An ionophore is a chemical species that reversibly binds ions. The ionophores are lipid soluble entities that transport ions across a cell membrane. Ionophores catalyze ion transport across hydrophobic membranes such as liquid polymeric membranes (carrier-based ion selective electrodes) or lipid bilayers found in the living cells or synthetic vesicles (liposomes).
The ionophores synthesized by microorganisms have been divided into two broad classifications: 
- Carrier ionophores that bind to a particular ion and shield its charge from the surrounding environment, that makes it easier for the ion to pass through the hydrophobic interior of the lipid membrane. These ionophores may be proteins or other molecules. Valinomycin, for example is a carrier ionophore that transports a single potassium cation.
- Channel formers that introduce a hydrophilic pore into the membrane, allowing ions to pass through without coming into contact with membrane’s hydrophobic interior. These ionophores are usually large proteins. Say, Gramicidin A from Bacillus brevis (linear peptide of 15 residues) is a channel former ionophore.
The following are some biological ionophores with the ions they act upon: [13, 23]
|Ionophores||Ions they act uopn||Antibacterial properties against|
|Monensin||Na+, K+, Rb+, Li+, H+||Gram-positive bacteria.|
|Narasin||K+, Na+ and Rb+||Gram-positive bacteria.|
|Beauvericin||Ca++, Ba++||Gram-positive bacteria and mycobacteria|
|Calcimycine||Mn++, Ca++, Mg++||Gram positive bacteria|
|Enniatin||NH4+||Gram positive bacteria|
|Gramicidin A||H+, Na+, K+||Gram-positive bacteria, except for the Gram-positive bacilli, and against selected Gram-negative organisms, such as Neisseria bacteria|
|Lasalocid||K+, Na+, Ca++, Mg++||Gram-positive bacteria, but not against Gram-negative bacteria.|
|Nigericin||K+, H+, Pb++||Gram positive bacteria|
|Salinomycin||K+, Na+ and Rb+||Gram-positive bacteria, but not against Gram-negative bacteria|
Of the above listed ionophores, monensin, a macrolid has been widely used against coccidia since 1971. Coccidiosis has been a major cause of poor performance and lost productivity in poultry and other farm animals, since many years. The disease is caused by protozoan parasites of the genus Eimeria, the oocysts of which can be present in the environment wherever poultry are raised. Attention to improvements in management and hygiene, were the early recommendations for controlling coccidiosis. However, the intensive nature of the poultry industry, the fact that birds are usually reared in direct contact with their feces, and the resilience of oocysts ensures the continued presence of coccidia wherever poultry are raised, and thus attempts to eradicate the disease have been unsuccessful. With the introduction of sulfaquinoxaline in 1940s, it was tried to transform this issue by incorporating sulfaquinoxalne in feed during the rearing phase of poultry production, by which means infection could be prevented. [5, 9]
In 1967, the structure of monensic acid (subsequently known as monensin) was described and the compound was reported to have broad-spectrum anticoccidial activity. Monensin is an antibiotic produced as a byproduct of fermentation by Streptomyces cinnamonensis and belongs to family polyether antibiotics aka ionophores. Monensin was followed by lasalocid, narasin, salinomycin and semduramicin, which are other ionophores that have a similar mode of action . Salinomycin is derived from Streptomyces albus.
Structure and Mechanism of Action of Ionophores
In the process of ion transport, transmembrane ion concentration gradients (membrane potential) are required for living organisms. Ionophores can disrupt the membrane potential by conducting ions through a lipid membrane, and thus could exhibit cytotoxic properties. The ionophores produced naturally by a variety of microbes act as a defense against competing microbes.
Macrolide antibiotics are ionophores, of which some exhibit high affinities for Na+, and others high affinities for K+. Ionophores have been used to modify the permeability of biological membranes toward certain ions.
Monensin conforms a cyclic structure in which the oxygen atoms are at the center of a doughnut-shaped structure in which they can complex to suitable cations (Figure 2B). The alkyl groups are spread out over the outer surface and render the complex highly soluble in the lipid component of cell membranes. Thus ionophores can enter cell
membranes and conduct cations across them by the process of passive diffusion.
According to Chapman (2016) , once an ionophore is taken up in the membrane of the sporozoite, it effectively short-circuits the sodium pump. Thus, as fast as the sodium pump removes Na+, the ionophore allows it to leak back in again. The parasite’s cell responds by continuing to pump out Na+, but eventually runs out of energy. This results in an uncontrollable uptake of water into the sporozoite, which swells up and then bursts (Figure 3).
The Antibiotic Free Poultry Production:
“UK Five Year Antimicrobial Resistance Strategy 2013 to 2018”, the report states “Increasing scientific evidence suggests that the clinical issues with antimicrobial resistance that we face in human medicine are primarily the result of antibiotic use in people, rather than the use of antibiotics in animals.” This agrees with previous manuscripts that have shown that antibiotic resistance in a community closely matches antibiotic use in people from the same community .
The main challenges faced by producers of Antibiotic Free (ABF) chickens or turkeys are undoubtedly related to intestinal health, and more specifically to the prevention and control of coccidiosis and NE i.e., necrotic enteritis . The prevention of these two diseases is closely related as intestinal lesions induced by coccidiosis (especially E. maxima), whether due to field challenge or live coccidiosis vaccine are a well-known predisposing factor for clinical outbreaks of NE .
It is because of the slow resistance to monensin and other ionophores; the coccidiosis has been so able to be controlled till date. Removing ionophore anticoccidials and antibiotic feed additives is certain to cause problems in controlling coccidial parasites and bacterial organisms, in particular, Clostridium perfringens, the causative agent of NE . In absence of ionophores in a coccidial prevention program, control of coccidiosis will have to be achieved with synthetic “chemical” anticoccidials or live coccidiosis vaccines or, more than likely, rotations between the two. On the other hand, use of synthetic chemical anticoccidial (except nicarbazin) result a problem i.e., they are only highly effective for a limited period of time.
Live coccidiosis vaccines have been used in rotations with chemical anticoccidials to seed the chicken houses with vaccine strains of Eimeria oocysts that are fully susceptible to all anticoccidials . Yet still, the problem with the live coccidiosis vaccines is that they induce immunity by replicating and cycling a number of times through the intestines, when the parasites also cause damage to the epithelium of the intestinal tract and this predisposes the birds to outbreaks of NE caused by C. perfringens type A (a bacteria normally present in the hind gut of chickens). This is the main reason for which live coccidiosis vaccines work more effectively when an antibiotic feed additive with good anticlostridial activity can be added to the feed . Even so, the use of antibiotics like Virginiamycin (potent antibiotic against C. perfringes) in feed is not an option available because of the issue of ABF chickens.
Although we are urging the ban of antibiotics in poultry, there are some issues on the other side of coin that should also be thought of. Going for alternatives to in-feed antibiotics has helped in reducing the problems associated with producing chickens without antibiotics, yet in the best-case scenario, we would find moderate increases in enteric disease and treatment costs; moderate losses in daily gain, uniformity, and carcass yield; and resultant moderate increases in production costs, together with moderate increases in feed conversion ratios. After a large number of alternatives to antibiotic feed additives ranging from prebiotics and probiotics to live E. coli vaccines, oregano and organic acids were field tested, the author concluded that none of them worked like an antibiotic feed additive or were not cost effective .
However, there have been confusions within countries about the ABF poultry production. “A European producer said the ban on the use of so-called growth-promoting antibiotics has not restricted the use of ionophore coccidiostats and ionophores have been helpful not just for controlling coccidiosis, but also for the antibacterial affect that they have in the gut of the bird”, Terrence O’Keefe. However, in U.S. antibiotic-free poultry production generally means the use of a coccidiosis vaccine. With vaccination, birds are exposed to live coccidia from drug-sensitive strains at hatch. This gives the birds the coccidiosis challenge earlier in their life than when exposure occurs later in the poultry house. The impact of the bird’s immune response at an earlier age is less costly in terms of lost performance than if the exposure occurs later. Thus, vaccination shifts the peak challenge from four weeks after placement to two weeks. Inspite of that, a significant problem that may appear is emergence of necrotic enteritis during the earlier age of birds.
Antimicrobial resistance happens when microorganisms (such as bacteria, fungi, viruses, and parasites) change when they are exposed to antimicrobial drugs (such as antibiotics, antifungals, antivirals, antimalarials, and anthelmintics). Antibiotic resistance occurs when bacteria change in response to the use of these medicines. Bacteria, not humans or animals, become antibiotic-resistant. These bacteria may infect humans and animals, and the infections they cause are harder to treat than those caused by non-resistant bacteria.
Ionophores are technically antibiotics as they are produced as a by-product of bacterial fermentation. An important distinction, however, is that ionophores are unrelated to the antibiotics used to increase the rate of weight gain and improve feed efficiency. Ionophores are not used in human medicine and, therefore, cannot contribute to perceived issues relating to drug resistance in man. Monensin is not used in human medicine.
Medications like monensin, salinomycin, lasalocid, to name a few, are among the shrinking list of in-feed antibiotics not considered medically important to human medicine by the World Health Organization. About 31 percent of the total amount of antibiotics used in food animals is ionophores – compounds not used in human medicine and thus not contributing to the burden of antibiotic resistance in humans.
The VKM Report,2015 states that “Development of resistance in coccidia to all eleven coccidiostats viz. ionophores (Narasin, Monensin sodium, Lasalocis sodium, Salinomycin sodium, Maduramicin ammonium, Semduramicin sodium) and non-ionopores (Robenidine hydrochloride, Decoquinate, Diclazuril, Nicarbazin, Halofuginon), has been described in the scientific literature, but the prevalence of resistance is unknown. Cross-resistance between various ionophore coccidiostats has also been shown, i.e. development of resistance to one ionophore may also render the coccidia resistant to another ionophore. In the Norwegian surveillance programme NORM-VET during the years 2002 – 2013, between 50 – 80 % of the tested flocks had narasin resistant faecal enterococci, which are bacteria that are part of the normal intestinal microbiota. However, the pathogenic bacterium C. perfringens has not been shown to be resistant against any ionophore. As coccidiostats are not used to treat infectious diseases in humans, concern of resistance is related to possible cross- or co-resistance with antibacterials considered important in human medicine. Such resistance has so far not been confirmed.”
A question is raised if coccidiostat exert a growth promoting effect. However, there is no clear-cut difference between coccidiostat and Antibiotic Growth Promoters as both groups of feed additives have prevented disease and therefore also improved production efficiency, by mitigating the risks the diseases otherwise would viz. impairment of growth and feed utilization.
The following is the list of resistance to coccidiostats in bacteria: 
- All five coccidiostats (Narasin, Monensin sodium, Lasalocis sodium, Salinomycin sodium, Maduramicin ammonium) approved for use in Norway are ionophores that also display an antibacterial effect, mainly against Gram-positive bacteria. Resistance to four of them in enterococci and a few other bacteria has been reported.
- Resistance in C. perfringens, the causative bacterium for the poultry disease necrotic enteritis, has not been reported.
- A limited amount of data may indicate an association between narasin and resistances against bacitracin, as well as between narasin and vancomycin.
- The additional six coccidiostats which are approved in the EU display little or no antibacterial effect, and antibacterial resistance is therefore not considered to be a relevant subject.
Studies on the drug resistance of ionophores via poultry meat in man should be continued. The probability for human exposure to resistant bacteria is also negligible in heat treated food since heat treatment kills the bacteria. Also, judicious use of antibiotics is the first precedence required by veterinarians taking in care the proliferating antibiotic resistance by microorganisms. The issues of controlling the disease of high economic importance with proper drug, solving the issue of antibiotic resistance together, without harming the animal welfare and letting farmers gain the high profit with low cost of poultry production is challenging and complex. Also, it has been contradictory whether or not antibiotic-free poultry production costs more. These all issues let future researchers a long way through to study upon whether ionophores should or not be banned with regards to antibiotic resistance factor.
- Agtarap, A., Chamberlin, J. W., Pinkerton,M. & Steinrauf,L. (1967). The structure of monensic acid, a new biologically active compound. J. Am. Chem. Soc. 89:5737–5739.
- Al-Sheikhly, F. & Al-Saieg,A. (1980). Role of coccidia in the occurrence of necrotic enteritis of chickens.Avian Dis. 24:324–333.
- Antibiotic Resistance. (2016). World Health Organization. Fact Sheet.
- Antimicrobial Resistance. (2016). World Health Organization. Fact Sheet.
- Campbell, W. C. (2008). History of the discovery of sulfaquinoxaline as a coccidiostat. J. Parasitol.
- Cervantes, H. (2007). ANTIBIOTIC FEED ADDITIVES: POLITICS AND SCIENCE. Phibro Animal Health, Watkinsville, Georgia, USA
- Cervantes, H.M. (2015). Antibiotic-free poultry production: Is it sustainable?. JAPR: Review Article. Poultry Science Association, Inc.
- Channel Forming Ionophores || Ion carriers for membrane ion transport. (2013). Available at http://agscientific.com/blog/2013/04/10/channel-forming-ionophores/ Accessed on 6th January, 2017
- Chapman, H. D. (2009). A landmark contribution to poultry science— Prophylactic control of coccidiosis in poultry. Poult. Sci.88:813–815.
- Chapman, H. D., Jeffers, T. K.& Williams, R.B. (2010). Forty years of monensin for the control of coccidiosis in poultry.Poultry Science 89 :1788–1801. doi: 10.3382/ps.2010-00931
- Chapman, H. D. (2016). How ionophores control coccidiosis.
- Going Mainstream: Meat and Poultry Raised Without Routine Antibiotics Use. (2015). NRDC.
- Ionophore. (2016). Wikipedia, the free Encyclopedia.
- Ionophores, ion carriers & membrane ion transport. (2011). Available at http://agscientific.com/blog/ index.php / 2011/11/16/ionophores-ion-carriers-membrane-ion-transport/. Accessed on 6th January, 2017
- Keefe, T.O. (2014). Antibiotic-free broiler production requires a paradigm shift. Poultry Health and Nutrition. Watt Poultry USA.
- Landers, T.F. et. al., .(2012). A Review of Antibiotic Use in Food Animals: Perspective, Policy, and Potential. Public Health Rep. 2012 Jan-Feb; 127(1): 4–22. PMCID: PMC3234384
- Magee, J. T., E. L. Pritchard, K. A. Fitzgerald, F. D. J. Dunstan, and A. J. Howard. (1999). Antibiotic prescribing and antibiotic resistance in community practice: retrospective study, 1996–8. Brit. Med. J. 319:1239–1240.
- Mathis, G. F., and C. Broussard. (2006). Increased level of Eimeria sensitivity to diclazuril after using a live coccidial vaccine. Avian Dis. 50:321–324.
- Monensin. (2007). Summary Report. COMMITTEE FOR MEDICINAL PRODUCTS FOR VETERINARY USE, European Medicines Agency Veterinary Medicines and Inspections. EMEA/CVMP/185123/2007-Final May 2007
- Questions and Answers about Antibiotics in Chicken Production. (2014). National Chicken Council.
- Smith, J. A. (2011). Experiences with drug-free broiler production. Poultry Science (2011) 90 (11):2670-2678. doi: 10.3382/ps.2010-01032
- The case for ionophores: How they’re different from other antibiotics – and why it matters. (2015). Poultry Health Today. Available at http://www.thepoultrysite.com/poultrynews/36040/the-case-for-ionophores-how-theyre-different-from-other-antibiotics-and-why-it-matters/. Accessed on 6th January, 2017
- VKM. (2015) The risk of development of antimicrobial resistance with the use of coccidiostats in poultry diets. Opinion of the the Panel on Animal Feed of the Norwegian Scientific Committee for Food Safety, ISBN: 978-82-8259-185-0, Oslo, Norway.
- Tempelaars, M.H., S. Rodrigues, and T. Abee. (2011). Comparative Analysis of Antimicrobial Activities of Valinomycin and Cereulide, the Bacillus cereus Emetic Toxin Appl. Environ. Microbiol. April 2011 vol. 77 no. 8 2755-2762 doi:10.1128/AEM.02671-10
- UK Five Year Antimicrobial Resistance Strategy 2013 to 2018, First Published: September, 2013, Department of Health, Richmond House, 79 Whitehall, London SW1A 2NS. Available at www.gov.uk/dh. Accessed on 8th January, 2017
- Use of Ionophores in Poultry Production. (2012). Available at http://articles.extension.org/pages/ 66414/use-of-ionophores-in-poultry-production. Accessed on 6th January, 2017
- Volume and Rate of Antibiotic Use in Animals and Humans. Available at http://www.togetherabx.com/8 . Accessed on 9th January,2017
B.V.Sc. & A.H. Internee
Institute of Agriculture and Animal Science (IAAS),
Rampur Campus, Chitwan, Nepal
International Veterinary Students’ Association Nepal