by salt (Table 7.4). The rate of growth and toxin production at the lower
temperature limit is slow and will be reduced still further by any other
factors adverse to growth. Experimental studies have indicated that
storage periods of 1–3 months are necessary for toxin production at
3.3 1 C, although this period can be markedly reduced at higher temper-
atures still within the chill range. Vacuum-packed herrings inoculated
with 100 spores per pack became toxic after 15 days storage at 5 1 C.
Most cases of botulism in humans are due to toxin types A, B or E and
the incidence of other types in human illness is extremely rare. Group III
strains producing toxin types C and D are usually associated with illness
in animals and birds. Type G toxin has been incriminated in human illness
largely as a result of its isolation at autopsy from people who had died
suddenly and unexpectedly. Since botulism was not necessarily the cause
of death in these cases, and there have been no reports of the presence of
type G in foods, its role in foodborne illness is questionable. Group IV
strains which produce type G toxin and some non-toxigenic clostridia
have been re-designated as a new species,Clostridium argentinense.
Although it is found occasionally, growing in the alimentary tract of
birds and mammals,C. botulinumis essentially a soil saprophyte. It
occurs widely, although the geographical distribution is not uniform.
Surveys conducted in the United States found type A to be the most
common in the Western States, rare in the Mississippi Valley but less so
along the Eastern Seaboard where type B was predominant. This distri-
bution was reflected in outbreaks of botulism in the United States in the
period 1950–1979; when 85% of those west of the Mississippi were due to
type A toxin and 63% of those to the east were due to type B. In
European soils type B tends to be more common than type A.
Aquatic muds provide a moist, anaerobic, nutrient-rich environment in
which clostridia can flourish, so isolation ofC. botulinumfrom these sources
is more frequent than from soils. The psychrotrophic type E has been
particularly associated with this environment in regions such as western
North America, Japan and the Baltic sea coasts. As a consequence, type
E is often responsible for outbreaks of botulism where fish is the vehicle.
The minimum pH at whichC. botulinumwill grow depends very much
on factors such as temperature, water activity and the acid used to adjust
the pH. The consensus has long been that a pH around 4.7 represents an
absolute minimum and this fact has had important practical implications
for the canning industry (see Chapter 4). Non-proteolytic strains have a
lower acid tolerance and are generally inhibited at pH 5.0–5.2. Reports
have appeared of growth and toxin production at pH values as low as 4.0
in protective, high-protein containing media but this does not reflect the
situation in acid canned foods which are generally low in protein. In
cases where botulism has occurred in foods where acidity is an important
protective hurdle, such as canned fruits, it has been as a result of other
Chapter 7 201