Food Microbiology &
Helen Billman-Jacobe
Foodborne diseases Intoxications
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Ray and Bhunia Ch 24-25
Intended learning outcomes
• Know the main types microorganisms causing foodborne disease, and understand their significance.
• Understand the main categories of foodborne diseases.
• Be able to describe Staphylococcus aureus and the foodborne diseases it causes (example of an intoxication).
• Be able to describe Clostridium botulinum and the foodborne disease it causes, botulism (example of an intoxication).
Botulism: Clostridium botulinum Three different types of infections:
2. Infant botulism 3. Wound infection
• Symptoms caused by neurotoxin
• Extremely potent, often leads to death
• Acts on nerve synapses, blocks muscle contraction
1. Food consumption with (pre-formed) toxin produced by Clostridium botulinum
Botulism: Clostridium botulinum
Neurotoxin (A,B subunits) ingested with food (inactive form)
Proteases in GIT convert it to active form
Absorbed in small intestine -> bloodstream -> through body
• B-subunit binds to nerve endings at nerve-muscle junction
• A-subunit taken up by nerve cells; metalloprotease; interferes with Acetylcholine release at neuromuscular junction
• BLOCKS release of acetylcholine -> flaccid muscle paralysis
• Often 12 – 48 hours after food consumption, but can be just 2 hr Symptoms:
• Abdominal cramps, vomiting, difficulty swallowing/speaking, double vision, breathing difficulty > respiratory failure -> DEATH
Botulinum toxin (botox)
Clostridium botulinum (for interest only)
Infant botulism – different to adult disease
• Infants 2-6 weeks old, particularly when non-milk foods introduced (e.g. honey!)
• Gut colonisation with in situ toxin production
– because less competitive gut microflora in infants
– pH of stomach not as low as in adults
• Ingestion of spores can be via environmental sources or by ingestion of low levels of spores in foods.
– Note: in adults colonisation does not occur and disease is caused by preformed toxin in foods.
• Symptoms are similar to classical botulism, but harder to recognise because they are so young, and become progressively weaker.
Clostridium botulinum: Gram +ve rods
Clostridium botulinum
Gram positive rods
Anaerobic (obligate)
Spore forming (terminal)
Mainly found in soils, muds (anaerobic)
Sensitive to low pH, low Aw (0.93) and salt
Spores do not germinate when nitrite present Spores heat resistant (115°C); vegetative sensitive Neurotoxins (proteins) produced during cell growth
• Heat sensitive! 90°C for 15 min destroys them
• Released from cells when they lyse
Clostridium botulinum
Isolation and identification:
Often found in low numbers in food (toxin so potent)
Enrichment necessary (e.g. cooked meat medium)
Then inoculated onto HBA (horse-blood agar)
Finally, tested for toxin production/type using specific antisera for each of the 8 types. Needs to be done under appropriate biosafety level; staff need to be vaccinated.
Clostridium botulinum
Strains of this species are classified into types according to the type of toxin they produce
• 7 types (A – G): A,B,E,F cause human disease
• Usually one toxin per strain (more info)
• Type E strains psychrotropic, grow at 3.3 °C (opt 30°C), and most often associated with fish products Extremely potent – miniscule amounts kill!
Human median lethal dose (LD50) of 1.3–2.1 ng/kg
Botulinum toxin
Protein with two subunits
3D structure known
A and B subunits (heavy/light)
Botulism: Clostridium botulinum
Treatment:
• Antitoxin, mechanical breathing, etc.
• May take months to recover
• Antibiotics no use if food poisoning (only wound infections)
Clostridium botulinum: growth conditions
Optimum conditions for toxin production:
• Anaerobic conditions (e.g. canned food)
• Temperature ≥ 4°C
Preventative conditions:
Any or all of the following (hurdles):
• pH 4.6, Aw 0.93, NaCl 5.5%, low °C
• Cooking before eating (toxin is heat sensitive!)
Botulism: food associations
Steps: food contamination, survival of spores during processing, ability to grow
Most outbreaks are fruits and vegetables: home canned vegetables very common source Second level – fish products
Glass jars of Jalapeño peppers that were the source of a botulism outbreak in Pontiac, Michigan, April, 1977.
Botulism: Clostridium botulinum
Prevention of botulism:
• Proper time and temperature during home canning of “low-acid” foods
• Commercial processing: 12D concept (chpt 33)
General directions:
1. proper cooking of foods
2. refrigerate after cooking (not stored too long) 3. pH < 5 (+chilled storage)
4. NaCl of ≥ 3.5% (+chilled storage)
5. Aw ≤ 0.97 (+chilled storage)
Read the “Analysis of a foodborne botulism case” at the end of chpt 25.
There are many other similar examples you can find in PubMed or in textbooks.
Low-acid foods have a pH level greater than 4.6, which means they are not acidic enough to prevent the growth of botulinum bacteria. Examples are:
Asparagus Green beans Beets
Some tomatoes* Figs
Fish and seafood
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