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Effects of Biocides on antibiotic resistance

3. Is there evidence that bacteria resistant to biocides are emerging?

  • 3.1 How can bacterial resistance to biocides be determined?
  • 3.2 Has resistance to biocides been observed in health care applications?
  • 3.3 Has resistance to biocides been observed in consumer products?
  • 3.4 Has resistance to biocides been observed in the food production chain?
  • 3.5 Has resistance to biocides due to discharges to the environment been observed?

3.1 How can bacterial resistance to biocides be determined?

Whether or not a biocide is effective depends to a large extent on the concentration of the active molecule in the product.

To measure resistance, bacteria are exposed to a biocide for a set period of time. Standard strains of bacteria are killed or stop growing while resistant strains are unaffected.

In many reports, bacterial resistance to biocides is determined by the minimum concentration of a biocide needed to stop the bacteria from growing (the minimum inhibitory concentration, or MIC). However, a better measure of resistance is the minimum bactericidal concentration (MBC) that would kill the micro-organisms after a certain period of exposure. Monitoring changes in the MIC is still useful to detect strains of bacteria that are beginning to develop tolerance to a biocide.

Bacterial resistance was already reported in the 1950s and, in many of the early cases, resistance developed because the biocides were used or stored incorrectly, so that the concentration of the biocide in the product was too low to be effective. Since then, the number of reports of resistance to biocides and to all known preservatives has increased.

In health care facilities, bacteria resistant to biocides have long been found. Some biocides currently used in hospitals were found to be ineffective against bacteria that grow as biofilms attached to surfaces, and this may have an important role in the transmission of hospital-acquired infections. More...

3.2 Has resistance to biocides been observed in health care applications?

Bacteria resistant to the biocides present in medical products have long been observed.

Silver has antibacterial properties and has traditionally been added to compresses applied to burn wounds to prevent infection. However, in the 1960s there were reports of bacteria resistant to silver. Over the years, different silver compounds were developed to overcome this problem, but bacteria developed resistance to those too. Today, resistant bacteria are reported for almost all biocides.

When a micro-organism becomes resistant to an antibiotic, the antibiotic can no longer be used to tackle it, and other antibiotics that are more expensive and might have more side effects need to be used.

However, unlike antibiotic resistance, the issues relating to biocide resistance are considered to have a very low profile and priority. Despite the widespread use of disinfectants and antiseptics in health care settings, emerging bacterial resistance has only been studied in the laboratory but not yet in practice.

Concentrations of biocide that are used in clinics and hospitals are so high that it is expected that they even kill bacteria that are less easily affected.

There is evidence of bacteria resistant to both biocides and antibiotics occurring in hospitals, and this resistance can be transferred to other bacterial strains. Therefore, further research is needed to see if the long-term use of biocides in hospitals has an effect on the emergence of resistance against antimicrobials including antibiotics and biocides. More...

3.3 Has resistance to biocides been observed in consumer products?

Biocides are added to cosmetics to prevent micro-organisms from growing on them and spoiling the product. Because only a few biocides are used extensively in many different products, bacteria are becoming resistant to them. For instance, many home and personal care products contain triclosan and the widespread use of this biocide may be associated with bacteria becoming resistant to it.

Resistant bacteria have been found in industrial plants where cosmetics are manufactured and in the cosmetic products themselves. Studies have focused on how these bacteria spoil the cosmetics and not on whether or not they can cause disease.

As a result to this resistance to specific preservatives, cosmetic products now contain a mixture of biocides to preserve them better, but this means that the consumer is exposed to larger amounts and more types of biocides.

There is accumulating evidence that biocide resistant bacteria can be found in consumer products, but to date there are no studies to indicate that they are linked to antibiotic resistance or the emergence of harmful micro-organisms. More...

3.4 Has resistance to biocides been observed in the food production chain?

Biocides are used widely in food production and there is evidence that some harmful bacteria found in food are becoming increasingly tolerant to biocides.

Bacteria can become resistant by biochemical (membrane changes) or genetic (new gene expression, mutations, etc) modifications. Genetic mutations or acquisition of external genetic materials (plasmids, transposons, etc.) that make bacteria resistant to biocides could also make them resistant to antibiotics. Given the increasing use of biocides in animal facilities, this issue is of growing concern.

There is a lot of research on whether using antibiotics in animals leads to the emergence of resistant bacteria. However, data on the role that disinfectant use may have in the emergence of bacterial resistance are scarce.

A study from 1998 compared different strains of bacteria found in a poultry farm. Those that were resistant to a specific biocide were also more resistant to several antibiotics, disinfectants and dyes. However, a study in 2005 from Denmark on the five most common disinfectants used in poultry farms did not find a link between biocide use and resistance in the Salmonella bacteria. Similar research in 2007 showed that using disinfectants stimulated bacteria to activate a defence mechanism that “pumps out” harmful chemicals from their cells. However, a single exposure to the disinfectant did not result in the selective survival of strains with resistance genes.

Laboratory studies show that biocide use could lead to antibiotic resistance, particularly when biocides are used improperly over a long period of time and at concentrations that are too low to be effective. However, to date this result has only been found in laboratory studies and not in working situations.

More research is needed to establish whether the current use of biocides in food production and in the disinfection and decontamination of foods of animal origin could lead to antibiotic resistance. More...

3.5 Has resistance to biocides due to discharges to the environment been observed?

Biocides are discharged with wastewater.
Biocides are discharged with wastewater.
Credit: Carl-Fredrik Runqvist

Once biocides have been used, they are discharged into wastewater and they can be found throughout the environment in concentrations possibly leading to the selective survival of resistant bacteria.

Studies on bacteria that form biofilms in sink drains found that exposure to a biocide did not change the total number of bacteria present, but those that were naturally resistant to it grew at the expense of bacteria that are more easily affected by it.

Another study found resistant bacteria on the factory floor of biocide manufacturers. Even resistant bacteria could be killed by the concentrations of biocide used in practice for disinfection, but they became resistant to some unrelated antibiotics.

A number of studies have investigated whether hospital wastewater, in which high concentrations of disinfectants and antibiotics are found, contains resistant bacteria. However, there are no studies on the possible emergence of biocide resistant bacteria in other hospital environments. More...

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