Submitted to: Federation of European Microbiological Societies Microbiology Reviews
Publication Type: Review Article
Publication Acceptance Date: 1/31/2003
Publication Date: 4/1/2003
Citation: RUSSELL, J.B., HOULIHAN, A.J. 2003. IONOPHORE RESISTANCE OF RUMINAL BACTERIA AND ITS POTENTIAL IMPACT ON HUMAN HEALTH. FEDERATION OF EUROPEAN MICROBIOLOGICAL SOCIETIES MICROBIOLOGY REVIEWS. 27:65-74. Interpretive Summary:
Technical Abstract: In recent years, there has been a debate concerning the causes of antibiotic resistance and 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 as therapy. 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 ionophore 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 a mutation per se. While it is conceivable that pathogens might have the same potential to become ionophore-resistant as ruminal bacteria, it should be noted that monensin resistant ruminal were not more resistant to therapeutic antibiotics.