The question of "why don't topical antibiotics cause resistance?" came up a little while ago, and while we were mostly correct, here are further clarifications in an excerpt from a recent article*:
"While there is continued concern that the overuse of systemic antibiotics will lead to increased antimicrobial resistance and eventually ineffective antibiotics, there is no evidence over the 3 decades of their extensive use worldwide that, with the exception of mupirocin, topical antibiotics used in the treatment of superficial cutaneous bacterial infections administered on an outpatient basis contribute significantly to any emerging resistance pattern. Specifically, there has been no demonstrated emergence of aminoglycoside-resistant pathogens (of which neomycin is a member) or pathogens resistant to the bacitracin and polymyxin classes of drugs. "Unlike with systemic antibiotics, the potential for the emergence of resistance with topical antibiotic therapy is low. The underlying reasons include: "Drug delivery of a topical formulation is orders of magnitude higher than the "breakpoint" level (population of susceptible bacteria vs resistant bacteria) set for resistance to systemic therapies, allowing topical drugs to overcome even a resistant-defined strain; "Episodic, rather than chronic exposure to topical antibiotics in an open (ie, nonhospital) setting is unlikely to lead to widespread resistance; "It is improbable that a genetic mutation will occur among the limited bacterial population that inhabits the area treated by topical agents that will approach the level needed for resistance to occur; and "Topical therapy bypasses the alimentary tract and the organisms that inhabit it, which decreases the population of organisms that come in contact with the agent that can potentially become resistant. "Mupirocin Resistance Increased use of mupirocin has been associated with the emergence of resistance. Mupirocin resistance has been noted in both MRSA and MSSA.# It appears to be associated with previous mupirocin use, particularly in patients treated for long periods of time (weeks to months). This is supported by data reported at one institution showing an increase in resistance of MRSA to mupirocin over 3 years of widespread use of mupirocin nasal ointment. In this study, all MRSA isolates obtained over a 3-year period at a public teaching hospital were tested for their susceptibility to mupirocin. The presence of mupirocin resistance increased dramatically over this period (from 2.7% in 1990 to 65% in 1993) in association with the increased use of the antibiotic intranasally as an adjunct to control nosocomial spread of S aureus infection. #["MRSA and MSSA" -- methicillin-resistant or -sensitive _Staphylococcus aureus_] "The reverse may also be true -- that is, rather than topical use promoting systemic resistance, systemic or oral therapy may be compromising the efficacy of topical care of MRSA over time.[24] As with all other antibiotics, continued surveillance is critical to early identification of resistance. "TAO Resistance [triple antibiotic ointment: bacitracin zinc + polymyxin B sulfate + neomycin sulfate] There is no evidence that TAO has contributed to any emerging resistance pattern over its more than 30-year duration of use. Topical use and triple components contribute to the extremely low likelihood of resistance. With a triple-component product, the chance of a mutation occurring against all 3 components is much less than that occurring against a single-component drug. Resistance involves an "antibiotic stress situation," in which bacterial enzymes or mutations are upregulated to produce inactivating enzymes against a particular class of agents. Mathematically, the probability of the genetic upregulation occurring against 3 classes of agents is extremely low. "Ongoing surveillance is necessary to document any resistance to TAO and to each component of TAO so that emerging resistance patterns, if they occur, are accurately identified...." *The Role of Topical Antibiotics in Dermatologic Practice Author: J.J. Leyden, MD Medical Writer: Linda Mattucci Schiavone MedScape Release Date: June 25, 2003 Debbi Gimme Some Skin Maru :) __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
