A 50-year-old male presents to the emergency room complaining of fever, shortness of breath, and diarrhea. He returned from a spa in the Rocky Mountains five days prior. He reports that over the past two days, he developed a fever, cough, dyspnea, and multiple watery stools. His past medical history is notable for major depressive disorder and peptic ulcer disease. He takes omeprazole and paroxetine. He does not smoke and drinks alcohol on social occasions. His temperature is 102.8°F (39.3°C), blood pressure is 120/70 mmHg, pulse is 65/min, and respirations are 20/min. Physical examination reveals dry mucus membranes, delayed capillary refill, and rales at the bilateral lung bases. A basic metabolic panel is shown below: Serum: Na+: 126 mEq/L Cl-: 100 mEq/L K+: 4.1 mEq/L HCO3-: 23 mEq/L Ca2+: 10.1 mg/dL Mg2+: 2.0 mEq/L Urea nitrogen: 14 mg/dL Glucose: 90 mg/dL Creatinine: 1.1 mg/dL Which of the following is the most appropriate growth medium to culture the pathogen responsible for this patient’s condition?

Charcoal yeast agar with iron and cysteine

A patient is hospitalized for pneumonia. Gram-positive cocci in clusters are seen on sputum gram stain. Which of the following clinical scenarios is most commonly associated with this form of pneumonia?

Hospitalized adult with development of pneumonia symptoms 2 weeks following a viral illness

Elderly patient who has trouble swallowing and poor dentition

An alcoholic with evidence of empyema and "currant jelly sputum"

An otherwise healthy young adult with a week of mild fatigue, chills, and cough

HIV positive adult with a CD4 count less than 150 and an impaired diffusion capacity

A 57-year-old HIV-positive male with a history of intravenous drug abuse presents to the emergency room complaining of arm swelling. He reports that he developed progressively worsening swelling and tenderness over the right antecubital fossa three days prior. He recently returned from a trip to Nicaragua. His past medical history is notable for an anaphylactoid reaction to vancomycin. His temperature is 101.4°F (38.6°C), blood pressure is 140/70 mmHg, pulse is 110/min, and respirations are 20/min. Physical examination reveals an erythematous, fluctuant, and tender mass overlying the right antecubital fossa. Multiple injection marks are noted across both upper extremities. He undergoes incision and drainage and is started on an antibiotic that targets the 50S ribosome. He is discharged with plans to follow up in one week. However, five days later he presents to the same emergency room complaining of abdominal cramps and watery diarrhea. Which of the following classes of pathogens is most likely responsible for this patient’s current symptoms?

Anaerobic flagellated protozoan

A 67-year-old man is brought to the emergency department because of severe dyspnea and orthopnea for 6 hours. He has a history of congestive heart disease and an ejection fraction of 40%. The medical history is otherwise unremarkable. He appears confused. At the hospital, his blood pressure is 165/110 mm Hg, the pulse is 135/min, the respirations are 48/min, and the temperature is 36.2°C (97.2°F). Crackles are heard at both lung bases. There is pitting edema from the midtibia to the ankle bilaterally. The patient is intubated and admitted to the critical care unit for mechanical ventilation and treatment. Intravenous morphine, diuretics, and nitroglycerine are initiated. Which of the following is the most effective method to prevent nosocomial infection in this patient?

Daily oropharynx decontamination with antiseptic agent

Daily urinary catheter irrigation with antimicrobial agent

A surgical ICU has implemented multiple interventions over 18 months: chlorhexidine bathing, antibiotic stewardship, contact precautions for MRSA, and environmental cleaning protocols. Despite these efforts, MRSA surgical site infection rates remain unchanged at 8 per 1000 surgical procedures. Universal MRSA screening shows 15% of admitted patients are colonized. Evaluate the most effective evidence-based strategy to reduce MRSA SSI rates further.

Universal MRSA decolonization for all surgical patients regardless of screening results

Targeted decolonization only for MRSA-positive patients with mupirocin and chlorhexidine

Isolation of all surgical patients in private rooms until discharge

Routine screening and decolonization of all healthcare workers

Extended vancomycin prophylaxis for all surgical patients for 48 hours postoperatively

Environmental reservoirs in healthcare settings for USMLE Step 2 CK: High-Yield Microbiology Lessons

Reservoirs & Fomites - The Usual Suspects

Hospital-acquired infection transmission pathways

Water Systems: Sinks, showers, ventilators, and ice machines.
- Legionella pneumophila (aerosolized water)
- Pseudomonas aeruginosa (biofilms)
- Acinetobacter baumannii
Dry Surfaces & Medical Equipment: Bed rails, stethoscopes, blood pressure cuffs, computers.
- MRSA (Staphylococcus aureus)
- VRE (Enterococcus spp.)
- Clostridioides difficile (spores)
Air/Ventilation: Dust from construction.
- Aspergillus spp.

⭐ C. difficile spores are highly resistant to alcohol-based sanitizers; handwashing with soap and water is required for mechanical removal.

Waterborne Pathogens - Just Add Water

Legionella pneumophila
- Reservoirs: Hospital water systems, cooling towers, showerheads, sink faucets.
- Transmission: Inhalation of contaminated aerosols, NOT person-to-person.
- Diseases: Legionnaires' disease (atypical pneumonia, often with hyponatremia) & Pontiac fever (self-limited, flu-like).
- Diagnosis: Urine antigen test; culture on buffered charcoal yeast extract (BCYE) agar.
Pseudomonas aeruginosa
- Reservoirs: Sinks, drains, ventilators, flowers/plants.
- High-Risk: Burn patients, cystic fibrosis, neutropenia, catheterized patients.
- Clues: Fruity, grape-like odor; blue-green pigment (pyocyanin).

⭐ Exam Favorite: Legionella infection classically presents with a combination of pneumonia, gastrointestinal symptoms (diarrhea), and neurological changes (confusion), often accompanied by hyponatremia (SIADH).

Surfaces & Spores - The Dry & Hardy Crew

Concept: Certain pathogens thrive on dry, inanimate surfaces (fomites) for extended periods, acting as persistent reservoirs for HAIs. Survival is often mediated by spore formation or inherent resilience.
Key Pathogens & Survival:
- Clostridioides difficile: Spores survive for >5 months.
- MRSA (S. aureus): Persists for weeks to months.
- VRE (Enterococcus): Survives for days to weeks.
- Acinetobacter baumannii: Extremely resilient; lasts for months.
Persistence Mechanisms:
- Endospores: Dormant, highly resistant bacterial forms (e.g., C. diff). Impervious to alcohol sanitizers.
- Biofilms: Protective matrix enhances surface survival.

⭐ High-Yield Fact: C. difficile spores are resistant to alcohol-based hand rubs. Handwashing with soap and water is crucial for mechanical removal of spores after patient contact.

Transmission & Control:

-   **Control:** Contact precautions, diligent hand hygiene (soap & water for spores), and use of sporicidal disinfectants (e.g., bleach).

SEM of C. difficile biofilm formation on surfaces

Disinfection & Control - The Clean-Up Crew

Sterilization: Kills all microbial life, including spores.
- Autoclave: Pressurized steam at 121°C for >15 min.
- Ethylene Oxide: For heat-sensitive items.
Disinfection: Eliminates most pathogens; spores may survive.
- High-level: Glutaraldehyde for semi-critical items (e.g., endoscopes).
- Intermediate-level: Alcohols, iodophors for non-critical surfaces.
- Low-level: Quaternary ammonium compounds (e.g., benzalkonium chloride).
Antisepsis: Disinfection applied to living tissue (e.g., chlorhexidine).

Microbial resistance to disinfectants hierarchy

⭐ Prions are highly resistant to standard sterilization. Decontamination requires specific protocols like a 1N NaOH soak followed by extended autoclaving at 134°C.

High‑Yield Points - ⚡ Biggest Takeaways

Legionella thrives in hospital water systems, causing pneumonia.

Pseudomonas aeruginosa is found in moist environments like sinks and ventilators.

Aspergillus spores are aerosolized during hospital construction.

C. difficile spores on surfaces require bleach disinfection and handwashing with soap and water.

Acinetobacter baumannii persists on dry surfaces and is often multidrug-resistant.

Medical equipment, like stethoscopes, can act as fomites for pathogens like MRSA.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play