Transmission of Enteric and Food-Borne Pathogens | Microbiology

Let us make an in-depth study of the transmission of enteric and food-borne pathogens.

Human intestinal tract contains millions of pathogenic and non-pathogenic micro organisms. Thus, the excreta of healthy or diseased individuals constitute the principle source of one or many spe­cies of virus, bacteria, protozoa or helminths. Some­times, some pathogens may be excreted in the in­fected urine (Escherichia coli, Proteus, Pseudomonas) which may be transmitted to others by drinking wa­ter polluted by this infected urine.

Feces or urine from the infected persons may be the primary source for:

(a) Drinking water to be polluted through the sew­age, (dysentery);

(b) Even food can act as vector for food borne diseases (cholera, bacillary and amoebic dysentery).

The following are the food borne pathogens:

(a) Family Enterobacteriaceae (Salmonella, Shigella);

(b) Family Bacillaceae (CIostridium);

(c) Family Micrococcaceae (Staphylococcus, Streptococcus);

(d) Family Brucellaceae (Brucella);

(e) Family Mycobacteriaceae (Mycobacte­rium tuberculosis, bovine or human type);

(f) Enterovirus (epidemic hepatitis virus)

(g) Protozoa (Entamoeba histolytica)

(h) Helminths (roundworm)

Pathogenic bacteria may grow in the improp­erly refrigerated or cooked food and may cause:

(1) Food infection, and

(2) Food poisoning or both.

Food Infection:

Food infection is due to the intake of bacteria along with the food (i.e., Typhoid fever due to Sal. typhi. The symptoms of food infection usually begin from 18 hours to several days after the ingestion of the infected food. During this time (incubation pe­riod), the organisms grow sufficiently to liberate the toxin responsible for the symptoms of the disease (Typhoid fever).

Food Poisoning:

Persons suffer from food poi­soning because of the consumption of preformed toxin by bacteria in food, i.e., toxin already produced outside the body in the food. In true food poisoning, there is no infection. In contrast to food infection, the symptoms of food poisoning generally develop within 12 hours or less, because the toxin which causes the symptoms is already formed in the food.

1. Food Infection:

The principle sources of infection are:

(a) Infected animals;

(b) Infected food handlers or cooks; and

(c) Pathogens in the food.

(a) Infected animals:

Food may be infected by two ways:

(i) Animal (rat or mouse) may pollute the food with their excreta.

Sal. typhimurium causes septicaemic lymphoid-like infection in the natural host (rat or mouse). Excreta of the infected mice may therefore infect the food with Sal. typhimurium which after ingestion causes in man “typical food poisoning” Charac­terised by vomiting, abdominal pain and diarrhea.

Flesh from infected animal used as food. Milk and milk products from tuberculous cattle may be highly infectious. Flesh and milk of cattle with brucella infection may cause human brucellosis. Sal. typhi, causal agent of typhoid fever, found in oysters and other sea foods from sewage polluted fishing ground may be infected with Sal. typhi. If these infected foods are eaten, Sal. typhi found in them may cause typhoid fever.

Ingestion of larva of Trichinella spi­ralis in the flesh of infected animal can cause trichinellosis in man. Thorough cook­ing may make these foods safer. Rats can transmit in the urine leptospira which causes leptospirosis or Weil’s disease (hemorrhagic jaundice) in man.

(b) Infected Carriers:

The hands or forearms of food handlers or cooks can be soiled by saliva droplets containing the organisms of common colds, influenza, scarlet fever, diphtheria or tuberculosis or by toxin pro­ducing staphylococci in the respiratory se­cretions.

Pimples or boils on the forearms of the cooks may contaminate the food. Besides sneezing, talking and coughing over the unwrapped food displayed in open cases in the restaurants and handling foods with soiled hands may infect the food. Food handlers may be carriers of intestinal pathogens (infectious hepatitis virus, salmonella, shig­ella, and cysts of Entamoeba histolytica) on the hands.

(c) Pathogens in the Foods:

Possible patho­gens (Proteus sp., Clostridium welchii, Pseudomonas aeruginosa, Streptococcus faecalis may grow in the food and decom­pose it. Some of the products of decom­position of food may cause irritation of the intestine causing gastroenteritis.

2. Food Poisoning:

Bacterial food poisoning is due to toxin liber­ated in the food during their growth in the food, not in the patient:

(a) Staphylococcal food poisoning due to pre- formed enterotoxin produced in the food.

(b) Botulism caused by CI. botulinum.

(c) CI. welchii food poisoning due to toxin of some strain of CI. welchii.

(d) Ptomaine poisoning.

(a) Staphylococcal Food Poisoning:

Staph, aureus are Gram-positive cocci arranged in clusters. They are most pathogenic and produce an enzyme called coagulase which causes citrated blood plasma to co­agulate. In addition to this enzyme, they produce also a potent toxin in the improp­erly precooked food outside the body.

Later, it is called enterotoxin as it acts on the intestinal mucosa. Enterotoxin is an intestinal toxin formed by 30-50 per cent of strains of Staph, aureus. It is heat stable and withstands an exposure at 100°C for a few minutes; when ingested the food in which Staph, aureus was growing profusely and pro­ducing toxin; this enterotoxin causes nau­sea, vomiting and diarrhea within 6 hours, which is called Staphylococcal food poisoning. Five types of enterotoxin — A to E — are produced. Staphylococcal food poisoning is rarely fatal.

(b) Botulism is caused by CI. botulinum:

This causal agent is strictly anaerobic, Gram- positive bacillus. It bears oval, sub-terminal bulging spore. It is motile and has no cap­sule. It is widely distributed saprophytic organism in the soil, on the vegetable fruits, leaves, produces preformed toxin (neurotoxin’s food).

Its spores withstand moist heat at 100°C for several hours. In­sufficient heating in the process of pre­serving foods is an important factor in the causation of botulism. Thus sufficient heat­ing should be achieved in all parts of the can or tin food in canning factories. The appearance, taste, odour of the food is often deceiving.

Human cases of botulism originate after inges­tion of variety of preserved spoiled foods (ham, sau­sage, home canned meat, fish, vegetable etc.). The intoxication follows absorption from the intestine of toxin performed by this bacillus in the food, but there may be also some formation of a powerful exotoxin by the organism after ingestion. This toxin is destroyed at a temperature of 80°C for 30-40 minutes.

The esti­mated lethal dose of this potent toxin for one mouse is 0.000000033 mg. In other words, one kilogram of this potent toxin may kill the world population. It ap­pears that this toxin interferes with the mechanism of acetylcholine release from the endings of the motor nerve cells of parasympathetic system.

The early clinical symptoms may suggest a coro­nary thrombosis. The neurotoxic signs and symptoms are vertigo, oculomotor and pharyngeal paralysis, dif­ficulty in speech, swallowing and breathing, loss of voice (aphonia). Death due to respiratory failure oc­curs a few days after the onset of symptoms, because of the thoracic muscle involvement.

(c) CI. welchii Food Poisoning:

CI. welchii is Gram positive bacillus and bears sub-ter­minal spore. In contrast to CI. botulinum, it is non-motile and is capsulated. It is dis­tributed in soil, faeces. Strains of CI. welchii conforming in most respects to type A, but producing non-haemolytic or feebly haemolytic colonies on horse blood agar and strains which do not produce theta toxin are associated with a mild form of food poisoning.

The exact mechanism of CI. Welchii food poisoning is not well un­derstood, but it depends upon the ability of the food poisoning producing strains to produce an enterotoxic exotoxin. In­gestion of large number of viable organ­isms in the contaminated food is neces­sary for the production of typical food poisoning in man.

Typical symptoms are abdominal cramps starting after 8 to 12 hours after ingestion, followed by diarrhea. Fever and vomiting are not typically en- countered. The condition is usually tran­sient and normally subsides within 24-48 hours.

CI. welchii food poisoning may be avoided by clean preparation of food and by prompt eating, or refrigeration of freshly prepared foods. In preparing canned foods, the same precaution should be taken as in the prevention of botulism.

Ptomaine Poisoning:

Ptomaine’s are products of protein decomposition that liquefies the food, which is offensive to sight, taste and olfactory sense. Such putrefied matter is never accepted as food. Acute gastroenteritis associated with ptomaine poisoning does not simulate the symptoms of botulism.

Prevention of Food Infection and Food Poisoning:

Cooking:

Cooking may or may not prevent food infection and food poisoning, because vegetative cells of CI. botulism, CI. welchii, Staph, aureus, Salmo­nella, Shigella, Vibrio cholerae and CI. welchii are all inactivated by boiling (100°C) for 10 minutes. Spores of CI. botulinum will survive oven baking and steam pressure cooking (autoclaving) unless prolonged at high temperature.

Spores of CI. welchii are also very thermo-stable. Staph, aureus enterotoxin will resist boiling for one hour or more. Because of the variable effect of cooking on bacterial cells and spore, food to be eaten uncooked should be selected with care. Never eat or serve food kept open or handled in warm place for more than 4 hours. The organisms will not grow in acid foods, dry foods (salted nuts and cookie) but will multiply in gravies, in cheese, cream, bread or rice pudding.

Refrigeration:

Efficient and prompt refrigera­tion of foods just after preparation prevents the bac­terial spoilage and growth of harmful bacteria which entered into the food during handling. Sterilised ba­bies’ bottles can be refrigerated with complete safe­ty.

Freezing is another method of inhibiting the growth of micro organisms. Frozen foods are not nec­essarily sterile; but most will keep for months or years; because all the metabolic activities of micro organ­isms are stopped. Medicines in solution are liable to deterioration or microbial spoilage are preserved by freezing and refrigeration (caution: Aqueous fluids may break glass containers on freezing).

Supervision of Food Handlers:

Before appoint­ing food handlers or cooks in hotels, restaurants, they should be examined for general cleanliness, enteric infections, the presence of boils or sores due to sta­phylococci or diphtheria, tuberculosis or syphilis. The person harbouring infectious disease organisms should not be allowed to handle the food in the kitchen or drinking place. The hands should always be washed thoroughly before preparing or serving the food. After blowing the nose and after using the toilet the hands should be well washed.

Proper Dishwashing:

The method of proper dishwashing has great bacteriological importance and it is of no use from the standpoint of preventive medi­cine and public health, but it is too common a prac­tice. Washing dishes in a pan of lukewarm water with a dirty mop with a detergent or soap water merely distributes the bacteria over them, but it will not de­stroy them.

Hand dishwashing. The proper hand dishwashing of spoons, forks, cups and glasses with plenty of scald­ing water with detergent is essential. After hand dishwashing, the dishes should be rinsed in a basin of cool, clean water containing 50 parts per million of free chlorine.

Alternatively, the dishes can be dipped in boiling water for 5 minutes. Organisms associated with pneumonia, diphtheria, tuberculosis, Vincent’s angina, scarlet fever, sore throat, influenza can be transmitted from one person’s mouth to another’s by poorly washed hands or forks. Machine dish washer is a very effective method and much hotter water can be used. Dishes and uten­sils of the patient should be boiled so to prevent the transmission of the disease by saliva and sputum.

Milk-Borne Disease:

Tuberculous cows excrete Mycobacterium tuberculosis bovine type through their milk. Rabies virus, Coxiella burnetti and Brucella organisms are disseminated through the milk of in­fected cows. Strept. pyogenes causing septic sore throat and scarlet fever may gain entrance into the milk directly from the infected persons who handle the milk or indirectly by means of cow’s udder in­fected by infected milker.

Since cows are not nor­mally infected by shigella, salmonella, and diphtheria bacilli, these bacilli may find their way into the milk after it has been drawn. Milk kept in warm place is the best medium for the growth of micro organisms ex­cept C. burnetti and possibly Mycobacterium tuberculosis bovine type. Milk from mastitis (infected ud­der) cases due to Staph, aureus may contain pre­formed toxin (enterotoxin) causing severe food poi­soning in man, in spite of pasteurization, since sta­phylococcal enterotoxin is heat stable.

Pasteurization:

Both holder method (62°C for 30 minutes) and flash method (80°C for 15-30 sec­onds) or high temperature, short time (HTST) process can kill all pathogenic microorganisms in dairy prod­ucts. Many harmless bacteria may survive, so pasteurization disinfects but does not sterilize. So, after pasteurization, the milk should be refrigerated promptly and bottled under hygienic conditions.