Effects of Biocides on antibiotic resistance

7. What explains resistance to both biocides and antibiotics?

• 7.1 How can biocide exposure lead to antibiotic resistance?
• 7.2 In practice, does resistance emerge in homes and the environment?
• 7.3 How can dissemination of resistance genes lead to resistance?
• 7.4 How can the formation of biofilms lead to resistance?

7.1 How can biocide exposure lead to antibiotic resistance?

In laboratory experiments, bacteria exposed to biocides were found to develop antibiotic resistance in five different ways:

1. Cross-resistance: Some bacteria have genes that make them resistant to a biocide, and which also happen to make them resistant to one or more antibiotics. There are several classes of antibiotics and all antibiotics in the same class work in the same way. Therefore, bacteria that are resistant to an antibiotic are also resistant to all the antibiotics in the same class. Some bacteria are resistant to both antibiotics and biocides, for instance because they have developed a mechanism that “pumps out” such unwanted chemicals from within their cells.
2. Change in the physiological response: As a result of exposure to a biocide, some bacteria change the way they respond to it and that makes them less easily affected by either the biocide or antibiotics.
3. Co-resistance : Some resistance genes can be transferred from one bacterium to another whether or not it is related. Some bacteria are simultaneously resistant to biocides and antibiotics because the genes that confer resistance to one are found near the genes that confer resistance to the other so that the two sets of genes are transferred together.
4. Indirect selection: Exposing a population of bacteria to a biocide wipes out those which are easily affected (susceptible bacteria) and only resistant bacteria remain. These surviving bacteria are less easily controlled by either biocides or antibiotics and over time the resistant bacteria are more common in the population.
5. DNA repair: exposing bacteria to biocide can activate mechanisms of DNA repair.

Unfortunately, researchers have limited their investigations to one or two of these ways and not to all five at the same time, potentially missing some important information on a possible link between biocide and antibiotic resistance. More...

7.2 In practice, does resistance emerge in homes and the environment?

In homes, repeated exposures of bacteria to biocides in cleaning products, disinfection products and other relevant products could be considered to be a continuous selective pressure. This could lead to resistant bacteria surviving better than standard bacteria, and to the emergence of resistant strains. In natural environments, bacteria are continually exposed to low concentrations of biocides present in air, water and soil which might contribute to the emergence of resistance.

Laboratory studies show that bacteria exposed to non-lethal concentrations of biocide can develop resistance quickly. However, it is difficult to tell how widespread the development of bacterial resistance is in practice because there is very little information available.

One of the most important factors in the development of resistance is the concentration of the biocide so it is crucial to determine actual exposure. It has been impossible to get any information on the production volumes and uses of the various biocides. Exposures of bacteria to biocides in homes and the environment has to be estimated by other means, for instance based on the concentration and frequency of use of cleaning products and concentrations measured in the environment. More...

7.3 How can dissemination of resistance genes lead to resistance?

Bacteria can pass sections of DNA to each other via genetic mobile elements (plasmids, transposons, etc.), even among different species. These genetic mobile elements can confer useful properties to the bacteria receiving it such as the ability to grow in the presence of antibiotics, biocides or heavy metals.

A 2002 study investigated whether bacteria resistant to a particular biocide were also resistant to antibiotics. They found that some of the genes that conferred resistance to the biocide, also conferred resistance to antibiotics. Biocide-resistant strains were more likely to be antibiotic-resistant than standard strains and some strains were multi-resistant. This shows that resistance to antibiotics and to biocides were linked at the genetic level and that the presence of biocide-resistance genes led to the selective survival of antibiotic-resistant bacteria.

The transfer of resistance genes together with other useful functions has been observed in several bacteria species that can cause disease in humans. The uncontrolled use of biocides could therefore lead to the selective survival of bacterial strains with resistance genes. Bacteria could not only pass these resistance genes to their offspring but also to neighbouring bacteria of the same or of different species. More...

7.4 How can the formation of biofilms lead to resistance?

Bacteria can adapt to changes in nutrient availability, environmental stresses, and presence of toxic compounds. One particularly important example of bacterial adaptation is the ability for a group of bacteria to grow as a biofilm attached to a surface. It is now recognized that many bacterial diseases involve the formation of a biofilm in the affected part of the body or implant.

Bacteria living as a biofilm are able to resist to biocides and to antibiotics more effectively than those living as free organisms and they withstand considerably higher doses of antimicrobial products. This simultaneous resistance to both antibiotics and to biocides could be explained if the underlying mechanism is effective against both antimicrobials. For instance, biofilms are encased in polysaccharide layers that reduce the diffusion of antimicrobials. Compared to free cells, bacteria in biofilms are more concentrated, grow more slowly and are in a different physiological state; and all this could affect their susceptibility to antimicrobial products. However, there is very little information on the cross-resistance of bacteria in biofilms to antibiotics and biocides.

In one study, exposing biofilms of bacteria to biocides used commonly to treat drinking water, did not increase antibiotic resistance. More...