Foodborne Listeriosis
595
(REA), pulsed field gel electrophoresis (PFGE), restriction fragment length polymorphisms (RFLP),
and ribotyping.
GROWTH
The nutritional requirements of listeriae are typical of those for many other Gram-positive bacteria.
They grow well in many common media such as brain heart infusion, trypticase soy, and tryptose
broths. Although most nutritional requirements have been described for L. monocytogenes, the other
species are believed to be similar. At least four B vitamins are required—biotin, riboflavin, thiamine,
and thioctic acid (α-lipoic acid; a growth factor for some bacteria and protozoa)—and the amino
acids cysteine, glutamine, isoleucine, leucine, and valine are required. Glucose enhances growth of
all species, and l(+)-lactic acid is produced. Although all species utilize glucose by the Embden–
Meyerhof pathway, various other simple and complex carbohydrates are utilized by some. Listeria spp.
resemble most enterococci in being able to hydrolyze esculin, and grow in the presence of 10 or 40%
(w/v) bile, in about 10% NaCl, 0.025% thallous acetate, and 0.04% potassium tellurite, but unlike the
enterococci, they do not grow in the presence of 0.02% sodium azide. The listeriae possess a bile-salt
hydrolyase, which permits their growth in the gall bladder. Unlike most other Gram-positive bacteria,
they grow on MacConkey agar. Although iron is important in its in vivo growth, L. monocytogenes
apparently does not possess specific iron-binding compounds, and it obtains its needs through the
reductive mobilization of free iron, which binds to surface receptors.
Effect of pH
Although the listeriae grow best in the pH range 6–8, the minimum pH that allows growth and
survival has been the subject of a large number of studies. Most research has been conducted with L.
monocytogenes strains, and whether the findings for this species are similar for other listerial species
can only be assumed. In general, some species/strains will grow over the pH range of 4.1 to around
9.6 and over the temperature range of 1
◦ C to around 45
◦ C. (Details of these parameters follow.)
In general, the minimum growth pH of a bacterium is a function of temperature of incubation,
general nutrient composition of growth substrate, water activity (a w ), and the presence and quantity
of NaCl and other salts or inhibitors. Growth of L. monocytogenes in culture media has been observed
at pH 4.4 in less than 7 days at 30
◦ C,
45 at pH 4.5 in tryptose broth at 19
◦ C,
12 and at pH 4.66 in 60
days at 30
◦ C.
18 In the first study, growth at pH 4.4 occurred at 20
◦ C in 14 days and at pH 5.23 at 4
◦ C
in 21 days.
45 In the second study, growth at pH 4.5 was enhanced by a restriction of oxygen. In the
third study, growth of L. monocytogenes was observed at pH 4.66 in 60 days at 30
◦ C, the minimum at
10
◦ C was pH 4.83, whereas at 5
◦ C, no growth occurred at pH 5.13. In yet another study, four strains
of L. monocytogenes grew at pH 4.5 after 30 days in a culture medium incubated at 30
◦ C,
102 but no
growth occurred at pH 4.0 or lower. pH values of 3.8–4.0 were more destructive to one strain than pH
4.2–5.0 when held in orange serum at 30
◦ C for 5 days (Figure 25–2).
When the pH of tryptic soy broth was adjusted with various acids, minimum pH for growth of four
strains of L. monocytogenes was shown to be a function of the acid employed. At the same pH, the
antimicrobial activity was acetic acid > lactic acid > citric acid > malic acid > HCl.
120 Growth occurred
at a pH of 4.6 at 35
◦ C between 1 and 3 days, and some strains grew at pH 4.4. The growth of two
strains of this species in cabbage juice containing no added NaCl has been observed at pH 4.1 within
595
(REA), pulsed field gel electrophoresis (PFGE), restriction fragment length polymorphisms (RFLP),
and ribotyping.
GROWTH
The nutritional requirements of listeriae are typical of those for many other Gram-positive bacteria.
They grow well in many common media such as brain heart infusion, trypticase soy, and tryptose
broths. Although most nutritional requirements have been described for L. monocytogenes, the other
species are believed to be similar. At least four B vitamins are required—biotin, riboflavin, thiamine,
and thioctic acid (α-lipoic acid; a growth factor for some bacteria and protozoa)—and the amino
acids cysteine, glutamine, isoleucine, leucine, and valine are required. Glucose enhances growth of
all species, and l(+)-lactic acid is produced. Although all species utilize glucose by the Embden–
Meyerhof pathway, various other simple and complex carbohydrates are utilized by some. Listeria spp.
resemble most enterococci in being able to hydrolyze esculin, and grow in the presence of 10 or 40%
(w/v) bile, in about 10% NaCl, 0.025% thallous acetate, and 0.04% potassium tellurite, but unlike the
enterococci, they do not grow in the presence of 0.02% sodium azide. The listeriae possess a bile-salt
hydrolyase, which permits their growth in the gall bladder. Unlike most other Gram-positive bacteria,
they grow on MacConkey agar. Although iron is important in its in vivo growth, L. monocytogenes
apparently does not possess specific iron-binding compounds, and it obtains its needs through the
reductive mobilization of free iron, which binds to surface receptors.
Effect of pH
Although the listeriae grow best in the pH range 6–8, the minimum pH that allows growth and
survival has been the subject of a large number of studies. Most research has been conducted with L.
monocytogenes strains, and whether the findings for this species are similar for other listerial species
can only be assumed. In general, some species/strains will grow over the pH range of 4.1 to around
9.6 and over the temperature range of 1
◦ C to around 45
◦ C. (Details of these parameters follow.)
In general, the minimum growth pH of a bacterium is a function of temperature of incubation,
general nutrient composition of growth substrate, water activity (a w ), and the presence and quantity
of NaCl and other salts or inhibitors. Growth of L. monocytogenes in culture media has been observed
at pH 4.4 in less than 7 days at 30
◦ C,
45 at pH 4.5 in tryptose broth at 19
◦ C,
12 and at pH 4.66 in 60
days at 30
◦ C.
18 In the first study, growth at pH 4.4 occurred at 20
◦ C in 14 days and at pH 5.23 at 4
◦ C
in 21 days.
45 In the second study, growth at pH 4.5 was enhanced by a restriction of oxygen. In the
third study, growth of L. monocytogenes was observed at pH 4.66 in 60 days at 30
◦ C, the minimum at
10
◦ C was pH 4.83, whereas at 5
◦ C, no growth occurred at pH 5.13. In yet another study, four strains
of L. monocytogenes grew at pH 4.5 after 30 days in a culture medium incubated at 30
◦ C,
102 but no
growth occurred at pH 4.0 or lower. pH values of 3.8–4.0 were more destructive to one strain than pH
4.2–5.0 when held in orange serum at 30
◦ C for 5 days (Figure 25–2).
When the pH of tryptic soy broth was adjusted with various acids, minimum pH for growth of four
strains of L. monocytogenes was shown to be a function of the acid employed. At the same pH, the
antimicrobial activity was acetic acid > lactic acid > citric acid > malic acid > HCl.
120 Growth occurred
at a pH of 4.6 at 35
◦ C between 1 and 3 days, and some strains grew at pH 4.4. The growth of two
strains of this species in cabbage juice containing no added NaCl has been observed at pH 4.1 within