Time and Binary Fission
Binary fission literally means splitting into two and is the term for how bacteria grow
or multiply. Under ideal conditions bacterial will double in size every 10 to 20
minutes. Large numbers of bacteria can grow in a very short period of time, for
example, one bacterial cell can become 1 billion in as little as five hours.
Spores
Spores are a resistant resting stage of two bacterial genera, namely Clostridium and
Bacillus. Examples of these two genera have already been covered, C perfringens,
C botulinum and B cereus. Further examples of non-food safety spores include B
anthracis (Anthrax) and C tetani (Tetanus). Spores can survive high temperatures,
up to 3500°C, which is equivalent to the temperature of a rocket’s thrust. They can
survive very low temperatures down to as little as -150°C. They can survive
chemicals such as disinfectants and they can survive dehydration. Spores are very
difficult to kill; in fact spores have been known to exist for millions of years. Several
years ago scientists found spores in the intestines of bees that had been preserved
in amber. The age of the bees was estimated at 42 million years. The spores were
extracted from the intestines and revived. Spores are harmless under most
circumstances apart from infant botulism, which was described earlier. Spores are
only a problem when they germinate. Think of spores as dried, un-germinated seeds.
If kept dry they will survive indefinitely and not cause a problem. Spores are a
problem when they germinate because they will then vegetate as bacterial cells. We
ingest spores on a daily basis, via the environment and on our food. As we have an
acidic stomach, the spores will not germinate and will pass through our GI tract to be
excreted. In a very young infant’s stomach, the parietal cells, which excrete the
hydrochloric acid for digestion, have not yet fully developed and the spores would
germinate, vegetate and cause poisoning.
The spore forming bacteria are a problem in the canning industry. For example, if C
botulinum spores are present in a sealed can of food they could germinate and
vegetate. Clostridia are the only bacteria that do not need oxygen or air to grow.
Therefore in the sealed can, which is a vacuum, spores can germinate and vegetate
to large numbers in a short period of time. As the bacteria grow they release waste
materials called toxins, in this case Botox, a deadly neurotoxin. The canning industry
has come up with a way of destroying C botulinum spores. They use a commercial
sterilisation temperature of 121°C for at least 3 min. This time temperature
combination will destroy the spores.
Rice Cooking
Spores will remain dormant in dry conditions; they require moisture in order to
germinate. They also need a warm temperature, an ideal temperature being 37°C.
All rice must be considered to contain B cereus spores. As the raw, uncooked rice is
dry, the spores are harmless. If the rice is cooked, the spores remain intact as the
temperature of the cooking rice is too high for germination. If the rice is drained and
served immediately or retained above 63°C, the spores will not germinate. During
digestion, the spores will not germinate due to the temperature of the food and the
acidity of the stomach.
If rice is to be served cold or used the next day, for example in stir fried dishes, it
must be cooled as quickly as possible after cooking. After draining the hot, cooked
rice, it must be held under cold running water until cool to the touch. The rice is then
drained and refrigerated until required for use. The spores have not had enough
time to germinate as the rice was brought through the temperature danger zone
quickly.
Many takeaways and restaurants do not, however, adopt this approach. After
cooking, the rice remains in the kitchen area to cool down overnight. During the slow
cooling period, the spores will start to germinate at 37°C. As there is moisture and
protein in rice the germinated spores will vegetate. During growth, B cereus release
two exotoxins into the rice. The exotoxins are heat stable and cannot be destroyed
by cooking. When the rice is used the next day it contains B cereus bacteria and
exotoxins. If the rice is reheated, the bacteria turn back into spores, which will not
cause a problem, but the poisons will remain, which will cause food poisoning.
Even if the rice is produced correctly by a takeaway, food poisoning can still result.
For example, you have bought some special fried rice, but could not eat it all and
want to finish it the next day. Always ensure the rice is refrigerated, not left out
overnight, as the spores will germinate as above.
Toxins
Toxins are defined as organic poisons i.e., they are produced by living organisms
such as animals, plants, and bacteria. Bacteria produce two types of toxins, namely
exotoxins and endotoxins. S aureus and C botulinum produce exotoxins. Exo
means produced outside the body. Therefore an exotoxin is a poison produced by
bacteria and then ingested.
Another type of poison is known as an endotoxin which is produced inside the body
from dying bacteria or from bacteria that produce spores. Gram negative bacteria
have material called LPS on their external membrane which produces poisoning. An
example of gram negative bacteria that release endotoxin are Salmonella spp.
Exotoxins are more toxic than endotoxins. The most virulent toxin on the planet is
Botox, an exotoxin, produced by C botulinum. It is classed as a neurotoxin which
acts on the synapses at neuron/muscle junctions, causing flaccid muscle paralysis.
Death is usually caused by respiratory failure.
Exotoxins come in three categories:
- Cytolytic, which causes cell death (lysis) by breaking down cell membranes.
- AB, where the B sub-unit attaches to the cell, allowing entry of the A sub-unit into the cell causing damage and lysis.
- Superantigen, which causes the immune system cells to become overstimulated thus producing inflammation and tissue damage.
Enterotoxins are exotoxins that affect the Gastrointestinal (GI) Tract, causing an influx of fluids from body cells and blood into the intestines causing diarrhoea and/or vomiting.
Examples of enterotoxin producing bacteria include: S aureus, C perfringens, B cereus, E coli and S enteritidis. Endotoxins have less severe effects than exotoxins with the symptoms including fever, vomiting and diarrhoea.