Antimicrobials

Antimicrobials US Medical PG Question 1: A 64-year-old woman with a past medical history of poorly managed diabetes presents to the emergency department with nausea and vomiting. Her symptoms started yesterday and have been progressively worsening. She is unable to eat given her symptoms. Her temperature is 102°F (38.9°C), blood pressure is 115/68 mmHg, pulse is 120/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for left-sided costovertebral angle tenderness, and urinalysis demonstrates bacteriuria and pyuria. The patient is admitted to the hospital and started on IV ceftriaxone. On day 3 of her hospital stay she is afebrile, able to eat and drink, and feels better. Which of the following antibiotic regimens should be started or continued as an outpatient upon discharge?

• A. Amoxicillin
• B. Nitrofurantoin
• C. Ciprofloxacin
• D. Ceftriaxone
• E. Trimethoprim-sulfamethoxazole (Correct Answer)

Antimicrobials Explanation: ***Trimethoprim-sulfamethoxazole*** - This patient presents with **acute pyelonephritis**, indicated by fever, CVA tenderness, bacteriuria, and pyuria, requiring a 7-14 day course of treatment. - After 3 days of IV ceftriaxone with clinical improvement, **step-down to oral therapy** is appropriate. - **Trimethoprim-sulfamethoxazole** is a first-line oral agent for completing treatment of uncomplicated pyelonephritis and is preferred when **local resistance rates are <20%** and no specific resistance data is available. - It achieves adequate concentrations in renal tissue and is cost-effective for outpatient therapy. *Amoxicillin* - **Amoxicillin** is not recommended for pyelonephritis due to **high resistance rates** among common uropathogens like *E. coli* (often >30%). - It does not achieve adequate concentrations in the renal parenchyma to effectively treat **upper urinary tract infections**. *Nitrofurantoin* - **Nitrofurantoin** is concentrated in the bladder and is highly effective for **lower urinary tract infections (cystitis)**. - It is **contraindicated for pyelonephritis** because it does not reach therapeutic levels in the renal parenchyma or bloodstream. - Additionally, it should be avoided in patients with impaired renal function. *Ciprofloxacin* - **Ciprofloxacin** is also a first-line oral agent for pyelonephritis and would be an appropriate choice for step-down therapy. - However, when no culture or susceptibility data is available, **TMP-SMX is generally preferred** as an initial oral choice due to concerns about promoting fluoroquinolone resistance and potential adverse effects (tendonitis, QT prolongation, C. difficile). - Fluoroquinolones are particularly valuable when reserved for cases with known resistance to other agents or specific patient contraindications. *Ceftriaxone* - **Ceftriaxone** is an IV third-generation cephalosporin used for initial empiric treatment of acute pyelonephritis in hospitalized patients. - Once the patient is **clinically stable, afebrile, and tolerating oral intake**, transitioning to oral therapy is standard practice. - Continued IV therapy as an outpatient is unnecessarily invasive and costly when oral options are effective.

Antimicrobials US Medical PG Question 2: A 77-year-old woman is brought to the emergency department from her nursing home because she was found down overnight. On presentation she was found to be delirious and was unable to answer questions. Chart review shows that she is allergic to cephalosporins. Her temperature is 102.2°F (39°C), blood pressure is 105/52 mmHg, pulse is 94/min, and respirations are 23/min. Physical exam reveals a productive cough. A metabolic panel is obtained with the following results: Serum: Na+: 135 mEq/L Cl-: 95 mEq/L K+: 4 mEq/L HCO3-: 19 mEq/L BUN: 40 mg/dL Creatinine: 2.5 mg/dL Glucose: 150 mg/dL Based on these findings two different drugs are started empirically. Gram stain on a blood sample is performed showing the presence of gram-positive organisms on all samples. One of the drugs is subsequently stopped. The drug that was most likely stopped has which of the following characteristics?

• A. Resistance conveyed through acetylation
• B. Associated with red man syndrome
• C. Single-ringed ß-lactam structure (Correct Answer)
• D. Causes discolored teeth in children
• E. Accumulates inside bacteria via O2-dependent uptake

Antimicrobials Explanation: ***Single-ringed ß-lactam structure*** - The patient presents with **sepsis** due to **pneumonia** likely caused by **gram-positive organisms**. Given a cephalosporin allergy, **aztreonam** (a monobactam) would be an initial empirical antibiotic choice to cover gram-negative bacteria, alongside a drug for gram-positive coverage (like vancomycin). - Since the **blood cultures** confirmed **gram-positive organisms**, the drug covering gram-negative bacteria (aztreonam) would be stopped. Aztreonam is characterized by its **single-ringed β-lactam structure**. *Resistance conveyed through acetylation* - This mechanism of resistance is typical of **aminoglycosides** (e.g., gentamicin) and **chloramphenicol**. - Aminoglycosides were unlikely to be one of the empirically started drugs, as they are often used in combination with β-lactams, and this patient has a cephalosporin allergy. *Associated with red man syndrome* - **Red man syndrome** is a common adverse effect associated with **vancomycin** administration, especially with rapid infusion. - Vancomycin would likely be continued, as it effectively targets gram-positive organisms, including **MRSA**, and is a suitable alternative given the cephalosporin allergy. *Causes discolored teeth in children* - This is a characteristic side effect of **tetracyclines** (e.g., doxycycline), which are contraindicated in young children and pregnant women due to their effects on bone and teeth development. - Tetracyclines are not typically first-line empiric therapy for severe pneumonia or sepsis, especially in an elderly patient. *Accumulates inside bacteria via O2-dependent uptake* - This describes the mechanism of uptake for **aminoglycosides**. Their entry into bacteria is an **energy-dependent process** requiring oxygen. - As mentioned, aminoglycosides are less likely to be the initial drug stopped in this scenario, as they target gram-negative bacteria.

Antimicrobials US Medical PG Question 3: A 64-year-old female with type 2 diabetes mellitus comes to the physician because of a 1-week history of painful red swelling on her left thigh. Examination shows a 3- x 4-cm, tender, fluctuant mass. Incision and drainage of the abscess are performed. Culture of the abscess fluid grows gram-positive, coagulase-positive cocci that are resistant to oxacillin. Which of the following best describes the mechanism of resistance of the causal organism to oxacillin?

• A. Degradation of the antibiotic
• B. Decreased uptake of the antibiotic
• C. Decreased activation of the antibiotic
• D. Altered target of the antibiotic (Correct Answer)
• E. Acetylation of the antibiotic

Antimicrobials Explanation: ***Altered target of the antibiotic*** - The organism described (gram-positive, coagulase-positive cocci, oxacillin-resistant) is **methicillin-resistant *Staphylococcus aureus* (MRSA)**. - MRSA achieves oxacillin (and other beta-lactam) resistance by acquiring the ***mecA* gene**, which encodes for a **modified penicillin-binding protein (PBP2a)** with reduced affinity for beta-lactam antibiotics. *Degradation of the antibiotic* - This mechanism, primarily through the production of **beta-lactamase enzymes**, can degrade beta-lactam antibiotics. - While *Staphylococcus aureus* can produce beta-lactamases, oxacillin (a **penicillinase-resistant penicillin**) is specifically engineered to be stable against these enzymes. *Decreased uptake of the antibiotic* - Reduced permeability of the bacterial cell wall can lead to decreased uptake, a mechanism more commonly associated with **gram-negative bacteria** due to their outer membrane. - This is not the primary mechanism of resistance for MRSA to oxacillin. *Decreased activation of the antibiotic* - Some antibiotics are prodrugs that require activation by bacterial enzymes, and resistance can arise from mutations affecting this activation. - Oxacillin is active in its administered form and does not require bacterial activation. *Acetylation of the antibiotic* - **Enzymatic modification**, such as acetylation, adenylylation, or phosphorylation, is a common mechanism of resistance, particularly against **aminoglycoside antibiotics**. - This specific mechanism is not responsible for oxacillin resistance in MRSA.

Antimicrobials US Medical PG Question 4: Three days after admission to the hospital following a motor vehicle accident, a 45-year-old woman develops a fever. A central venous catheter was placed on the day of admission for treatment of severe hypotension. Her temperature is 39.2°C (102.5°F). Examination shows erythema surrounding the catheter insertion site at the right internal jugular vein. Blood cultures show gram-positive, catalase-positive cocci that have a high minimum inhibitory concentration when exposed to novobiocin. Which of the following is the most appropriate pharmacotherapy?

• A. Clarithromycin
• B. Vancomycin (Correct Answer)
• C. Metronidazole
• D. Penicillin G
• E. Polymyxin B

Antimicrobials Explanation: ***Vancomycin*** - The description of **gram-positive, catalase-positive cocci** that are **novobiocin-resistant** (high MIC) strongly points to **Staphylococcus epidermidis** or other coagulase-negative staphylococci. *S. epidermidis* is the most common cause of **catheter-related bloodstream infections** and is often **methicillin-resistant**, making **vancomycin** the drug of choice. - The presence of erythema at the catheter site and fever in a patient with a central venous catheter indicates a **central line-associated bloodstream infection (CLABSI)**, for which empiric coverage with vancomycin is standard until sensitivities are known. - The novobiocin resistance test helps differentiate *S. epidermidis* (resistant) from *S. saprophyticus* (sensitive). *Clarithromycin* - **Clarithromycin** is a macrolide antibiotic primarily used for respiratory tract infections and *Mycobacterium avium complex*. - It is **not effective** against methicillin-resistant staphylococci and would not be appropriate for a suspected CLABSI. *Metronidazole* - **Metronidazole** is an antibiotic mainly used for **anaerobic bacterial infections** and certain parasitic infections. - It has **no activity** against gram-positive cocci like staphylococci. *Penicillin G* - **Penicillin G** is a narrow-spectrum penicillin effective against some gram-positive cocci, like **Streptococcus pyogenes**. - However, virtually all staphylococci, especially those causing hospital-acquired infections, are **resistant to penicillin G** due to beta-lactamase (penicillinase) production. *Polymyxin B* - **Polymyxin B** is an antibiotic primarily effective against **gram-negative bacteria**, particularly those with multi-drug resistance such as **Pseudomonas aeruginosa** and **Acinetobacter baumannii**. - It has **no significant activity** against gram-positive cocci like staphylococci.

Antimicrobials US Medical PG Question 5: A scientist is studying the mechanisms by which bacteria become resistant to antibiotics. She begins by obtaining a culture of vancomycin-resistant Enterococcus faecalis and conducts replicate plating experiments. In these experiments, colonies are inoculated onto a membrane and smeared on 2 separate plates, 1 containing vancomycin and the other with no antibiotics. She finds that all of the bacterial colonies are vancomycin resistant because they grow on both plates. She then maintains the bacteria in liquid culture without vancomycin while she performs her other studies. Fifteen generations of bacteria later, she conducts replicate plating experiments again and finds that 20% of the colonies are now sensitive to vancomycin. Which of the following mechanisms is the most likely explanation for why these colonies have become vancomycin sensitive?

• A. Point mutation
• B. Gain of function mutation
• C. Viral infection
• D. Plasmid loss (Correct Answer)
• E. Loss of function mutation

Antimicrobials Explanation: ***Plasmid loss*** - The initial **vancomycin resistance** in *Enterococcus faecalis* is often mediated by genes located on **plasmids**, which are extrachromosomal DNA. - In the absence of selective pressure (vancomycin), bacteria that lose the plasmid (and thus the resistance genes) have a **growth advantage** over those that retain the energetically costly plasmid, leading to an increase in sensitive colonies over generations. *Point mutation* - A **point mutation** typically involves a change in a single nucleotide and could lead to loss of resistance if it occurred in a gene conferring resistance. - However, since there was no selective pressure for loss of resistance, it is less likely that 20% of the population would acquire such a specific point mutation to revert resistance. *Gain of function mutation* - A **gain of function mutation** would imply that the bacteria acquired a *new* advantageous trait, not the *loss* of resistance. - This type of mutation would not explain why some colonies became sensitive to vancomycin after the drug was removed. *Viral infection* - **Viral infection** (bacteriophages) can transfer genes through transduction or cause bacterial lysis, but it's not the primary mechanism for a widespread reversion of resistance in the absence of antibiotic pressure. - It would not explain the observed increase in vancomycin-sensitive colonies due to evolutionary pressure. *Loss of function mutation* - While a **loss of function mutation** in a gene conferring resistance could lead to sensitivity, it's generally less likely to explain a 20% shift without selective pressure than **plasmid loss**. - Plasmids are often unstable and are easily lost in the absence of selection, whereas a specific gene mutation causing loss of function would need to arise and become prevalent in the population.

Antimicrobials Flashcard 1 of 5

Question: In addition to increased risk for C. difficile infection, patients on proton pump inhibitors are also at increased risk for _____

Answer: pneumonia

Antimicrobials Flashcard 2 of 5

Question: Fluoroquinolones are _____ due to them causing cartilage damage to the fetus

Answer: teratogenic

Antimicrobials Flashcard 3 of 5

Question: Isoniazid works to treat tuberculosis by inhibiting mycobacterial _____ production

Answer: cell wall

Antimicrobials Flashcard 4 of 5

Question: Which antimicrobials displace unconjugated bilirubin from serum albumin in the blood?_____

Answer: Sulfonamides

Antimicrobials Flashcard 5 of 5

Question: What antimicrobial is similar to sulfonamides, inhibiting dihydropteroate synthase?_____

Answer: Dapsone