Food poisoning/ Food intoxication

Food intoxication is a health condition which caused by consuming contaminated food that contains certain toxins. It can be caused by

 - Due to naturally occurring toxins in some foods. Ex: Endemic ascitis (Pyrrolizidine alkaloids)

 - Due to toxins produced by certain bacteria. Ex: Botulism, Staphyloccal toxins

 - Due to toxins produced by some fungi. Ex: Aflatoxin, Fusarium toxins

 - Due to toxins produced by some algae like dinoflagellates, Diatoms, Cyanobacteria

-  Due to food-borne chemical poisoning

However, most common food intoxication is caused by the toxins produced by certain bacteria. It involves food poisoning in which the organism grows in food and releases a toxin from the cells. The presence of organism in the food is irrelevant to disease production. It is the toxin that gives rise to the disease. Bacterial toxins that produce intoxications are exotoxins of either enterotoxin (affecting the gut) or neurotoxin (affecting the nervous system).

Type of food poisoning	Causative agent	Type of toxin	Mode of transmission	Food commonly involve
Staphylococcus food poisoning	Staphylococcus aureus	Enterotoxin	Sources from where microorganisms enter food	Meat, fish, milk, poultry, custards, cream
Botulism	Clostridium botulinum	Neurotoxin	Spores are transferred from soil into food	Damaged leaky and rusty cans or cans with broken seals
Bacillus cereus food poisoning	Bacillus cereus	Enterotoxin	Spores from soil to food such as cereals and grains	Chinese fried rice spices, reheated boiled rice, corn flour sauce
Perfringes	Clostridium perfringens	Enterotoxin	From human feces to food via hand, flies	Meat dishes, reshuffled dishes, reheated dishes, pie salads

Staphylococcus food poisoning

The causative agent Staphylococcus aureus is gram positive coccus that occurs in singles, pairs, chains, tetrads and irregular shaped clusters. It is present in all places in the environment. Only those strains that produce enterotoxin can cause food poisoning. Food is usually contaminated from infected food handler with an active lesion or carriage can contaminate food.

Pathogenesis: When the food is stored at room temperature for a considerable time period the organism may multiply in the food and produce toxins. S.aureus produces six serologically different types of enterotoxins (A, B, C, C2, D and E). Most food poisoning is caused by enterotoxin A. Isolates commonly belong to phage type III. These enterotoxins are known to be heat stable. The type B is the most heat resistant. Enterotoxins that are inactivated in low temperatures can undergo reactivation in some food. Ingestion of as little as 23 µg of enterotoxin can induce vomiting and diarrhoea. Staphylococcal enterotoxins act as super antigens, binding to MHC II molecules and stimulating T cells to divide and produce lymphokines such as IL-2 and TNF-alpha, which induces diarrhea. The toxin acts on the receptors in the gut and sensory stimulus is carried to the vomiting center in the brain.

Incubation period: Usually 1-6 hours as it’s a preformed toxin.

Clinical features: The onset is sudden and is characterized by vomiting and diarrhea but no fever. The illness lasts less than 12 hours. There are no complications and treatment is usually not necessary.

Botulism

The causative agent is a gram positive anaerobic spore bearing bacillus that is widely distributed in soil, sediments of lakes and ponds, and decaying vegetation.

Pathogenesis: There are seven toxigenic types of the organism exist, each producing an immunologically distinct form of botulinum toxin. These neurotoxins are designated A, B, C1, D, E, F, and G. The spores may germinate and produce botulinum toxin when contaminated food has been inadequately sterilized or canned improperly. The toxin is released only after the death and lysis of cells. The toxin resists digestion and is absorbed by the upper part of the GI tract and then into the blood. It then reaches the peripheral neuromuscular synapses where the toxin binds to the presynaptic stimulatory terminals and blocks the release of the neurotransmitter acetylcholine. This results in flaccid paralysis. Even 1-2 µg of toxin can be lethal to humans.

Clinical features: Vomiting, thirst, dryness of mouth, constipation, blurred-vision, difficulty in speaking, breathing and swallowing. Coma or delirium may occur in some cases. Death may occur due to respiratory paralysis within 7 days.

Bacillus cereus food poisoning

The causative agent Bacillus cereus is a gram positive aerobic spore bearing bacillus. It is found abundantly in environment and vegetation.

Pathogenesis: Sporulation is associated with toxin production. This bacterium causes two types of food-borne intoxications; the ‘emetic-type’ or the short incubation type and ‘diarrheal form’ or the long-incubation type. The short-incubation form is caused by a preformed heat-stable enterotoxin. This is most often associated with fried rice that has been cooked and then held at warm temperatures for several hours. The toxin is heat-stable, and can easily withstand the brief high temperatures used to cook fried rice. The longincubation form of illness is mediated by a heat-labile enterotoxin which activates intestinal adenylate cyclase and causes intestinal fluid secretion. This is frequently associated with meat or vegetable-containing foods after cooking.

Clinical features: The short incubation form has an incubation period of 1 to 6 hours. It is characterized by nausea, vomiting and abdominal cramps and resembles S. aureus food poisoning in its symptoms and incubation period. The second type is manifested primarily by abdominal cramps and diarrhea with an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery and it resembles food poisoning caused by Clostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset.

Perfringes

The causative agent Clostridium perfringens is a gram positive anaerobic spore bearing bacilli that is present abundantly in the environment, vegetation, sewage and animal feces.

Pathogenesis: The bacterium is known to produce at least 12 different toxins. Food poisoning is mainly caused by Type A strains, which produces alpha and theta toxins. Spores in food may survive cooking and then germinate when they are improperly stored. When these vegetative cells form endospores in the intestine, they release enterotoxins. The toxins result in excessive fluid accumulation in the intestinal lumen.

Incubation period: 8-24 hours

Clinical features: Illness is characterized by acute abdominal pain, diarrhea, and vomiting. Illness is self-limiting and patient recovers in 18-24 hours.

Different kinds of bacteriophages (phages)

M13

Filamentous.

About 870nm in length and 6nm in width.

Consists of single stranded DND (ssDNA).

Three kinds of capsomeres build the capside.

Infects E. coli by adsorbing to the cell and entering through F pilli. Therefore only infect F⁺ and HFr cells. Also male specific.

Does not kill the host. Particles are released by budding, therefore when the particles are released, the host cell is alive.

An efficient vector in gene cloning as it can hold longer pieces of foreign DNA.

T phage

Structure is composed of icosahedral head, double stranded DNA and a tail.

Infects E.coli

 Main types of T phages are T2, T4 and T12.

In the infection linear DNA of the phage is released to the host cell and becomes circular by replicating that later produces a long DNA chain known as ‘Concatamen’. This coils into the phage’s head by headfull mechanism while packaging.

Infection kills the host cell as the new particles are released outside by bursting the host cell.

Lambda (λ)

Composed of head and tail.

Head is consisting of double stranded linear DNA.

At both 5’ ends of the DNA strand, 12 complementary base pair, single stranded segments are present. These two ends are known as “cos ends”.

Because of the cos ends, phage chromosome circularizes before replication. Concatamen is produced during the replication and during the packaging, Terminase enzyme cuts off the cos ends.

Host cell is E.coli.

MS2

Contains the smallest known genome.

Super coiled single stranded DNA.

Infect only through sex pilli. Therefore male specific.

Infect E.coli.

Phi×174(ΦX174)

Contains a single stranded circular DNA.

After adsorption, synthesizes the complementary strand and becomes double stranded.

Use as a positive control in DNA sequencing.

G4

Structurally similar to ΦX174 phage.

Can infect susceptible E.coli cells. 

Diversity of bacteria according to nutritional requirments, temperature, Oxygen conditions and water activity

Nutritional type	Energy source	Carbon source	Examples
Photoautotroph	Light	Inorganic carbon, i.e. CO2	Some purple and green bacteria (Chromatium)
Photoheterotroph	Light	Organic compounds	Some purple and green bacteria (Rhodospirillum)
Chemoautotroph (Lithotroph, Lithoautotroph)	Inorganic compounds; H2, H2S, NH3	CO2	Many Archaea and few bacteria (Nitrosomanas)
Chemoheterotroph (Heterotroph)	Organic compounds	Organic compounds	Few Arcaea and many bacteria (Pseudomonas)

Type	Minimum (0C)	Optimum (0C)	Maximum (0C)
Psychrophile	Below 0	10-15	Below 20	Contain unsaturated fatty acids in plasma membrane to tolerate.
psychrotroph	0	15-30	Above 25	Able to grow at low T but prefer moderate T
Mesophile	10-15	30-40	Below 45	Most bacteria especially the ones associated with warm-blooded animals.
Thermophile	45	45-70	Above 100	Contain Saturated fatty acids in plasma membrane. High glucose and carbon content as well as high melting point for DNA.
Hyperthermophile	80	80-115	Above 115	Contain phytane and modified proteins in plasma membrane. High glucose and carbon content as well as high melting point for DNA.

Type	Aerobic condition	Anaerobic condition
Obligate aerobe	Growth	No growth
Microaerophiles	Growth; when the O2 is at very low level	No growth
Obligate anaerobe	No growth; O2 is toxic	Growth
Facultative anaerobe/ facultative aerobe	Growth; Not essential to grow but utilized when available	Growth
Aerotolerant anaerobe	Growth; neither essential nor utilized	Growth

Type	Speciality
Halophile	Require NaCl for growth
Halo tolerant	Able to grow at moderate salt concentrations but grow best in the absence of NaCl
Osmophile	Able to grow in high levels of suger
Xerophile	Able to grow in dry conditions