Chapter 12. Food Microbiology I. H. Siddique

 

12.1 CHAPTER REVIEW:

The foods serve as an important media for transmission of the organisms (present in foods) to the consumers. However, most of the microorganisms contained in properly handled and prepared food are harmless, although improper storage may lead to food spoilage. If pathogenic bacteria are present on food, they may cause food poisoning of food infection or both. By the same token, pathogenic organisms other than bacteria may likewise be transmitted when present in foods. Food poisoning is caused by toxins produced by an organism; food infections are caused by pathogens in food coming from infected animals or contaminated by infected persons during the processes prior to their consumption. The most commonly found food-borne pathogens, the microorganisms important in food microbiology, the various types of food infections, food poisons and methods of prevention are discussed in this chapter. 

12.2 Introduction: 

Role of Foods in Microbial Disease

Most foods contain viable bacteria unless they have just been thoroughly heated or made sterile. Usually the microorganisms in foods are harmless if the foods are properly handled and prepared. Many of the non-pathogenic organisms may grow in the foods (which usually serves as a good medium) and cause spoilage; pathogenic organisms may cause food poisoning or food infection or both. Most commonly found food-borne pathogens are:

A. The important food-borne bacteria are:

l. Genus Salmonella

2. Genus Shigella

3. Genus Escherichia (Enterotoxigenic E. coli)

4. Genus Campylobacter causes gastroenteritis

5. Genus Clostridium: Cl. botulinum causes botulism, a dangerous form of food poisoning and Cl. perfringens causes acute but transitory gastroenteritis.

6. Genus Staphylococcuscauses one form of food poisoning by a toxin known as enterotoxin. Genus Streptococcus - (beta type) sometimes contaminates milk and other foods.

7. Genus Brucella- cause undulant fever or brucellosis in man.

8. Genus Mycobacterium - bovine and human tuberculosis via meat and milk.

B. The important viruses which are transmitted via foods are :

Enteroviruses and epidemic hepatitis virus

C. The important protozoa which are transmitted thru foods are: 

Entamoeba histolytica, some helminths and tapeworms

D. Other important microorganisms which are important in food microbiology are:

l. Molds.

Common term applied to certain multicellular, filamentous fungi whose growth on food usually are readily recognized by a fuzzy or cottony appearance.

2. Yeast and Yeast-like Fungi.

Yeasts are defined as fungi whose usual dominant growth form is unicellular. They may be useful or harmful in foods. Yeast fermentations are involved in the manufacture of foods like bread, beer, wines, vinegar, surface-ripened cheese and yeasts grown for enzymes and foods. Some yeasts are undesirable as they cause spoilage.

 

12.3 Bacteria

and their relationship to the preservation or spoilage of foods.

A. The presence of capsules or slim layers could account for ropiness of a food. Also, the capsules increase resistance of bacteria to adverse conditions like heat or chemicals.

B. Endospores of Bacillus and Clostridium are more resistant than vegetative cells.

C Formation of cell aggregates - it is more difficult to kill all bacteria in interwined chains or sizable clumps than to destroy separate cells.

12.4 Cultural Characteristics in Food Bacteriology -

Bacterial actions on foods make it unattractive in appearance. Pigmented bacteria cause discoloration on the surface of foods. In liquid foods the growth of bacteria may result in undesirable cloudiness or sediment. 

12.5 Physiological Characteristics in Food Bacteriology

The presence and growth of bacteria produces chemical changes in foods, such as hydrolysis of complex carbohydrates to simple ones, break down of proteins to polypeptides, amino acids, ammonia or amines, and of fats to glycerol and fatty acids. During these actions product such as alcohols, organic acids, aldehydes, ketones and gases are formed.

 12.6 Genera of Bacteria which are important to Food Bacteriology

A. Family Pseudomonadaceae includes the following genera:

l. Pseudomonas - spoilage bacteria

2. Acetobacter - vinegar bacteria

3. Photobacterium - luminescent bacteria

4. Halobacterium - are chomogenic and cause discoloration of foods,

B. Family Spirallaceae

includes genus Vibrio which may be present in salt and fish and meat brines.

C. Family Achromobacteraceae.

There are 3 genera of this family.

1. Alcaligene

produce alkaline reaction-ropiness of milk and comes from manure, water, dust and feeds, etc.

2. Achromobacter

unpigmented bacteria are confused with

pseudomonas; cause spoilage of meats, poultry, fish and produces slimy growth on foods.

3. Flavobacterium

yellow to orange pigmented species cause discolorations on surfaces of meat, butter, poultry, milk, etc.

D. Family Enterobacteriaceae

includes the following genera:

Genus Escherichia,

Aerobacter, Klebsiella

Paracolobacterum.

These Coliform organisms are indicative of sewage contamination. Genus Erwina - the species of this genus are plant pathogens that damage plants and fruits. In vegetables, markets, the disease is called soft rot.

2. Genus Proteus causes food poisoning.

3. Genus Salmonella may cause food infections.

4. Genus Shigella causes bacillary dysentries.

E. Family Brucellaceae includes

Franciesella tularensis (human tuleremia from rabbit) and brucella species from food animals.

F. Family Micrococcaceae

includes Genus Micrococcus which ferments sugars and produce acids and has the following:

1. Staphylococcus aureus produces enterotoxin which causes food poisoning.

G. Family Lactobacillaceae

includes Genus Lactobacillus. This are classified into:

Homofermentative which produce acetic acid, CO2 and

Heterofermentative which produce volatile products, including alcohol, in addition to lactic acid.

H. Family Corynebacteriaceae

includes genus Corynebacterium and Arthrobacter.

I. Family Propionibacteriaceae

the members of this family ferment lactic acid from carbohydrates. They are used in Swiss cheese to aid eyes and holes and contribute to various flavors.

J. Family Bacillacceae

have genus Bacillus and Clostridium. (B. stearo-thermophilus, B. coagulans, B. macerans, etc. and C. butyricum, C. ther-mosaccharolyticum, C. putrafaciens etc.)

12.7 Food Poisoning

The kinds of illnesses which may occur by ingestion of foods are summarized below.

A. Individual idiosyncrasies

B. Toxemia

from foods such naturally poisonous or accidentally contaminated with poisons such as arsenic; foods which contain poisons of bacterial infections such as staphylococci.

C. Foodborne infections

such as Salmonella infections.

D. Hypersensitivity

or allergic reactions may be caused by some foods.

E. The following types of food poisoning of bacterial origin may occur in humans.

1. Botulism

A result of ingestion of performed toxins of the types of Clostridium botulinum in foods. These toxins are neurotoxin and are among the most potent poisons known. The toxin is formed when the organisms finds a suitable anaerobic environment. Botulism in this country is mainly due to types A and B; however, type E botulism has involved fish and fish products. Symptoms of human botulism follow within a day or two after ingestion of food containing the toxin. Unlike tetanus toxin, which irritates motor nerve cells, producing titanic convulsions, botulinal toxin blocks nerve terminals (myoneural junctions) so that paralysis occurs. This paralysis progresses downward from eye, to face, throat, speech, swallowing, arms and so on. Prognosis depends largely on the amount of the toxin swallowed. When the thoracic muscles become fully involved, respiration is impossible and in the absence of artificial respiration, death supervenes. The appearance, taste, or odor of the poisoned food may or may not be bad. Food often appears to be normal and gives little or no hint or the presence of toxins that cause botulism. 

Prevention of Botulism

Protection against botulism is easy because the toxin (not the spores) is destroyed by 10 minutes of boiling. For absolute safety, all home canned foods should be held at boiling, after opening the can, for at least 10 minutes before eating. Botulism from commercial canned foods is uncommon in the United States.

 

Botulism Antitoxin

There is an efficient antitoxic serum, which may be used for the prevention of botulism. This trivalent botulinum antitoxin against A, B, and E toxins is available to physicians from the Centers for Disease Control in Atlanta, Georgia. There are six (or seven) serologic types of botulinal neurotoxins. A, B, and E are most common, in human outbreaks; F less so. C type causes botulism in fowl and D most often affects cattle. For therapeutic and preventive purposes polyvalent serum, effective against several types, is generally used. In the United States, human botulism is most often due to types A and B, less frequently to type E (found principally in improperly processed smoked fish as just noted).

2. Staphylococcal Food Poisoning

Another important and common cause of food poisoning in the toxigenic Staphylococcus. It has been found that staphylococci grow in many foods, especially precooked hams, milk, custards, cream fillings, salads and the like. Some strains or species of these staphylococci produce a powerful exotoxin (enterotoxin) when they grow in food. When the food is eaten, food poisoning results, with severe but transitory gastroenteritis, nausea, vomiting, diarrhea, and marked weakness or prostration. Severity depends on the amount of toxin swallowed. No infection occurs. These symptoms come on usually within 2 to 12 hours after eating the toxin. This permits differentiation from salmonellosis, which produces very similar symptoms, but only after a necessary incubation period, usually of 12 to 24 hours. Staphylococcal food poisoning is rarely fatal.

 Properties of Staphylococcal Enterotoxin

Staphylococcal poisoning is due to toxins secreted in foods when the causative organism grows therein. The toxins have been shown to be proteins of low molecular weight, 30,000-35,000, consisting only of amino acids. The amino acid composition and the terminal amion acids of the toxins are known; but the particular sequence of amino acids in the molecules is not known.

It has been found that large numbers of Staphylococcus aureus must be present in foods to cause poisoning. It must be remembered that Staphylococcus aureus may grow to large numbers in foods without causing changes in the odor, taste or physical appearance, thereby providing no warning signal to the consumer of such foods.

 

Effect of Temperature on Staphylococcal Toxin

In contrast to the heat-labile botulinal toxin, staphylococcal enterotoxin can resist 100 C for at least 30 minutes and probably longer. Once formed in food, cooking does not necessarily destroy it, though the staphylococci that produced it may easily be killed by heat.

Table 2: Staphylococcal Enterotoxin B in Phosphate - Saline Buffer (pH 7.4)

Time to inactivate 50% of Toxin (min) Partially purified Temperature Crude toxin toxin 600C (1400F) 204 204 800C (1760F) 14 12 1000C (2120F) 37 24 1100C (2300F) 17 16

Immunological Tests to Determine the Presence of Staphylococcal Enterotoxin

 

Bacteriological tests to determine enterotoxic staphylococci do not provide an absolute indication of their presence in foods, hence certain immunological tests for the presence of toxin have been developed. At the present time the purified toxins (the antigen) of types A, B, C, and D are available and also the purified antitoxins (antibodies) corresponding to these toxins.

The suspected food must be extracted to obtain the toxin and the extract further purified and concentrated by extracting the food with 0.2 M sodium chloride. The extract is then adsorbed on a column of Sephadex G-100 and eluted with 0.2 M NaCl and the eluate concentrated by dialyzing polyethylene glycol.

A modified Ouchterlony technique may is used to identify the toxin. A center well is prepared in an agar plate to which a mixture of the various antitoxins is added. Peripheral wells containing a known toxin, the unknown extract, a known toxin of another type and so are prepared. If after incubation a confluent line of precipitation is obtained with a known antigen or toxin and the extract, then the extract is identified as containing that particular toxin. It is known that four different types of enterotoxin, A, B, C and D can be produced by different strains of Staphylococcus aureus.

3. Clostridium Perfringens.

It is not as strict an anaerobe as Cl. botulinum. Some strains of this organism appear to be a cause of food poisoning characterized by acute gastroenteritis, usually beginning 8 to 14 hours after eating and lasting for 6 to 12 hours.

The toxins A, B, C, D, and E presumably are destroyed by heating, in this resembling Cl. botulinum toxin. Poisoning by Cl. perfringens type A strains (at least nine antigenic types are known) may therefore be avoided by clean preparation of foods and by prompt eating or refrigeration of freshly prepared foods. Poisoning by Cl. perfringens is often associated with cooked meats. In preparing canned foods the same precautions should be used as for prevention of botulism. Relative heat-resistance of common food borne microbial pathogenic agents are given in Table 3.

4. Mycotoxins

are poisonous by-products of the growth of certain eucaryotic fungi in foods. They have been causes of serious economic losses to stock and poultry growers when the animals have been fed moldy products, mainly vegetables, such as ground peanuts, rice and corn. The role of mycotoxins in human disease may be more important than is generally known. Among the mycotoxins are the aflatoxins, produced in foods by Aspergillus flavus and a group of toxins, the rubratoxins, produced by Penicillium purpurogenum and P. rubrum.

5. Foodborne Bacterial Infections

The principal sources of pathogens in foods are infected animals, infected persons who handle or inspect foods, or contaminated ingredients in the foods. The food borne infections are of two general types:

a. Diseases transmitted by foods

are largely enteric diseases which infect through oral route with the production of characteristic symptoms of the diseases as bacillary dysentery typhoid and paratyphoid infections, etc.

b.Food poisoning syndrome

Foodborne infections are characterized by sudden onset, vomiting and diarrhea.

c.Salmonella food poisoning

The ingestion of Salmonella typhimurium and its varieties, Salmonella enteritidis, or Salmonella choleraesuis in food frequently results in the typical symptoms of food poisoning. Isolation and identification of the prganism involved is carried out in a routine laboratory procedure in such cases.

 

Table 3: Relative Heat Resistance of Common Foodborne Microbial Pathogenic Agents

Resistance to 100 C or Pathogenic Agent more for 30 minutes  Salmonella, Shigella, Vibrio cholerae, - Campylobacter enteroviruses, Coxiella, Mycobacterium, Cysts of Entamoeba histolytica Staphyloccus: enterotoxin + cells -  Clostridium botulinum and Cl. perfringens: toxin - cells - spores + Epidemic hepatitis virus + + resistance; - inactivated; + relatively resistant

d. Vibrio parahemolyticus

- is a gram-negative, halophilic marine bacterium which causes gastroenteritis in individuals eating contaminated seafood. In recent years it has been identified as a cause of food outbreaks in the U.S. related to the consumption of shellfish and crustaceans. The symptoms in patients are similar to those salmonellosis and shigellosis, leading frequently to arbitrary diagnosis of these diseases, even though no Salmonella or shigella strains were isolated.

e. Other Food Microorganisms

Proteus (commonly Proteus mirabilis), Pseudomonas aeruginosa, certain intestinal cocci (e.g., streptococcus faecalis Escherichea coli and Bacillus cereus have also been reported to cause infections and result in food poisoning. In addition, viruses (like infectious hepatitis) and parasites (e.g., Entamoega histolytica) may be transmitted through foods.

f. 'Ptomaine Poisoning'

Ptomaines are products of protein decomposition so extensive that the food is partly liquefied and offensive to sight, taste, and olfactory sense. Such putrefied matter is never accepted as food. The condition that used to be called "ptomaine poisoning" was either due infection by Salmonella species or other enterotoxigenic microorganisms.