Use of Antibiotics

Antibiotics are antimicrobial chemicals produced in nature by bacteria and fungi. These chemicals help the producing organism by killing or inhibiting its microbial competition to limited resources like water or food, especially in environments where these resources are less available. These chemicals have had an enormous impact on therapeutics over the past decades because of their proficiency in exclusively killing bacterial cells. While antibiotics in medicine are well known, antibiotics are used for a wide array of other purposes.

The uses of antibiotics

As mentioned, the most widely known use of antibiotics is in therapeutics to treat medical conditions caused by bacteria. However, this is not the only use. Antibiotics are used in microbial culture, farming, food preparation, building, domestic products and several other areas.

Antibiotic use in medicine

Antibiotics are used in medicine to treat bacterial infections; they function by either halting bacterial growth or outright killing the bacteria. The former group are termed bacteriostatic antibiotics and the latter bactericidal antibiotics.

The following table provides an overview of the main antibiotic classes used in therapeutics.

Incorrect medical use of antibiotics is one of the major sources of developing antibiotic resistance. Antibiotic resistance occurs when antibiotics are exposed to selection pressure, in this case, insufficient antibiotics to kill or prevent the growth and division of an entire population of bacteria. This often occurs due to using antibiotics to treat incorrect conditions, such as viruses, or because people don't take the full course of antibiotics prescribed to them. It may also occur randomly or when antibiotics are being used correctly, but this is much more limited. Using antibiotics appropriately to maintain their efficacy is termed antibiotic stewardship.

The uses of antibiotics in agriculture

Antibiotics are used and abused in many areas of farming, some for good reason and others less so. Around 66% of antibiotics manufactured yearly are used in animals. Antibiotic use in farming is another major source of antibiotic resistance.

Antibiotics in animal farming

Antibiotics are used in animal farming for the legitimate use of treating animal infections. In this case, antibiotics are used similarly to human medicine. The other major use of antibiotics in animal farming is as growth promoters. In this case, antibiotics are introduced at sub-therapeutic doses (lower than the medical dose) through the animals' food and water. This causes them to grow quicker. The exact mechanisms behind this are not precisely known; however, it is believed to be a result of either the killing of gastrointestinal bacteria, which consume nutrients that could otherwise be used for growth, or the prevention of infections that cause limited growth.

Antibiotics are also used prophylactically to prevent infection, operating on the principle that animals can't get sick if the bacteria that cause the infections are killed before they can cause an infection. This is used when animals are raised in tightly packed, less clean conditions.

Antibiotics in horticulture

Antibiotics are used within horticulture for the treatment of many plant diseases. They are generally applied prophylactically via sprays, as application after infection is generally not effective. Due to the introduction of antibiotics to the environment, this treatment presents another avenue for developing antimicrobial resistance. This release of antibiotics into the environment occurs due to overspray not landing on the plants or rain washing the drugs into the surrounding environment.

Antibiotics in fish farming

Fish farming, also known as aquaculture, is another area where antibiotics are generally administered as a preventative, prophylactic measure instead of a treatment. While aquaculture is often used to refer to fish farming, it describes the farming of any aquatic organism in salt, brackish or freshwater. Similar to antibiotic use in animal farming, this preventative measure protects profits by minimising sickness in farmed organisms. The chances of disease arising are exacerbated by the incredibly high stocking levels usually present in aquaculture. The density of the organisms plus the high levels of waste generated by them means an infection is highly likely to occur. When that is the case, the disease can spread rapidly through the population.

Like with all other non-medical uses of antibiotics, this use of antibiotics often leads to the release of the drugs into the surrounding environment. The organisms may be raised out of natural bodies of water in artificial habitats. Still, the likelihood of antibiotic release into the surrounding environment in these setups depends on the method used.

An alternative to artificial environments is enclosing natural water areas, either in shallow, near-shore waters or pens in deep, open water. These both present the largest risk of antibiotics being released into the surrounding ecosystem. The water is simply part of the natural body, separated only by the mechanism used to contain the farmed organisms.

Antibiotics in insect farming

Like animals, insects are susceptible to infection by bacterial pathogens. While a variety of insects are cultivated for the production of human foodstuffs, the most common globally is the honey bee. So these present the main use of antibiotics in insect farming. Insects are also farmed for non-human foodstuffs, including for use as animal feed, which may lead to antibiotics being inadvertently passed on to other areas.

Fig. 1 - The European honey bee, Apis mellifera, on a flower

Uses of antibiotics in shipping

Antibiotics, mainly tetracyclines, are often included in paint used for ships! Organisms such as barnacles or algae can attach to vessels by forming a bacterial biofilm. The added mass and friction due to the presence of this organism slows the ship and increases the amount of fuel used. The attachment of these organisms is known as biofouling, and so the paints used to prevent them are known as anti-fouling paints.

Fig. 2 - The structure of a biofilm

Biofouling occurs in several stages, gradually increasing in size of the organism. First organic polymers bind to the surface, providing a substrate for bacteria and diatoms to colonise, forming the previously mentioned biofilm. The biofilm then allows algae and protozoa to attach, encouraging larger organisms such as barnacles to colonise the surface. These large organisms have the largest impact on speed and fuel economy. The collective organisms on the surface are known as a fouling community. By adding antibiotics to the paint, the bacteria that form the biofilm may be killed, so the fouling community's formation halted or slowed!

The uses of antibiotics in microbial culture

Bacterial contamination presents a significant threat to the success of microbial cultures. Generally, the end goal of culturing microorganisms is to have a pure culture of a single microorganism, either for the microorganism itself or one of its products. Bacterial contamination can compete with other microorganisms, preventing or limiting their growth or contaminating the end product, rendering it unusable, or increasing the complexity and cost of purification. The use of antibiotics in microbial cultures may help prevent this.

Antibiotics are also used when modifying microorganisms. When inserting genetic material via vectors into a bacteria, an antibiotic that the target bacteria is susceptible to is used to select for bacteria that have taken up the genetic material. This is done by including a resistance gene for that antibiotic within the vector, meaning that bacteria containing the vector become resistant. The antibiotic kills any that did not take it up. This allows the microbial culture to have only successfully modified bacteria.

Pros of using antibiotics

Both the medical and non-medical applications of antibiotics present several benefits.

• Their use as a growth promoter increases the efficiency and profitability of animal farming.
• Prophylactic administration prevents disease in a variety of organisms and farming systems.
• The use of antibiotics in medicine revolutionised treatments and means that bacterial infections that were once incredibly dangerous are now mere inconveniences.
• Their use in shipping lowers cost and increases efficiency, thereby also decreasing emissions from shipping.
• Finally, their use in microbial culture allows for more efficient culture, preventing contamination and selecting successfully modified organisms.

Consequences of antibiotic use

Antibiotic use can lead to the development of resistances, where the drug's efficacy is lost. As previously discussed, this occurs due to incorrect doses, where a selection pressure is created, and naturally occurring mutations that convey resistance are selected. Their unsupervised use in agriculture and other areas increases the likelihood of incorrect doses being used, which significantly increases the likelihood of resistance occurring when antibiotics are used for this purpose compared to medicine.

Another significant consequence of non-medical antibiotic use is the release of antibiotics into the environment through agricultural runoff or other means. This is termed antibiotic pollution and may significantly alter the natural bacterial population in an area and increase the likelihood of resistance developing. Antibiotic use in the food chain can also lead to accidental exposure of humans to antibiotics through the contamination of foodstuffs. This is exacerbated by bioaccumulation, a phenomenon where a substance accumulates in an organism from its surrounding environment, food and water.