EXCESS OF METALS FAVOR RESISTANCE TO ANTIBIOTICS
Last month, I had a very interesting discussion with Agathe Romeo, a French scientist who explained to me that the use of excess of metals could trigger the development of resistance in antibiotics. That was new information for me and I believe it is important to spread this news if we want to fight effectively against antibiotics resistance. Her article was published in July 2014 in Pig International magazine.
Antimicrobials substances have been used worldwide in large quantities. For example, in 1994, antimicrobial consumption in Danish animal production was 200 tons; more than half of these 200 tons were supplemented as growth promoter in animal feed. Occurrence of antibiotic-resistant bacteria has become an important concern in recent years and has led to the ban of non-medicinal use of antibiotics in livestock production in European Union. Other substances are known for the antimicrobial properties and for their growth promoting effect. Zinc (especially zinc oxide) and copper are supplemented in excess in pig diets to improve intestinal health, but their impact in the fight against antimicrobial resistance is not entirely positive. CURRENT SITUATION Zinc is an essential trace element for the life of numerous organisms. Its positive effect on growth performance has long been recognized in swine production:. According to the National Research Council, pig requirements are between 50 to 100ppm in relation to the age and weight of the animal. As the zinc content in feeding stuffs is low, zinc is usually supplemented in pig diets. In the EU, the maximum content in monogastrics feeds is 150ppm, but some countries authorize pharmacological dosages of zinc (3000ppm), in medicated premix, during the post-weaning period. Like zinc, copper is an essential trace element. It is a cofactor for numerous enzymes and a famous growth promoter. Pig requirements are low (4 to 10 ppm), but copper is often supplied in excess. Maximum copper contents in European feeds are 170ppm for piglets and 25ppm for pigs. These trace elements are used at high levels for their antimicrobial properties. As a result of the common use of drugs in livestock production, bacteria have developed four main strategies against antibiotics:
Reduction of membrane permeability (lower porin expression)
Drug inactivation (production of enzymes)
Alteration of cellular targets (modification of binding sites of the antibiotics)
Efflux of toxic elements (via efflux pumps)
In Denmark, a glycopeptide named avoparcin was used as growth promoter until 1995; at that time, 21 percent of Enterococcus faecium (E.Faecium) obtained from pig carcasses were resistant to avoparcin. Fully aware of the problem, Denmark banned the non-medical use of antibiotics in swine production in 1998 for adult pigs and in the 1999 for piglets. In 2006, the withdrawal of the antibiotics used as a growth promoter was implemented in the EU. Copper and zinc can regulate the intestinal microflora and reduce diarrhea associated to post-weaning period: they are therefore able to partly replace antibiotics. However, excess of copper and excess of zinc have a negative impact on the environnement and select copper- and zinc-resistant bacteria. In addition, we can observe a strong correlation between resistance to metals and resistance to anbitiotics. CROSS-RESISTANCE Multi-resistances can be explained by cross-resistance. In this case, a single generic determinant leads to resistance to several elements (metals and antibiotics). For example, bacteria may synthetize transport proteins that carry both antibiotics and metals. In the zinc-resistant microorganisms, efflux pumps decrease intracellular zinc from the cell; these pumps can be specific to zinc or can accept other molecules. Consequently, zinc-resistant bacteria can be also resistant to one or more antibiotics.
CO-RESISTANCE Multi-resistance can also be associated to co-resistance. In this case, different resistance genes take place in the same genetic element, in general in a mobile element like plasmid. Some association’s metal-antibiotics are studied in the literature; for example zinc-resistance linked to methicillin-resistance in Staphylococcus aureus (S.aureus) or copper-resistance linked to macrolide and glycopeptide resistances in E.faecium. RESISTANCE TO ZINC A gene named czrC confers cadmium and zinc resistance in S.aureus. A study on pig isolates from different countries demonstrated that the gene czrC was found in 95 percent of the zinc-resistant isolates. In contrast, less than 1 percent of the isolates involving crzC were susceptible to zinc. A strong correlation between zinc-resistance and methicillin-resistance was observed; among the methicillin-susceptible isolates, all were susceptible to zinc chloride, and among the methicillin-resistant isolates, more than 90 percent were resistant to zinc chloride.
- THERE IS A STRONG CORRELATION BETWEEN RESISTANCE TO METALS AND ANTIBIOTICS -
In another study, with Danish pig isolates, none of the methicillin-susceptible isolates and 74 percent of the methicillin-resistant isolates were resistant to zinc. In average, the minimum inhibitory concentration (MIC) of zinc for these resistant bacteria is 4 times higher than the normal MIC: 8 mM for the methicillin-resistant S.aureus (MRSA) versus 2mM for the methicillin-susceptible S.aureus (MSSA). In a word, selecting zinc-resistant bacteria means selecting antibiotic-resistant bacteria. In Europe, in Canada and the US (Midwest), emergence of MRSA with a similar genetic profile (complex clonal 398) is facilitated by the use of tetracycline and by the use of zinc oxide. This strain has also been found recently in Australia and in New Zealand. SPREAD OF RESISTANCE Resistance genes are generally located in mobile genetic elements (plasmid, transposons), which facilitate horizontal transfers. Bacteria may be able to accept plasmids or transposons of other cells, by mating (conjugation); they could also to incorporate exogenous genetic material on their own genome (transformation). Specifically, DNA may be transferred from resistant bacteria to another by a phage (transduction). Next, bacteria that become resistant may transfer their genes to daughter cells during cell division. Consequently, the use zinc or copper in excess creates a new generation of metal- and antibiotic-resistant bacteria.