Laboratory Exercise 5: Chemical Antimicrobial Treatments (SS2018) 1. Evaluate th
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Laboratory Exercise 5: Chemical Antimicrobial Treatments (SS2018) 1. Evaluate the effectiveness of common household disinfectants/antiseptics. 2. Explain why (and how) phenol coefficients are determined 3. Do a routine test for checking the effectiveness of antibiotics Objectives: After completing this exercise you should be able to Background Control of MOs can be divided into physical and chemical methods (see lecture notes indiscrim and Lab 9 for a review). Physical means of sterilization usually kill all cells and cannot be used in many situations. (You would not want to flame or autoclave your hand!) Therefore, a wide variety of chemicals have been developed for controlling bacterial growth where complete sterilization is not feasible. Disinfectants are chemicals used on inanimate objects. Antiseptics are chemicals that are used on the skin or wounds to decrease the number of microorganisms. If antimicrobial agents act by actually killing the bacteria, they are re called bactericidal agents. If they act by causing temporary inhibition of growth, they a x relationship that depends on many variables of any agent is a comple rd to assign a simple number to it. One approach to measuring the The killing effect and this makes it ha effectiveness of a chemical agent is to determine its phenol coefficient. This is simply a comparison of the killing action of the compound to that of phenol under the same conditions. Phenol ("carbolic acid") is a toxic compound and is not as widely used today, but it was historically important in the development of aseptic surgical techniques, and provides a well- defined "yardstick" by which to evaluate other disinfectants. In this exercise, we will monitor the viability of a culture to determine the phenol coefficient of a test compound Today's phenol coefficient demonstration was done by mixing the chemical being tested with Staphylococcus aureus. At the listed times, a loopful of the mixture was transferred into a sterile nutrient broth tube, and incubated. To evaluate these tubes, look for growth in the tube ANY cloudiness = growth. Be sure to look and see if any of the growth has settled to the bottom of the tube. On your lab report, record each test tube as (+)for growth or (-) for no growth. Chemotherapeutic agents or antibiotics are antimicrobial compounds that can be safely absorbed or taken internally. The effectiveness of antibiotics can be evaluated different ways. The first method, the tube dilution, consists of a serial dilution of the antibiotic, to which a standard amount of bacteria is added to each tube. After incubation, the highest dilution of antibiotic with no growth is the minimum inhibitory concentration (MIC). MIC is the exact value of the smallest amount of antibiotic that kills/inhibits the MOs. The ffectiveness of an antibiotic is more conveniently measured by the Kirby-Bauer agar diffusion method. A micro-organism is first spread over the surface of a petri plate. Paper discs impregnated with various antibiotics are then placed on the surface of the agar. During incubation, the antibiotic diffuses from the disk, formin itself: high concentra area. If the antibiotic inhibits the clear zon appear around the disk as a confluent lawn of The size of this zone roughly species since zone size can also be affected by many different factors, ncluding the rate of diffusion, the size of the inoculum, the growth sta zones of inhibition are m bacteria is sensitive, intermediate, or resistant to g a concentration gradie nt in the plate tion near the disk and decreasi ng concentration out into the surrounding organism, a clear zone of inhibition (area of no growth) will cells develops everywhere else on the plate indicates the effectiveness of that antibiotic against that bacterial ge of the bacteria... The diameters of the easured and compared to a standard chart to determine if this an antibiotic.Explanation / Answer
The distinction between bactericidal and bacteriostatic agents appears to be clear according to the in vitro definition, but this only applies under strict laboratory conditions and is inconsistent for a particular agent against all bacteria
Clinical Relevance of Bacteriostatic versus Bactericidal Mechanisms of Action in the Treatment of Gram-Positive Bacterial Infections
The distinction between bactericidal and bacteriostatic agents appears to be clear according to the in vitro definition, but this only applies under strict laboratory conditions and is inconsistent for a particular agent against all bacteria. The distinction is more arbitrary when agents are categorized in clinical situations. The supposed superiority of bactericidal agents over bacteriostatic agents is of little relevance when treating the vast majority of infections with gram-positive bacteria, particularly in patients with uncomplicated infections and noncompromised immune systems. Bacteriostatic agents (e.g., chloramphenicol, clindamycin, and linezolid) have been effectively used for treatment of endocarditis, meningitis, and osteomyelitis—indications that are often considered to require bactericidal activity
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