Bacillus cereus has been recognized as an agent of food
poisoning since 1955. There are only a few outbreaks a year reported by
CDC. Between 1972 and 1986, 52 outbreaks of food-borne disease
associated with B. cereus were reported to the CDC (in 2003,
there were two), but this is thought to
represent only 2% of the total cases which have occurred during these
periods.
It is not a reportable disease, and usually goes undiagnosed.
B.
cereus causes two types of food-borne illnesses. One type
is characterized by nausea and vomiting and abdominal
cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus
aureus food poisoning in its symptoms and incubation period. This
is
the "short-incubation" or emetic form of the disease.
The second type
is manifested primarily by abdominal cramps and diarrhea following an
incubation
period of 8 to 16 hours. Diarrhea may be a small volume or profuse and
watery.
This type is referred to as the "long-incubation" or diarrheal form
of the disease, and it resembles food poisoning caused by Clostridium
perfringens. In either type, the illness usually lasts less than 24
hours after onset. In a few patients symptoms may last longer.
The short-incubation form is caused by a preformed, heat-stable emetic toxin, ETE. The mechanism
and site of
action of this toxin are unknown, although the small molecule forms ion
channels and holes in membranes. The long-incubation form of illness
is
mediated by the heat-labile diarrheagenic enterotoxin Nhe and/or hemolytic enterotoxin HBL, which
cause
intestinal fluid secretion, probably by several mechanisms, including
pore formation and activation of adenylate cyclase enzymes.
Bacillus
cereus produces
one emetic
toxin (ETE) and three different enterotoxins: HBL, Nhe,
and EntK.
Two of the
three enterotoxins are involved in food poisoning.
They both consist of three different protein subunits that act
together. One of
these enterotoxins (HBL) is also a hemolysin; the second enterotoxin
(Nhe) is not a hemolysin. The third enterotoxin (EntK) is a
single
component protein that has not been shown to be involved in food
poisoning. All three enterotoxins are cytotoxic and cell membrane
active toxins that will make holes or channels in membranes.
The emetic
toxin (ETE) is a
ring-shaped structure of three repeats of four amino acids with a
molecular weight of 1.2 kDa. It is a K+ ionophoric
channel, highly resistant to pH between 2 and 11, to heat, and to
proteolytic cleavage.
The
nonhemolytic enterotoxin (Nhe) is one of the three-component
enterotoxins responsible for diarrhea in Bacillus
cereus
food
poisoning. Nhe is composed of NheA, NheB and NheC. The three genes
encoding the Nhe components constitute an operon. The nhe genes have been
cloned separately,
and expressed in either Bacillus
subtilis
or Escherichia
coli.
Separate expression showed that all three components are required for
biological activity.
The hemolytic enterotoxin, HBL is encoded by the hblCDA operon. The three
protein components, L1, L2 and B, constitute a hemolysin. B is for
binding; L1 and L2 are lytic components. This toxin also has
dermonecrotic and vascular permeability activities, and it causes fluid
accumulation in rabbit ileal loops.
B. cereus food poisoning occurs year-round and is without any
particular geographic distribution. The short-incubation form is most
often associated with rice dishes that have been cooked and then held
at
warm temperatures for several hours. It is often associated with
Mexican
and Chinese restaurants, but in one reported outbreak, macaroni and
cheese
made from powdered milk turned out to be the source of the bacterium.
"Mac and cheese"- you can't get much more American than that.
Long-incubation B. cereus food poisoning is frequently
associated with meat or vegetable-containing foods after cooking. The
bacterium has been isolated from 50% of dried beans and cereals and
from 25% of dried
foods such as spices, seasoning mixes and potatoes. One outbreak of the
long-incubation form was traced to a "meals-on-wheels" program in which
food was held above room temperature for a prolonged period before
delivery to consumers.
The short-incubation or emetic form of the disease is diagnosed by
the isolation of B. cereus from the incriminated food. The
long-incubation or diarrheal form is diagnosed by isolation of the
organism from stool
and food. Isolation from stools alone is not sufficient because 14% of
healthy adults have been reported to have transient gastrointestinal
colonization with B. cereus. Because B. cereus
gastroenteritis is generally a benign, self-limited illness,
antimicrobial agents are of no value in
management. Since the bacteria grow best at temperatures ranging from
40 to
140°F,
infection may be prevented if cold food is refrigerated and if hot food
is
held at greater than 140°F before serving.
CASE STUDY
Summary and Analysis of a
Report of Bacillus cereus Food Poisoning Associated with Fried
Rice at
Two Child Day Care Centers -- Virginia, 1993 (from CDC).
Summary
On July 21, 1993, a regional public health facility received reports of
acute gastrointestinal illness that occurred among children and staff
at
two jointly owned child day care centers following a catered lunch.
The catered lunch was served on July 21 to 82 children aged less
than
or equal to 6 years, and to nine staff; dietary histories were obtained
for 80 persons. 67 ate the catered lunch. A case was defined as
vomiting by a person who was present at either day care center on July
21. Fourteen (21%) persons who ate the lunch became ill, compared with
none of 13 who did not. Symptoms included nausea (71%), abdominal
cramps or pain (36%), and diarrhea (14%). Twelve of the 14 cases
occurred among children aged 2.5-5 years,
and two occurred among staff. The median incubation period was 2 hours
(range: 1.5-3.5 hours). Symptoms resolved a median of 4 hours after
onset (range: 1.5-22 hours).
Chicken fried rice prepared at a local restaurant was the only food
significantly associated with illness; illness occurred in 14 (29%) of
48 persons who ate chicken fried rice, compared with none of 16 who did
not.
The rice had been cooked the night of July 20 and cooled at room
temperature before refrigeration. On the morning of the lunch, the rice
was pan-fried in oil with pieces of cooked chicken, delivered to the
day care centers
at approximately 10:30 a.m., held without refrigeration, and served at
noon without reheating.
Following the outbreak, health officials recommended to day care
staff and restaurant food handlers that the practice of cooling rice or
any food at room temperature be discontinued, food be maintained at
proper temperatures (i.e., below 41 F {5 C} or above 140 F {60 C}), and
a thermometer be used to verify food temperatures.
Analysis
The emetic ("short incubation") form of the disease, which occurred in
this outbreak, is mediated by a highly stable toxin that survives high
temperatures and exposure to trypsin, pepsin, and pH extremes; the
diarrheal syndrome is mediated by a heat- and acid-labile enterotoxin
that is sensitive to proteolytic enzymes.
The diagnosis of B. cereus food poisoning can be confirmed
by the isolation of greater than or equal to 105B.
cereus organisms per gram from epidemiologically- implicated food.
Underreporting of such outbreaks is likely because illness associated
with B. cereus is usually
self-limiting and not severe. In addition, findings of a recent survey
about
culture practices for outbreaks of apparent foodborne illness indicate
that
20% of state public health laboratories do not make B. cereus
testing
routinely available.
Fried rice is a leading cause of B.
cereus emetic-type food
poisoning
in the United States. B. cereus
is frequently present in uncooked rice,
and heat-resistant spores may survive cooking. If cooked rice is
subsequently held at room temperature, vegetative forms multiply, and
heat-stable toxin is produced that can survive brief heating, such as
stir frying. In the
outbreak described in this report, vegetative forms of the organism
probably
multiplied at the restaurant and the day care centers while the rice
was
held at room temperature.
The day care staff and restaurant food handlers in this report were
unaware that cooked rice was a potentially hazardous food. This report
underscores the ongoing need to educate food handlers about basic
practices for safe food handling.
