crenated colonies which retain the turquoise-blue of the pH indicator
(bromothymol blue) due to their inability to ferment mannitol, they are
surrounded by a zone of egg-yolk precipitation caused by lecithinase
activity. Pyruvate in the medium improves the egg-yolk precipitation
reaction and a low level of peptone enhances sporulation. Colonies of
B. cereuscan be confirmed by a microscopic procedure combining a
spore stain with an intracellular lipid stain. Spores appear green in a cell
with red vegetative cytoplasm and containing black lipid globules.
Biochem-ical confirmation can be based on an isolate’s ability to produce
acid from glucose but not from mannitol, xylose and arabinose.
Commercial kits are available which claim to detect the diarrhoeal
enterotoxin though they have limited use. One detects the L 2 unit of
HBL, though some outbreak strains do not produce this toxin. The other
detects a protein present in NHE but not in the HBL complex.
7.2.5 Association with Foods
The ability to produce spores resistant to factors such as drying and heat
means that the food-poisoning bacilli are widely distributed in foods. In
most circumstances however they are only a small part of the total flora
and are not present in numbers sufficient to cause illness.
Heat processing will select for spore formers and a number of surveys
have reported a higher incidence ofB. cereusin pasteurized and other
heat-processed milks (typically 35–48% of samples positive) compared
with raw milk (B9% positive). In most of these cases the numbers
detected were low (o 103 ml^1 ), but when pasteurized milk or cream are
stored at inadequate chill temperaturesB. cereuscan grow and cause the
type of spoilage known as ‘sweet curdling’ or ‘bitty cream’. Despite this,
milk and dairy products are rarely associated with illness caused by
B. cereus, although dried milk has been implicated in outbreaks when
used as an ingredient in vanilla slices and macaroni cheese. A possible
explanation is that though liquid milk is an excellent growth medium for
the organism, toxin production is not favoured. One study in Sweden has
linked this with the low aeration in static packs of milk.
The ability of spores to resist desiccation allows their survival on dried
products such as cereals and flours. In the Norwegian outbreaks de-
scribed above (Section 7.2.1), the cornflour used to thicken the vanilla
sauce was implicated. Moderate heating during preparation would not
inactivate the spores and subsequent extended storage of the high-aw
sauce at ambient temperature was conducive to spore germination and
outgrowth.
The emetic syndrome is particularly associated with starchy products
such as rice and pasta dishes. In the UK, its association with cooked rice
has been sufficiently marked for it to earn the soubriquet ‘Chinese
Chapter 7 189