When antimicrobials are used to treat an infection, the antimicrobial used is based on selective toxicity. Selective toxicity is when the agent selected kills or inhibits the bacteria without causing serious harm to the host. The agent must interact with a function or structure of the microbe that is not present or is very different from the host. There are two different types of antimicrobial agents: antibiotics and antimicrobial chemicals. Antibiotics are substances that are produced as metabolic products of one microorganism, which inhibits or kill other microorganisms. Antimicrobial chemicals are chemicals that are synthesized in the laboratory, which can be used therapeutically on microorganisms.
If an antimicrobial is effective against a large variety of Gram-positive and Gram-negative bacteria, it is considered broad spectrum. Some examples of broad-spectrum antimicrobials include tetracycline, streptomycin, cephalosporins, ampicillin, and sulfonamides. Broad-spectrum antimicrobials are beneficial when the source of an infection is not yet known but it is critical to treat the infection immediately (Broad Spectrum Antibiotics, 1995-2010). However, indiscriminate use of broad-spectrum antimicrobials can lead to super infections (Alonzo, 2014, p. 190). When an antimicrobial is effective against just Gram-positive, just Gram-negative or only a few species of microbes it is a narrow-spectrum antibiotic. Some examples of common narrow-spectrum antimicrobials are penicillin G, clindamycin, and gentamicin. Narrow-spectrum antibiotics are the preferred method of treatment due to them being less destructive to the body’s normal flora (Alonzo, 2014, p. 190). However, narrow-spectrum antimicrobial agents are not effective unless the source of the infection is known.
Antimicrobial resistance can emerge for a variety of reasons. Bacteria can become resistant by producing enzymes that either detoxify or inactivate the antibiotic. The bacteria can alter the target site of the antibiotic to reduce of block the binding of the antibiotic. The bacteria can also prevent the transport of the antimicrobial agent into the bacterium but altering the outer membrane or the cytoplasmic membrane. Bacteria can develop an alternate metabolic pathway to bypass the metabolic step that is blocked by the antimicrobial agent and overcome antimicrobials that resemble substrates and tie up bacterial enzymes. The bacterium may also increase the production of certain bacterial enzymes which then overcome the antimicrobials by tying up the bacterial selection of antibiotic resistant pathogens at the site of infection (Alonzo, 2014, p. 190).
In this lab, a Kirby-Bauer test was performed for S.epidermidis with Gentamicin, Penicillin and Novobiocin. After incubation for forty-eight hours, the following results were observed. S. epidermidis was found to be resistant to Penicillin with no zone diameter standards. Gentamicin produced a 15mm zone diameter, making S.epidermidis susceptible to this antimicrobial. Novobiocin produced an even larger zone diameter at 21 mm. However, these results show that S.epidermidis has an intermediate susceptibility to Novobiocin.
Alonzo, C. M. (2014). Experiement: Antibiotic Sensitivity. Retrieved March 12, 2015, from LabPaq:The Best Way to Learn Science: https://srm–c.na13.content.force.com/servlet/fileField?id=0BEa00000008f6B Betsy, Tom, Keogh, James. (2005). Microbiology Demystified. New York: McGraw-Hill Professional Publishing. Broad Spectrum Antibiotics. (1995-2010). Retrieved March 13, 2015, from SRS Pharmaceuticals Pvt. Ltd.: http://www.srspharma.com/broad-spectrum-antibiotics