Discussion Disinfectants are chemical agents used on inanimate objects to lower the level of microbes present on the object. Antiseptics are chemicals used on living tissue to decrease the number of microbes present in that tissue.Disinfectants and antiseptics affect bacteria in many ways. Those that result inbacterial death are called bactericidal agents. Those causing temporary inhibition ofgrowth are bacteriostatic agents.No single antimicrobial agent is most effective for use in all situations - differentsituations may call for different agents. A number of factors affect selection of thebest agent for any given situation - Antimicrobial agents must be selected withspecific organisms and environmental conditions in mind. Additional variables toconsider in the selection of an antimicrobial agent include pH, solubility, toxicity,organic material present, and cost.Once an agent has been selected, it is important to evaluate its effectiveness. Inevaluating the effectiveness of antimicrobial agents, the concentration, length ofcontact, and whether it is lethal (-cidal) or inhibiting (-static) at that concentration andexposure are the important criteria.One method of measuring the effectiveness of a chemical agent is to determine itszone of inhibition. In the agar diffusion method, one species of bacteria is uniformlyswabbed onto a nutrient agar plate. Chemicals are placed on paper disks. These discsare added to the surface of the agar. During incubation, the chemical diffuses from thedisk containing the agent into the surrounding agar. An effective agent will inhibitbacterial growth, and measurements can be made to quantify the size of the zones ofinhibition around the disks. The relative effectiveness of a compound is determined bycomparing the diameter of the zone of inhibition with values in a standard table.The agar diffusion test is not used to determined whether a chemical is bactericidal(kills bacteria) or bacteriostatic (inhibits bacteria) - instead this characteristic isdetermined by the dilution method. In this method the bacterium of interest is placedin a tube containing the chemical which is being tested. The bacterium is then added(subcultured) onto a nutrient agar plate. If the bacterium grows on the nutrient agarthe chemical is bacteriostatic; if not, it was killed by the chemical which is thentermed "bactericidal."
, the effectiveness of a chemical in sensitivity testing is based on the size of thezone of inhibition. But the zone of inhibition varies with the relative rate ofdiffusion of the agent, the size of the inoculum, the type of medium, and manyother factors. Therefore, zone size alone cannot be used to determine the absoluteeffectiveness of an antibiotic, antiseptic, or disinfectant, but can certainly be usedto test relative effectiveness.Two important rules to remember concerning the use of antiseptics anddisinfectants are (1) always use the most concentrated form that will cause the leastdamage to the tissue or inanimate surface, and (2) if long-term use is necessary,apply either a combination of agents or frequently change to another effectiveagent. The continued application of a single agent (especially at lowconcentrations) will select for microbial mutants that are resistant to the agent.The risk is minimized when you use higher concentrations and is almost eliminatedwhen you use a combination of effective agents. These same rules apply toantibiotics. In this experiment you will assess the relative effectiveness of certainantibiotics, antiseptics, and disinfectants in killing bacteria. The bacteria testedwill be bacteria common in various parts of your body. One simple way to evaluatethe relative effectiveness of antimicrobial agents is to use the zone-of-inhibitionmethod. With this method, you apply the chemical to a freshly inoculated plate,incubate the culture, and then look for a zone of inhibition. The presence of such aclear zone (lack of growth) surrounding the chemical shows either the cells havebeen killed or that their growth has been inhibited (but you cannot tell which). Inother words, a zone of inhibition does not discriminate between bacteriostatic andbactericidal chemicals.
he bacteria of interest is swabbed uniformly across a culture plate. Then a filter-paper disk,impregnated with the compound to be tested, is placed on the surface of the agar. Thecompound diffuses out from the filter paper into the agar. The concentration of thecompound will be higher next to the disk, and will decrease gradually as distance from thedisk increases. If the compound is effective against bacteria at a certain concentration, nocolonies will grow wherever the concentration in the agar is greater than or equal to thateffective concentration. This region is called the "zone of inhibition." Thus, the size of thezone of inhibition is a measure of the compounds effectiveness: the larger the clear areaaround the filter disk, the more effective the compound. If bacteria growth was not inhibitedaround this piece of filter paper, the zone of inhibition is the diameter of the filter paper. Labelthe filter papers and their respective antibiotics with the smallest zones of inhibition as"Resistant," the ones with the largest zones as "Sensitive" and those in between "Intermediate."Read more: How to Calculate the Zone of Inhibition |eHow.com http://www.ehow.com/how_5845724_calculate-zone-inhibition.html#ixzz1vZsAQVpQRead more: How to Calculate the Zone of Inhibition |eHow.com http://www.ehow.com/how_5845724_calculate-zone-inhibition.html#ixzz1vZs2MmNbAfter overnight incubation, examine your plates (keep them covered at all times) tomeasure the zone of inhibition. a. The control plates should show uniform colonies over the entire surface of the plate. If the distribution is highly uneven, you will need to improve your innoculation technique and repeat the experiment. The filter disks should not impede bacterial growth, since they contained only water. b. If your disinfectants are effective at the concentrations you tested, you should see zones of inhibition around the disinfectant disks. The clear zones around each disk should have a uniform diameter, since diffusion of the compounds through the agar should be uniform in every direction. If this is not the case, suspect either your impregnation technique, or poor contact of the filter paper with the agar. 2. Measure the diameter of the zone of inhibition around each disk. Keeping the lid of the plate in place, use a ruler to measure the diameter of the clear area in millimeters. You will get four separate measurements for each dilution of each disinfectant—one from each quarter section of the test plate. The length of this clear zone is the zone of inhibition.all plates should be disinfected for safe disposal. 1. The best way to dispose of bacterial cultures is to pressure-sterilize (autoclave) them in a heat-stable biohazard bag.
2. If autoclaves or pressure cookers are not available, an alternative is to bleach the plates.References 1. http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p013.shtml 22nd May 2012 2. http://biology.bard.edu/ferguson/course/bio112/Lab/Lab_04_Antibiotics_Antiseptics_Disinfect ants.pdfNew antibiotics are constantly being discovered to replace older types that no longer fightbacteria. Unfortunately, the longer an antibiotic is used, the greater chance the harmful bacteriawill become immune to the antibiotic.met largely by semisynthetic tailoring of natural product scaffolds discovered in ththe middle of the 20 century. More recently, however, advances in technologyhave sparked a resurgence in the discovery of natural product antibiotics frombacterial sources. In particular, efforts have refocused on finding new antibioticsfrom old sources (for example, streptomycetes) and new sources (for example,other actinomycetes, cyanobacteria and uncultured bacteria). This has resulted inseveral newly discovered antibiotics with unique scaffolds and/or novelmechanisms of action, with the potential to form a basis for new antibiotic classesaddressing bacterial targets that are currently underexploitedThe clinical need for new classes of antibiotic continues to grow, as drug resistance erodes theefficacy of current therapies. Historically, most antibiotics were discovered by random screeningcampaigns, but over the past 20 years, this strategy has largely failed to deliver a sufficientrange of chemical diversity to keep pace with changing clinical profiles. A more rationalapproach to drug hunting has been greatly potentiated by the availability of bacterial genomicinformation. The rapid progress in sequencing and analysis of these small, prokaryotic genomeshas enabled the concomitant development of powerful new technologies that are alreadyenhancing the potential utility of genomic information. The future promises versatile and precisetools to understand what makes a successful antibiotic and moreover the means to identify andevaluate novel classes of drug.