Effects of Biocides on antibiotic resistance

6. What are the potential threats of biocide use in terms of bacterial resistance?

• 6.1 How might the use of biocides constitute a direct or indirect threat?
• 6.2 What are potential threats of using biocides in veterinary settings?
• 6.3 What are potential threats of using disinfectants in health care settings?
• 6.4 What are potential environmental threats of using biocides?

6.1 How might the use of biocides constitute a direct or indirect threat?

Using biocidal products could potentially lead to bacteria that are resistant to biocides, antibiotics or both. These successful bacteria would reproduce more than other strains and if they were transmitted to humans, could pose a direct health threat.

Bacteria can also pose an indirect threat if they develop genes that confer resistance to antibiotics and biocides. These resistance genes can move between different bacteria and can thus transfer to a bacterium that could previously be controlled by antimicrobials. Genetic material can also pass between bacteria that are resistant to different substances so that they achieve an even higher level of resistance. This process can take place anywhere: in the environment, in food, animals or in the human body. More...

6.2 What are potential threats of using biocides in veterinary settings?

The use of biocides in intensive, industrial-scale farming could potentially lead to the emergence of strains of bacteria that are resistant to the disinfectants used, particularly when the concentration of the biocide is lower than it should be. This can happen when large animal houses are washed and disinfected and some areas are not cleaned thoroughly enough, or in disinfectant baths outside of animal houses that can easily be diluted by rainfall. Some of these baths also contain other chemicals that could make the biocide less effective. According to the findings of laboratory studies, the use of common farm disinfectants can enable strains of bacteria that are resistant to one antibiotic or to several classes of antibiotics to become more common than susceptible bacteria. This could be a stepping stone to a higher level of antibiotic resistance. In addition, low concentrations of biocides can favour genes involved in resistance and their expression.

If the bacteria found in animals can also cause disease in humans, the emergence of resistant strains would compromise antibiotic treatments used in humans.

There is a need for further studies on whether the intensive, and in some cases long-term use of biocides in animal facilities, can lead to antimicrobial resistance. This is also important because modern, intensive animal husbandry relies increasingly on the use of antibiotics; and harmful resistant micro-organisms can pass between animals and humans by direct contact and via the food chain. More...

6.3 What are potential threats of using disinfectants in health care settings?

Biocide-resistant micro-organisms are increasingly found in hospitals and there is a possible link between the use of biocides and antibiotic resistance in health care settings. However, evidence of such a link has only been found in some cases of antibiotics. In addition, both bacteria that are resistant to antibiotics and those that are not are killed by disinfectants at the concentrations used in practice in health care facilities.

Some of these biocide-resistant bacteria cause serious concerns. A particularly challenging problem is the presence of resistant bacteria that grow as biofilms on surfaces such as walls or floors, and on medical devices; and which are the cause of most hospital-acquired infections. Even high-level disinfectants are less effective against bacteria when these grow as biofilms. Despite this, most laboratories do not use biofilms to test the efficacy of biocides and there are no European standards for such tests.

More research is needed to see if there is a link between the use of biocides and antibiotic resistance, and researchers need to agree on how best to measure resistance to biocides. There should be a surveillance system to detect any emerging resistance to biocides, particularly for important micro-organisms that become resistant to antibiotics or those that are already multi-drug-resistant. More...

6.4 What are potential environmental threats of using biocides?

Biocides are very widely used in consumer products and in health care settings so they find their way into wastewater from homes and hospitals. Some biocides are used in such large volumes that even though sewage treatment plants remove large proportions of biocide in wastewater, significant amounts are still discharged. There is concern that these environmental concentrations might lead to resistance in micro-organisms.

A laboratory study showed that bacteria exposed to a range of triclosan concentrations below the level that would kill them did not become resistant to it, even when another triclosan-resistant bacterial species was present. The researchers concluded that environmental concentrations did not induce resistance to triclosan and that resistance was not transferred between different species of bacteria. In contrast, a recent report described that the exposure to triclosan at the minimum inhibitory concentration (MIC) during a short period of time is sufficient to activate the expression of resistance genes in two bacterial species.

A similar study investigated the effects of detergent containing another common disinfectant, on biofilms that form in household sink drains. They found that long term use of the biocide did not result in significant changes in resistance. The researchers concluded that although exposure to the biocide can lead to resistance in isolated bacteria, the same is not necessarily true for large groups of bacteria growing as a colony. More...