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?

Single-ringed ß-lactam structure

Resistance conveyed through acetylation

Associated with red man syndrome

Causes discolored teeth in children

Accumulates inside bacteria via O2-dependent uptake

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?

Altered target of the antibiotic

Decreased uptake of the antibiotic

Decreased activation of the antibiotic

A 27-year-old man presents to the emergency department with weakness and a fever for the past week. The patient is homeless and has a past medical history of alcohol and IV drug abuse. His temperature is 102°F (38.9°C), blood pressure is 107/68 mmHg, pulse is 120/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for a tremulous patient with antecubital scars and a murmur over the left lower sternal border. Blood cultures are drawn and the patient is started on vancomycin and ceftriaxone and is admitted to the ICU. The patient's fever and symptoms do not improve despite antibiotic therapy for which the initial identified organism is susceptible. Cultures currently reveal MRSA as one of the infective organisms. Which of the following is the best next step in management?

Transesophageal echocardiography

Nafcillin and piperacillin-tazobactam

An 11-year-old boy presents with a sore throat, fever, chills, and difficulty swallowing for the past 3 days. The patient’s mother says that last night he was short of breath and had a headache. Past medical history is unremarkable. The patient has not been vaccinated as his mother thinks it is "unnecessary". His temperature is 38.3°C (101.0°F), blood pressure is 120/70 mm Hg, pulse is 110/min, and respiratory rate is 18/min. On physical examination, the patient is ill-appearing and dehydrated. A grayish-white membrane and pharyngeal erythema are present in the oropharynx. Significant cervical lymphadenopathy is also present. A throat swab is taken and gram staining shows gram-positive club-shaped bacilli along with few neutrophils. Which of the following would most likely be the result of the bacterial culture of the throat swab in this patient?

Small black colonies on tellurite agar

Hemolytic black colonies on blood agar

Metallic green colonies on eosin-methylene blue agar

Greyish-white colonies on Thayer-Martin agar

Creamy white colonies on Loeffler's serum

MRSA and VRE for USMLE Step 1: High-Yield Microbiology Lessons

MRSA - The Staph Superbug

Mechanism: The mecA gene encodes Penicillin-Binding Protein 2a (PBP2a). This altered protein has a low affinity for β-lactam antibiotics, conferring resistance by preventing their binding.
Clinical Syndromes:
- Community-Associated (CA-MRSA): Presents as skin/soft tissue infections (abscesses, boils). Virulence is often driven by Panton-Valentine Leukocidin (PVL) toxin.
- Hospital-Associated (HA-MRSA): Causes more invasive disease (bacteremia, pneumonia, endocarditis) in patients with recent healthcare exposure.
Diagnosis:
- Rapid: NAAT for the mecA gene offers fast identification.
- Culture: Chromogenic agar for screening; Kirby-Bauer testing with a cefoxitin disk confirms resistance.

⭐ The PVL toxin in CA-MRSA is linked to severe, necrotizing pneumonia, classically seen in healthy patients following an influenza infection.

VRE - Vexing Vancomycin Villain

Vancomycin Resistance: VanA-type D-Ala-D-Lac Synthesis

Mechanism: Acquired vanA or vanB genes alter the peptidoglycan precursor target.
- Terminal $D-Ala-D-Ala$ is changed to $D-Ala-D-Lac$.
- This substitution prevents vancomycin from binding effectively.
Predominant Species:
- Enterococcus faecium is more frequently vancomycin-resistant than E. faecalis.
Key Risk Factors:
- Prolonged hospitalization or ICU stay.
- Long-term antibiotic therapy, especially with vancomycin.
Clinical Syndromes:
- Nosocomial infections, including UTIs, bacteremia, and endocarditis.

⭐ VRE can transfer its resistance genes to other bacteria, such as S. aureus, leading to the emergence of Vancomycin-Resistant Staphylococcus aureus (VRSA).

Treatment - The Resistance Rumble

Drug	Mechanism & Key Side Effect	MRSA	VRE
Vancomycin	Inhibits cell wall synthesis by binding D-Ala-D-Ala. → Red Man Syndrome, nephrotoxicity.	✅	❌
Daptomycin	Lipopeptide; depolarizes cell membrane. → Myopathy (monitor CPK).	✅	✅
Linezolid	Binds 50S subunit. → Thrombocytopenia, Serotonin Syndrome (risk with SSRIs).	✅	✅
Ceftaroline	5th-gen cephalosporin; binds PBP2a. → Hemolytic anemia.	✅	❌
Tigecycline	Binds 30S subunit. → Severe nausea/vomiting; ↑ mortality warning.	❌	✅

MRSA resistance stems from the mecA gene, encoding an altered penicillin-binding protein (PBP2a), making it impervious to most beta-lactams.

VRE resistance arises from the conversion of the peptidoglycan precursor from D-Ala-D-Ala to D-Ala-D-Lac.

Key MRSA treatments include Vancomycin, Linezolid, and Daptomycin.

For VRE, preferred agents are Linezolid and Daptomycin.

Both are significant causes of nosocomial infections, particularly in immunocompromised patients.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play

MRSA and VRE

MRSA and VRE

On this page

MRSA - The Staph Superbug

VRE - Vexing Vancomycin Villain

Treatment - The Resistance Rumble

Continue reading on Oncourse

Enjoying this lesson?