732 Modern Food Microbiology
BACTERIA
Enterobacter sakazakii
This bacterium, once classified as a yellow-pigmented Enterobacter cloaceae, has been identified
as the cause of neonatal necrotizing enterocolitis (NEC); neonatal meningitis; and sepsis, dating
back to 1961. The usual vehicle food is powdered milk formulas. Although it is considered to be an
opportunistic pathogen, some strains produce an enterotoxin, and they are lethal to suckling mice.
Citrobacter freundii has been identified as the cause of neonatal infections and the vehicle foods were
infant formulas (see reference 105).
In one study, 4 of 18 E. sakazakii strains produced enterotoxin, and at 10
8 cfu/mouse by the
intraperitoneal route, all 18 isolates were lethal to suckling mice (16–18 days old) and 2 were lethal
by the peroral route.
84 In addition to suckling mice, potential virulence is manifested in monolayers
of CHO, Vero, and Y-1 adrenal cells. Infant mortality rates range from 40 to 60%. In an outbreak of
12 cases in 1998 in an intensive care unit in Belgium, 2 infants died and E. sakazakii was recovered
from unused prepared formula and from unopened cans of a single batch.
110
In a study of enteric bacteria in 141 powdered milk formulas from 35 countries, 25% contained
Pantoea agglomerans, 21% E. cloaceae, and 14% E. sakazakii.
75 From a study of 120 dried infant
formulas in Canada, 8 (6.7%) contained E. sakazakii.
80 The minimum growth temperature of tested
isolates in the latter study was 5.5 to 8
◦ C, and the maximum growth temperature ranged from 41 to
45
◦ C with a mean of 42.5
◦ C for 11 isolates. No isolates grew at 4
◦ C.
79
In regards to the thermal resistance of E. sakazakii, it appears to be higher than that of most Gramnegative bacteria. In a study of 10 strains (5 clinical and 5 food isolates) in reconstituted dried-infant
formula, a mean D 60 ◦ C of 2.5 minutes and z = 5.82
◦ C were found.
80 The mean D 60 ◦ C for the 5 clinical
isolates was 2.15, and 3.06 for the 5 food isolates. In another study, the D 58 ◦ C of 12 strains in rehydrated
infant formula ranged from 30.5 to 591.9 seconds (0.508 to 9.865 minutes).
35 By adding the most
heat-resistant strain of E. sakazakii (z = 5.6
◦ C) found in the last study to dry infant formula rehydrated
at 70
◦ C, > 4-log reduction was achieved.
35 According to these investigators, if it is assumed that the
typical level of this organism in infant formula is 1 cfu/100 g of dry formula, a 4-D treatment should
assure its absence after cooling for infant feeding.
35 The heat-resistant strain employed was the most
resistant of 12 tested.
Stationary phase cells of E. sakazakii have been reported to be more resistant to osmotic and dry
stress than E. coli and some other bacteria; and this increased resistance appeared to be associated
with the accumulation of trehalose by stationary phase cells.
10 The latter investigators found D 58 ◦ C
values of 0.27–0.50 for 5 E. sakazakii strains compared to 0.40–0.50 for three salmonellae.
Histamine-Associated (Scombroid) Poisoning
Illness contracted from eating scombroid fish or fish products containing high levels of histamine
is often referred to as scombroid poisoning. Among the scombroid fishes are tuna, mackerel, bonito,
and others. In one report, histamine poisoning was associated with sailfish, a nonscombroid.
51 The
histamine is produced by bacterial decarboxylation of the generally large quantities of histidine in
the muscles of this group. Sufficient levels of histamine may be produced without the product being
organoleptically unacceptable, with the result that scombroid poisoning may be contracted from both
fresh and organoleptically spoiled fish. The history of this syndrome has been reviewed by Hudson
and Brown,
50 who questioned the etiological role of histamine. This is discussed further below.
BACTERIA
Enterobacter sakazakii
This bacterium, once classified as a yellow-pigmented Enterobacter cloaceae, has been identified
as the cause of neonatal necrotizing enterocolitis (NEC); neonatal meningitis; and sepsis, dating
back to 1961. The usual vehicle food is powdered milk formulas. Although it is considered to be an
opportunistic pathogen, some strains produce an enterotoxin, and they are lethal to suckling mice.
Citrobacter freundii has been identified as the cause of neonatal infections and the vehicle foods were
infant formulas (see reference 105).
In one study, 4 of 18 E. sakazakii strains produced enterotoxin, and at 10
8 cfu/mouse by the
intraperitoneal route, all 18 isolates were lethal to suckling mice (16–18 days old) and 2 were lethal
by the peroral route.
84 In addition to suckling mice, potential virulence is manifested in monolayers
of CHO, Vero, and Y-1 adrenal cells. Infant mortality rates range from 40 to 60%. In an outbreak of
12 cases in 1998 in an intensive care unit in Belgium, 2 infants died and E. sakazakii was recovered
from unused prepared formula and from unopened cans of a single batch.
110
In a study of enteric bacteria in 141 powdered milk formulas from 35 countries, 25% contained
Pantoea agglomerans, 21% E. cloaceae, and 14% E. sakazakii.
75 From a study of 120 dried infant
formulas in Canada, 8 (6.7%) contained E. sakazakii.
80 The minimum growth temperature of tested
isolates in the latter study was 5.5 to 8
◦ C, and the maximum growth temperature ranged from 41 to
45
◦ C with a mean of 42.5
◦ C for 11 isolates. No isolates grew at 4
◦ C.
79
In regards to the thermal resistance of E. sakazakii, it appears to be higher than that of most Gramnegative bacteria. In a study of 10 strains (5 clinical and 5 food isolates) in reconstituted dried-infant
formula, a mean D 60 ◦ C of 2.5 minutes and z = 5.82
◦ C were found.
80 The mean D 60 ◦ C for the 5 clinical
isolates was 2.15, and 3.06 for the 5 food isolates. In another study, the D 58 ◦ C of 12 strains in rehydrated
infant formula ranged from 30.5 to 591.9 seconds (0.508 to 9.865 minutes).
35 By adding the most
heat-resistant strain of E. sakazakii (z = 5.6
◦ C) found in the last study to dry infant formula rehydrated
at 70
◦ C, > 4-log reduction was achieved.
35 According to these investigators, if it is assumed that the
typical level of this organism in infant formula is 1 cfu/100 g of dry formula, a 4-D treatment should
assure its absence after cooling for infant feeding.
35 The heat-resistant strain employed was the most
resistant of 12 tested.
Stationary phase cells of E. sakazakii have been reported to be more resistant to osmotic and dry
stress than E. coli and some other bacteria; and this increased resistance appeared to be associated
with the accumulation of trehalose by stationary phase cells.
10 The latter investigators found D 58 ◦ C
values of 0.27–0.50 for 5 E. sakazakii strains compared to 0.40–0.50 for three salmonellae.
Histamine-Associated (Scombroid) Poisoning
Illness contracted from eating scombroid fish or fish products containing high levels of histamine
is often referred to as scombroid poisoning. Among the scombroid fishes are tuna, mackerel, bonito,
and others. In one report, histamine poisoning was associated with sailfish, a nonscombroid.
51 The
histamine is produced by bacterial decarboxylation of the generally large quantities of histidine in
the muscles of this group. Sufficient levels of histamine may be produced without the product being
organoleptically unacceptable, with the result that scombroid poisoning may be contracted from both
fresh and organoleptically spoiled fish. The history of this syndrome has been reviewed by Hudson
and Brown,
50 who questioned the etiological role of histamine. This is discussed further below.