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    Antibiotic resistance in the eyecare practice

    A most unwelcome guest—a superbug called mcr-1 positive—arrived in the United States in May 2016.1 That event, along with the increasing threat of Zika virus, has brought microbes to the forefront of our thinking.

    The current dilemma we face is that we have no viable treatments for many of these new microbes. Mcr-1 positive is a superbug, which means it has resistance to not just one class of drugs but several drug classes. The new superbug is resistant to even what is called our last line of defense, the antibiotic Colistin (colistimethate sodium, Taj Pharmaceuticals).

    Evolution of antibiotic resistance

    Microbial antibiotic resistance is the ability of microbes to survive the effects of drug compounds formulated to eradicate them.

    As a background, microbes have inhabited the earth for billions of years and may be the earliest life forms on the planet. They have the capacity to survive in the most extreme environments. The success of microorganism survival is due to their remarkable adaptability. Having the flexibility to change under stressful external environmental conditions, antibiotic exposure for example, ensures microbial survival.

    Antibiotic resistance is a natural phenomenon. During antibiotic challenge in the course of bacterial generations, those bacteria that are weak or sensitive to the drug will perish, and those resistant will continue to thrive and multiply. For example, bacteria which have undergone random genetic mutation that provides genetic material to encode for antibiotic resistance will continue to populate. This natural selection process ensures genetic survival.

    Different genetic mutations encode for different resistance mechanisms in bacteria. Some bacteria have genetic material that encodes for enzymes that deactivate the challenging antibiotic. Others remove landing sites for the antibiotic so that it is unable to exert its therapeutic effect. Bacteria can close membrane ports, blocking antibiotic access into the cell, and yet others develop pumps to pump the offending antibiotic out of the cell.

    When bacteria possess the genome to encode for resistance, they can pass it on vertically to subsequent generations or horizontally to its contemporaries through different mechanisms. Via transformation, bacteria incorporate free-floating DNA strands from their environment. In transduction, bacterial DNA is moved from one bacterium to another by a virus; in bacterial conjugation, direct cell-to-cell contact allows for bacterial DNA sharing or gene swapping,2 as seen with mcr-1 positive.

    Katherine M. Mastrota, MS, OD, FAAO
    Clinical director of Omni Center for Dry Eye Specialty Care in New York City

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