Toxins From Pathogenic Bacteria Can Be Destroyed By Freezing And Cooking

Toxins from pathogenic bacteria can be destroyed by freezing and cooking. True or False? It is false. Pathogenic bacteria are bacteria that cause illness in people.

These bacteria often cause the illness by the toxins that they produce. It is therefore very important that the bacteria and their toxins are destroyed so as to prevent people from becoming sick.

The problem is that there is a great deal of variation in how easily bacteria and toxins can be destroyed. Some can be more easily destroyed than others. This means that it is sometimes true that toxins can be destroyed by cooking bacteria. However, there are bacteria that can recover from such treatments to grow and produce toxins again.

An autoclave, used to kill bacteria by heat and pressure

Bacteria can either produce exotoxins or endotoxins. Exotoxins are secreted from the bacterium into the outside environment while endotoxins are a part of the cell wall. The toxins vary in how dangerous they are with some causing a mild fever, while in other cases, they can be deadly.

Various methods are used to try to control bacteria, from killing their cells and toxins to controlling their growth rates. Freezing and heating are used but do not both destroy bacteria.

Heat can kill bacteria while cold temperatures can stop them from growing. There are exceptions though with some species being very difficult to defeat since they produce resistant spores, e.g. Clostridium botulinum.

Bacterial toxins

Bacteria can produce exotoxins or endotoxins. Exotoxins are secreted by the bacterial cell into the external environment, while endotoxins are a part of the bacterial cell itself.

Both exotoxins and endotoxins are dangerous and can be deadly, though there are more deadly exotoxins than endotoxins.

Exotoxins

Exotoxins can be deadly, for instance, the C. botulinum cell produces the toxin botulism which causes the illness of the same name. Tetanus, diphtheria and anthrax are also caused by exotoxins produced by a bacterial species in each case. Exotoxins are usually produced by gram-positive bacteria.

Gram-positive bacteria are named for their response to a staining method. These bacteria have a specific cell wall structure that is different from gram-negative bacteria. They have techoic acid and a thick peptidoglycan wall that takes up the gram stain.

Endotoxins

Endotoxins are part of the cell wall of the bacterium, which means they are only released when the bacterial cell dies and disintegrates.

Lipoproteins and lipopolysaccharides both form the toxin, more specifically it is the lipid part that is the actual toxin while the polysaccharides and proteins may trigger the immune system of the host.

They are not as deadly as exotoxins but can produce a fever in an ill person as the person's immune system responds to these toxins. They can, however, sometimes cause death.

An example is the Escherichia coli bacterium, which can sometimes cause death if in the more deadly form. These bacteria that produce endotoxins are gram-negative bacteria.

Gram-negative bacteria have a thin cell wall and contain the lipopolysaccharides. These are absent in the gram-positive bacteria. The gram-negative bacteria can't retain the gram stain since they are thin and thus are known as gram-negative bacteria.

Many cases of food poisoning occur each year because of bacteria and their toxins. Improperly prepared food is a major cause of food poisoning.

Freezing and heating food is a good way to help avoid getting food poisoning by stopping bacteria growing and killing bacteria outright.

Freezing

Keeping foods at a cold temperature below 40 degrees Fahrenheit is recommended. Freezing food won't kill bacteria, but cold temperatures can stop them from growing, for instance, it controls the growth of Salmonella bacteria.

The bacteria, in general, grow on substrates and as temperature decreases, their ability to grow on a substrate also decreases as they are less able to take up nutrients for growth.

If you do freeze food, you do not kill the bacteria, they simply become dormant which means that they may reactivate once you thaw the food. This means that you need to be careful to not leave cold foods outside the fridge for too long.

For example, do not leave milk standing out for hours. During the time a food is left out, the temperature rises and bacteria can begin to grow again.

Some species can still grow in the refrigerator at temperatures that are slightly above freezing, for instance, Versinia enterocolitica and enteropathogenic E. coli.

These two species of bacteria can grow between 35 and 40 degrees Fahrenheit. Fortunately, these bacteria can normally be destroyed by cooking food at a high enough temperature.

Heating

Heating is a way to kill most bacterial cells and their associated toxins. It is recommended that food is heated above 140 degrees Fahrenheit (60 Celsius).

This is why eating raw foods such as raw seafood or raw meat is not a good idea since they may have high bacterial loads.

Some species can form heat-resistant spores, notably those bacteria that are in the Clostridium genus. Both C. botulinum and Clostridium perfringens can survive heating to 140 degrees Fahrenheit, and it is recommended therefore that food be heated to 165 degrees Fahrenheit (74 Celsius).

C. perfringens can occur in poultry, meat, and gravy; while C. botulinum can occur in food that is not properly canned.

Salmonella is another species that is hard to kill and will only be killed if food is heated above 150 degrees Fahrenheit (66 Celsius). This pathogen is found in eggs, poultry, and meat. These foods thus need to be heated adequately to ensure that no Salmonella bacteria survive.

Wet heat is even better at killing bacteria than dry heat. Autoclaves are instruments that are used to sterilize medical equipment.

These machines use a combination of high temperature and pressure to kill bacteria. Autoclaves are available in industrial and research institutions but are not available for the general public to use.

Other methods of bacteria contro l

Chemical agents can be used to kill bacteria and thus disinfect surfaces. Such chemicals include formaldehyde, glutaraldehyde, and chlorine compounds.

The problem is these chemicals are often toxic to people, with formaldehyde being a known carcinogen. The use of such chemicals is thus limited since they are so harmful to people and of course you can't use these agents to kill bacteria in food and drink.

References

DB Nedwell (1999). Effect of low temperature on microbial growth: lowered affinity for substrates limits growth at low temperature. FEMS microbiology ecology.
AB Wagner (2008). Food technology & processing: bacterial food poisoning. Retrieved from tamu.edu.
Editors of Encyclopedia Britannica (2018). Endotoxin. Retrieved from Britannica.com.
Editors of Encyclopedia Britannica (2018). Exotoxin. Retrieved from Britannica.com.
RS Weiser, CM Osterud (1945). Studies on the death of bacteria at low temperatures: I. the influence of the intensity of the freezing temperature, repeated fluctuations of temperature, and the period of exposure to freezing temperatures on the mortality of Escherichia coli. Journal of bacteriology.