Question

S. aureus (diameter in mm) E 15 mcg Sou o1 duy Sou 01 d 01. Are antibiotics considered to be effective against virus infections? Explain Q2. Excluding the situation with respect to allergies, is penicillin usually considered to be nontoxic to humans? Explain. (Think about selective toxicity.) Q3.What is beta lactamasc? Where are beta lactamases found in a bacterial cell- are they intracellular or extracellular (secreted) enzymes? Typically, what kind of genetic element carries the gene for this enzyme - is it on the chromosome, on a plasmid, etc.? 04. What does the abbreviation MIC stand for? Provide a brief explanation of its relevance to bacterial drug resistance. Explain what an E-test is and how it can be used to determine a MIC for a bacterial organism. Using the text, are there other tests that can be used to determine a MIC for a bacterial organism? Q5. What does the term Mueller-Hinton medium refer to? When/ why is this medium used? Note: For exam, it may be of use to know: i) what is a monobactam & why/when is it used? ii what is clavulanic acid & how/ why is clavulanic acid used iii) how can one relatively casily test for the ability of a bacterial cell to produce a beta lactamase? (In this case, you may have to look up this answer by Googling, ctc. - sometimes we get to do a demonstration so be on the lookout....) 24

4&5

Answer 1

4) MIC stands for Minimum Inhibitory Concentration. It is the minimum amount of drug required to inhibit bacterial growth. MIC breakpoint is a chosen concentration of antibiotic which defines whether a bacteria is resistant or susceptible to the antibiotic. If MIC is more than the breakpoint then the bacteria is resistant. Alternatively, if it is less than or equal to the breakpoint, the bacteria is susceptible to the drug.

Etest is a plastic reagent strip that has a predefined gradient of antibiotic, covering a continuous range, this determines the precise MIC value for a range of microorganisms. Other method to determine MIC is 'Broth dilution assay'. In this media, antimicrobial agent and microbe are the main reagents. Gradient solution of antimicrobial agent is made after mixing up with media. Inoculation is done at appropriate temperature as per the optimal growth conditions of bacteria. Turbidity of inoculation tubes determines MIC.

5) Mueller Hinton media is a cation adjusted media that is used in cultivation of wide variety of microorganisms. For sensitivity or susceptibility diffusion tests, it is recommended to used this media, as it supports growth of most pathogens and lacks inhibitors towards common antibiotics.

Mueller Hinton is commonly used as media in Broth Dilution Assay to determine MIC.