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2
How Clostridium Botulinum Can Poison Food
C. botulinum, commonly found in soil, grows best when oxygen and acid
levels are low. In an unfavorable environment, the bacteria can form
protective cases, thus becoming spores that survive in a dormant state until
conditions improve. Botulism, or botulinus poisoning, although rare, can be
1
Life stages
Spore-forming bacteria live
in the soil.
Bacteria form spores under
unfavorable conditions.
When conditions improve, spores
germinate into bacteria and release
toxins into food.
2
Contamination
Raw food is contaminated with
Clostridium botulinum bacteria;
spores form in the presence of
oxygen.
Contamination risk
Low High
Bacteria Spore Toxin
a special concern for sous vide or canning processes that eliminate oxygen.
If the procedure is done incorrectly, the spores can germinate within the
food. The resulting bacteria quickly multiply and produce a potent neurotoxin
that is among the most deadly known.
3a
3b
Contamination risk
Low High
Unsafe storage
If improperly canned or otherwise
stored in the absence of oxygen,
especially in a high-pH environment,
spores can germinate into
toxin-releasing bacteria.
Safe storage
Refrigeration, immediate serving,
and proper canning or sous vide
preparation can all curtail spore
germination.
Contamination risk
Low High
Bacteria Spore
Bacteria Spore
Toxin
Toxin
4a
Unsafe cooking
Most cooking conditions may
kill the bacteria, but the toxins can
remain.
Contamination risk
Low High
Bacteria Spore
Toxin
and roast a turkey that somehow has become
contaminated. If you follow the usual temperature
guidelines, the bird will be cooked to an internal
temperature of 74 °C / 165 °F. The high temperature
will kill the bacteria, but it also encourages
the germination of the remaining spores, which
leads to more toxin-producing bacteria.
If the turkey is eaten right away, there is no
problem. If the turkey is allowed to sit for too long,
however, bacteria from the germinated spores
could produce toxins that will sicken people who
eat it. This is true even if the turkey is refrigerated
whole because the center of the bird could take
hours to cool down.
Unless you take active steps to cool the turkey
quicklyin a blast chiller, for examplethe
temperature of the stuffing-filled body cavity
drops very slowly. And once the temperature falls
below 50 °C / 122 °F, C. perfringens can begin to
grow. If the interior temperature stays below that
threshold but above standard refrigeration
tem peratures for even a few hours, toxins can
begin to accumulate. Because C. perfringens
tolerates exposure to salt fairly well, similar
outcomes can result from inadequate cooling of
cooked hams, corned beef, and other cured meat
products.
The neat categories of invasive infections,
noninvasive infections, and food poisoning cover
most cases, but some modes of food poisoning
combine several mechanisms. A few bacteria have
become particularly adept at this combined
strategy, though much depends on the relative
health of their human targets and the particular
way they encounter the pathogen. In the typical
scenario, C. perfringens produces a toxin that
causes food poisoning. But if a victim ingests
a sufficiently large dose, say about 100 million
cells, the bacteria can noninvasively infect the
victim’s gut and begin secreting toxins there.
Under the right conditions, a person infected
with the type C strain of C. perfringens can fall ill
with a rare but very serious disease known as
pig-bel, enteritis necroticans, or necrotizing
enteritis. Under these circumstances, the dose of
toxin is high enough that intestinal tissues begin
to die (necrotize), which can lead to a very severe
and often fatal blood infection.
Physicians first diagnosed and named pig-bel in
New Guinea, where it sickened people who feasted
on whole pigs cooked in pits in the ground. Those
pigs went through the same heating and slow
cooling cycle described for turkey, although the
larger size of the pigs meant that the cycle was
extended even longer. A contributing factor in the
New Guinea cases may have been one of the other
dishes on the menu: sweet potatoes. Unfortunately,
the sweet potatoes probably contained a protein
that blocks the action of a stomach enzyme
that otherwise would help kill C. perfringens.
Clostridium perfringens bacteria (shown
below) make spores (shown on page 110)
that can get into a deep cut or a wound
with dead tissue, where they can
germinate and grow to produce the
condition called gas gangrene. Because
C. perfringens is anaerobic, it cannot
invade living tissue that is properly
oxygenated, so gas gangrene is a
noninvasive infection. This type of
infection, of course, is not foodborne,
illustrating the diverse ways that one
organism can cause trouble.
140 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 141