• Food borne disease are divided into two main types i.e. poisoning and infection.
• Food poisoning:
  • If disease is caused by infection of toxic chemical present in food, it is called food poisoning.
  • Food poisoning is further divided into two types i.e. chemical poisoning and food intoxication.
  • If poisoning is caused by injection of poison which is accidently or purposely added in food, it is called chemical poisoning. E.g. disease caused by injection of rat poison in food.
  • If poisoning is caused by injection of toxin secreted by living being, it is called food intoxication.
  • Sometimes tissue of food is itself toxic and causes intoxication. E.g. mushroom poisoning.
  • Some microorganisms contaminate food and produce toxin in the food.
  • If intoxication is caused by injection of microbial toxin in food, it is called microbial food intoxication.
• Food infection:
  • If disease is caused by ingestion of viable microorganisms in food, it is called food infection.
  • In food infection microorganisms multiplies in intestinal tracts and cause disease.
  • There are two types of food infection i.e. invasive type and enterotoxigenic type.
  • In enterotoxigenic type of food infection, microorganisms produce enterotoxin in intestinal tract that cause disease. E.g. food infection caused by EPEC, vibrio cholera.
  • In invasive type of food infection microorganisms cause disease by invading through intestinal tract. E.g. food infection caused by Shigella, EIEC.
  • In some food borne disease, food simply serves as carrier of microorganisms and microorganisms do not multiply in food. For e.g. in case of vibrio cholera, Corynebacterium diphtheria, Mycobacterium bovis.
  • In other food borne diseases food serves as cultivation medium for growth of microorganisms and microorganisms multiply in food. E.g. EPEC, vibrio parahaemolyticus.

Difference between food infection and food intoxication:

Food poisoning caused by Staphylococcus aureus (Staphylococcal food intoxication):

• Disease caused by Staphylococcus aureus is intoxication caused by ingestion of pre-formed enterotoxin in food.
• It is one of the most commonly occurring food borne disease.

Etiological agent:

• Staphylococcus aureus is gram positive cocci that occur in grape like cluster.
• Intoxication is caused by enterotoxin producing strain of S. aureus.
• Most enterotoxin producer are coagulase +ve, however, not all coagulase +ve Staphylococcus aureus produce enterotoxin.
• It can tolerate high concentration of salt 10-20% of NaCl and sugar (50-60%).
• It is nitrate tolerant and can grow in curing solution of meat.
• They are proteolytic but usually do not produce bad smell in food.
• Therefore, food spoilage caused by S. aureus is difficult to judge by its smell and appearance.
• Temperature range for growth and toxin production is (14-46)^oC and minimum value of A_w for growth is 0.86.
• Maximum pH for growth in food is 8.
• Natural habitat of S. aureus is skin and upper respiratory tract of human being and other animal.
• From these sources, it contaminates in food.

Enterotoxin:

• Staphylococcus aureus produces six antigenic type of enterotoxins.
• They include A, B, C₁, C₂, D and E, of which, type A and D are more frequently associated with food intoxication.
• Condition that favor multiplication of S. aureus also favor toxin production.
• Toxin is produced at an appreciable rate at (15.6-46.1)^oC and production is best at 40^oC.
• Under best condition, sufficient amount of toxin appears in food within 4-6 hrs.
• At lower temperature, it takes very long time to produce enough enterotoxin in food.
• Therefore, poisoning by S. aureus can be prevented by adequate refrigeration during storage of food.
• An important characteristic of enterotoxin is that it is heat stable.
• Therefore, this enterotoxin is not easily damage by normal cooking of food.
• So, food poisoning by S. aureus, occur when food is not sufficiently heated just before eating.

Food involved:

• Creamed filled bakery product are more commonly associated with S. aureus food poisoning.
• However, many other food products like meat and meat products, milk and milk products, vegetable, egg containing products etc. are also found associated.

Pathogenesis of staphylococcal food intoxication

• Staphylococcal food poisoning is caused by ingestion of pre-formed toxin in food.
• Enterotoxin acts on intestine to induce vomiting and diarrhoea.
• Production of diarrhoea by enterotoxin is not clear.
• Enterotoxin binds to the wall of small-intestine, stomach and large intestine with high affinity and it causes inflammation and irritation of lining of stomach and intestine.
• Induction of vomiting by enterotoxin occurs due to stimulation of central nervous system.
• Enterotoxin binds to receptor of vomiting in intestine, which stimulates vomiting center in brain via vagus and sympathetic nerves.

Disease symptoms of staphylococcal food intoxication :

• Symptom of the disease vary with amount of enterotoxin ingested and the immunity of the host.
• Staphylococcal food poisoning is characterized by very short incubation period of 2 – 4 hrs.
• Most common symptoms include vomiting, diarrhea, retching (sound in stomach), abdominal cramp, nausea.
• In severe case blood and mucus may be seen in stool and vomits.
• Mortality rate is extremely low and the disease is usually self-limiting.

Treatment of staphylococcal food intoxication:

• Treatment of diarrhea include giving salt solution to counteract loss of water and electrolyte during diarrhea.

Prevention and control of staphylococcal food intoxication:

• Staphylococcal intoxication can be prevented by:
• Preventing contamination of food:
  • Good personal hygiene of food handler is very important to prevent contamination because S. aureus enters into food from skin and upper respiratory tract of human being.
  • Person who is carrier of S. aureus should not be allowed to handle food.
• Preventing growth and toxin production:
  • Growth of S. aureus in food and toxin production can be prevented by adequate refrigeration of food, by lowering A_w value of food below 0.86, by adjusting pH at very low level etc.
  • Enterotoxin of S. aureus is heat stable. Therefore, it can be damaged by sufficient heating the food before eating.

Food borne disease: food poisoning and food infection with example

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