Case Studies – Antimetabolites

by Pravin Shukle, MD

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    00:00 Okay, let's go on to a question.

    00:03 An HIV positive patient presents with an atypical pneumonia and was treated appropriately with Sulfamethoxazole/trimethoprim.

    00:12 Which of the following organisms can be reasonably treated with this regimen in the HIV patient? Is it A) Pneumocystis.

    00:21 Is it B) Toxoplasmosis.

    00:24 Is it C) Microbacterium tuberculi.

    00:28 It is D) A and B which is both Pneumocystis and Toxoplasmosis.

    00:34 I've highlighted both of them there for you.

    00:36 And or is it A and C which is Pneumocystis and Microbacterium tuberculi.

    00:42 Good, you chose D which is A and B.

    00:46 So trimethoprim/sulfamehtoxazole is an effective treatment for pneumocystis carinii pneumonia or PCP as well as toxoplasmosis gondii.

    00:59 Both A and B are therefore correct.

    01:01 So therefore the answer is actually D.

    01:03 This drug combination acts against folic acid production.

    01:07 It is not effective against viruses or against tuberculosis.

    01:11 And it is rarely effective against fungal infections.

    01:14 Now when you're doing an exam question like this, remember that most of the board exams have dropped the K-type exam question.

    01:23 But some boards still use K-type exam questions.

    01:26 It is still worthwhile for you to keep your skills answering this type of question.

    01:31 But I will tell you that you probably will get this kind of K-type in the USMLE step one.

    01:39 Okay, let's go on to another question.

    01:41 Mr. Locutus recently had an implant.

    01:44 He developed an infection that is composed of multiple gram positive cocci and gram negative rods.

    01:50 He was started on ciprofloxacin and responded well.

    01:54 Three weeks later he returns.

    01:56 You are considering the same treatments.

    01:58 Which is not a mechanism of resistance with this medication? Is it A) Point mutations in the gene coding for the 3OS ribosomal subunit.

    02:11 Is it B) Changes in porins or pores in the cell membrane.

    02:15 Is it C) Point mutations in the DNA gyrase enzyme.

    02:19 And is it D) Enhanced ejection of the drug through new efflux mechanisms developed by organisms like Staphylococcus aureus.

    02:30 You chose A.

    02:32 Point mutations in the gene coding for the 3OS ribosomal subunit.

    02:37 So ciprofloxacin is an excellent broad spectrum antibiotic.

    02:40 It works well against a whole host of bacteria as well as some anaerobes both gram positives and gram negatives anaerobes.

    02:48 These are the types of infections we see in prosthetic infections, tunnel infections and dirty procedures.

    02:54 For example an emergency treatment of a motor vehicle accident.

    02:59 Resistance develops through the following mechanisms.

    03:02 Number 1, decreased intracellular accumulation due to changes in porins.

    03:08 Number 2, Efflux mechanisms in microbacterium tuberculosis, staphylococcus aureus, staphylococcus pneumoniae.

    03:16 And number 3, the changes in sensitivity of target enzymes like the DNA gyrase is also called topoisomerases via point mutations.

    03:26 Now cipro does not act through the 30S ribosomal subunit.

    03:30 The correct choice is therefore is A because that's the incorrect answer.

    About the Lecture

    The lecture Case Studies – Antimetabolites by Pravin Shukle, MD is from the course Antimicrobial Pharmacology.

    Included Quiz Questions

    1. Changes in sensitivity of target enzymes via point mutations
    2. Increased intracellular accumulation due to changes in porins
    3. Decreased intracellular accumulation due to changes in DNA gyrase
    4. Influx mechanisms in M. tuberculosis
    5. Influx mechanisms in S. aureus
    1. Toxoplasmosis
    2. P. aeruginosa
    3. Enterococcus faecalis
    4. Enterococcus faecium
    5. VRE

    Author of lecture Case Studies – Antimetabolites

     Pravin Shukle, MD

    Pravin Shukle, MD

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