Ionophore resistance of ruminal bacteria and its potential impact on human health

Abstract

In recent years, there has been a debate concerning the causes of antibiotic resistance and the steps that should be taken. Beef cattle in feedlots are routinely fed a class of antibiotics known as ionophores, and these compounds increase feed efficiency by as much as 10%. Some groups have argued that ionophore resistance poses the same public health threat as conventional antibiotics, but humans are not given ionophores to combat bacterial infection. Many ruminal bacteria are ionophore-resistant, but until recently the mechanism of this resistance was not well defined. Ionophores are highly lipophilic polyethers that accumulate in cell membranes and catalyze rapid ion movement. When sensitive bacteria counteract futile ion flux with membrane ATPases and transporters, they are eventually de-energized. Aerobic bacteria and mammalian enzymes can degrade ionophores, but these pathways are oxygen-dependent and not functional in anaerobic environments like the rumen or lower GI tract. Gram-positive ruminal bacteria are in many cases more sensitive to ionophores than Gram-negative species, but this model of resistance is not always clear-cut. Some Gram-negative ruminal bacteria are initially ionophore-sensitive, and even Gram-positive bacteria can adapt. Ionophore resistance appears to be mediated by extracellular polysaccharides (glycocalyx) that exclude ionophores from the cell membrane. Because cattle not receiving ionophores have large populations of resistant bacteria, it appears that this trait is due to a physiological selection rather than a mutation per se. Genes responsible for ionophore resistance in ruminal bacteria have not been identified, but there is little evidence that ionophore resistance can be spread from one bacterium to another. Given these observations, use of ionophores in animal feed is not likely to have a significant impact on the transfer of antibiotic resistance from animals to man.

Long-Term Effects of Feeding Monensin on Methane Production in Lactating Dairy Cows

“…Monensin is a carboxylic polyether ionophore antibiotic that is produced by fermentation of Streptomyces cinnamonensis (Russell, 2002). Benefits of feeding ionophores to lactating dairy cows include a shift in the acetateto-propionate ratio toward more propionate and an as-sociated decrease in methanogenesis (Russell and Houlihan, 2003), increased milk production (McGuffey et al, 2001), antiketogenic effects (Duffield and Bagg, 2000), improved BCS (Sauer et al, 1998;Duffield et al, 1999), control of legume bloat (Maas et al, 2002), and reduced risk of acidosis by inhibiting lactic acid production in the rumen (Tung and Kung, 1993;McGuffey et al, 2001).…”

Evaluating the effects of six essential oils on fermentation and biohydrogenation in in vitro rumen batch cultures

“…However, the use of antibiotics in animal feeds has been queried due to the potential of appearance of residues in animal products (Russell and Houlihan, 2003). For this reason, there is an increasing interest in evaluating the potential use of plants and plant extracts to modify rumen microbial fermentation .…”

A comparison of data analysis methods for determining gas phase stabilities by cid: Alkali metal complexes of polyether ionophore antibiotics

“…The antimicrobial properties of the ionophores are a result of their ability to move the otherwise insoluble cations through cell membranes, upsetting the natural cation gradient and ultimately causing cell death. Both gram-negative and gram-positive bacterial strains are sensitive to polyether ionophores; however, gram-positive species are typically more affected [5].…”

“…Polyether ionophores have found their most common use in the agricultural sector as anticoccidiostats in poultry as well as growth promoters in ruminant animals [28]. Their widespread production and sale as feed additives has prompted interest in monitoring the increase in microbial resistance in both humans and livestock [5]. There is also concern regarding the allowable residual levels found in dairy, poultry and beef products.…”