs over the range 7–
47.8
◦ C, and enterotoxins are produced between 10
◦ C and 46
◦ C, with the optimum between 40
◦ C and
45
◦ C.
98 These minimum and maximum temperatures of growth and toxin production assume optimal
conditions relative to the other parameters, and the ways in which they interact to raise minimum
growth or lower maximum growth temperatures are noted below.
EFFECT OF SALTS AND OTHER CHEMICALS
Although S. aureus grows well in culture media without NaCl, it can grow well in 7–10% concentrations, and some strains can grow in 20%. The maximum concentrations that permit growth depend
on other parameters such as temperature, pH, water activity (a w ), and oxidation–reduction potential
(Eh) (see below).
S. aureus has a high degree of tolerance to compounds such as tellurite, mercuric chloride, neomycin,
polymyxin, and sodium azide, all of which have been used as selective agents in culture media.
S. aureus can be differentiated from other staphylococcal species by its greater resistance to acriflavine.
In the case of borate, S. aureus is sensitive, whereas S. epidermidis is resistant.
59 With novobiocin,
S. saprophyticus is resistant, whereas S. aureus and S. epidermidis are not. The capacity to tolerate
high levels of NaCl and certain other compounds is shared by Micrococcus and Kocuria, which are
widely distributed in nature and occur in foods generally in greater numbers than staphylococci, thus
making the recovery of the latter more difficult. The effect of other chemicals on S. aureus is presented
in Chapter 13.