An 18-year-old girl is brought to the emergency department because of a 1-day history of severe headache with photophobia and diffuse myalgias. She is a college student and lives in a dormitory in a large urban area. She has not traveled recently. On arrival, she is lethargic. Her temperature is 39.3°C (102.7°F), pulse is 120/min, and blood pressure is 88/58 mm Hg. Examination shows scattered petechiae and ecchymoses on the trunk and lower extremities. There is decreased range of motion of the neck. Cerebrospinal fluid analysis shows a cell count of 1,600/μL (80% neutrophils) and a lactate concentration of 5.1 mmol/L. Which of the following is most likely to have prevented this patient's condition?

Polysaccharide conjugate vaccine

A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?

Oxidase-positive and ferments glucose and maltose

Oxidase-positive test and ferments glucose only

Catalase-negative and oxidase-positive

No growth on Thayer-Martin medium

A 71-year-old woman presents with high-grade fever and chills, difficulty breathing, and a productive cough with rust-colored sputum. She complains of a sharp left-sided chest pain. Physical examination reveals increased fremitus, dullness to percussion, and bronchial breath sounds on the lower left side. A chest X-ray shows left lower lobe consolidation. The offending organism that was cultured from the sputum was catalase-negative and had a positive Quellung reaction. The organism will show which gram stain results?

Cannot be seen with gram staining since the organism lacks a cell wall

Gram-positive cocci in clusters

You are seeing a 4-year-old boy in clinic who is presenting with concern for a primary immune deficiency. He has an unremarkable birth history, but since the age of 6 months he has had recurrent otitis media, bacterial pneumonia, as well as two episodes of sinusitis, and four episodes of conjunctivitis. He has a maternal uncle who died from sepsis secondary to H. influenza pneumonia. If you drew blood work for diagnostic testing, which of the following would you expect to find?

Abnormally low number of B cells

Abnormally low number of T cells

Abnormally high number of B cells

Abnormally high number of T cells

An 8-year-old girl is brought to the emergency room for a 6-hour history of fever, sore throat, and difficulty swallowing. Physical examination shows pooling of oral secretions and inspiratory stridor. Lateral x-ray of the neck shows thickening of the epiglottis and aryepiglottic folds. Throat culture with chocolate agar shows small, gram-negative coccobacilli. The patient's brother is started on the recommended antibiotic for chemoprophylaxis. Which of the following is the primary mechanism of action of this drug?

Inhibition of DNA-dependent RNA-polymerase

Inhibition of the 50S ribosomal subunit

Inhibition of prokaryotic topoisomerase II

Inhibition of the 30S ribosomal subunit

Inhibition of peptidoglycan crosslinking

A 27-year-old woman presents to the clinic with severe pain in her left knee of 1-day duration. Physical examination reveals a red, swollen, warm, and tender left knee with a decreased range of motion. The patient affirms that she has been sexually active with several partners over the last year and that 1 of her partners has complained of dysuria and yellow urethral discharge. An arthrocentesis was performed and showed a WBC count of 60,000/µL, with 90% polymorphonuclear leukocytes. Visualization of the patient's synovial fluid is provided in the image. Which of the following is a characteristic feature of the organism causing this condition?

It selectively grows on Thayer-Martin medium

It causes the Jarisch-Herxheimer reaction when treated with penicillin

It produces a heat-labile toxin that prevents protein synthesis

It is a gram-positive diplococcus

While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

Hemolytic reaction when grown on sheep blood agar

An 18-year-old female college student is brought to the emergency department by ambulance for a headache and altered mental status. The patient lives with her boyfriend who is with her currently. He states she had not been feeling well for the past day and has vomited several times in the past 12 hours. Lumbar puncture is performed in the emergency room and demonstrates an increased cell count with a neutrophil predominance and gram-negative diplococci on Gram stain. The patient is started on vancomycin and ceftriaxone. Which of the following is the best next step in management?

Add ampicillin to treatment regimen

Add ampicillin, dexamethasone, and rifampin to treatment regimen

Add dexamethasone to treatment regimen

Treat boyfriend with ceftriaxone and vancomycin

Haemophilus influenzae for USMLE Step 1: High-Yield Microbiology Lessons

Microbiology & Virulence - The Chocolate-Loving Vampire

Gram-negative coccobacillus.
Pleomorphic: can appear as long filaments, or coccoid.
Grows on chocolate agar, which contains heat-lysed blood cells.
- 📌 Requires both Factor V (NAD+) and Factor X (hematin) for growth - "The Vampire needs his X and V's".
Virulence Factors:
- Polysaccharide capsule: Antiphagocytic. Type b (Hib) is the most virulent, containing polyribosylribitol phosphate (PRP).
- IgA protease: Cleaves IgA at mucosal sites, facilitating colonization.

⭐ The Hib conjugate vaccine, which links the PRP capsule to a protein carrier (e.g., tetanus toxoid), has dramatically reduced the incidence of invasive H. influenzae type b diseases like meningitis and epiglottitis.

Haemophilus influenzae: Gram stain, SEM, and culture

Clinical Syndromes - From Ear to Everywhere

H. influenzae presents as two main clinical entities: non-typeable (NTHi) strains causing mucosal infections and encapsulated type b (Hib) causing invasive disease.

Non-typeable (NTHi) - Local Invasion
- Major cause of otitis media, sinusitis, and conjunctivitis.
- Frequently exacerbates COPD, leading to bronchitis and pneumonia.
Encapsulated (Hib) - Systemic Invasion (mainly in unvaccinated children)
- 📌 EMOP: Epiglottitis, Meningitis, Otitis media, Pneumonia.
- Acute Epiglottitis: Medical emergency! Characterized by dysphagia, drooling, and distress. Cherry-red epiglottis.
- Meningitis: Previously the leading cause in children aged 3 months to 3 years.
- Septic arthritis, Sepsis.

⭐ The Hib conjugate vaccine, given between 2 and 15 months, has dramatically reduced invasive disease. Consequently, NTHi is now the predominant strain causing infections in the US.

Lateral neck X-ray: Epiglottitis "thumb sign"

Diagnosis - Lab Detective Work

Specimen: CSF, blood, sputum, or pus.
Microscopy: Gram-negative coccobacilli or pleomorphic rods.
Culture: Requires chocolate agar (heated blood) to release factors X (hematin) and V (NAD+).
- 📌 "Satellitism": H. influenzae grows around S. aureus colonies on blood agar, as S. aureus lyses RBCs and provides the necessary V factor.

Gram stain of Haemophilus influenzae coccobacilli

Quellung Reaction: For capsular serotyping (e.g., type b); capsule swells with specific antiserum.

⭐ Antigen detection in CSF via latex agglutination is a rapid method for diagnosing H. influenzae meningitis.

Treatment & Prevention - Drugs, Jabs, and Victory

Systemic Disease (Meningitis, Epiglottitis):
- IV Ceftriaxone is the empirical treatment of choice.
Mucosal Infections (Otitis Media, Sinusitis):
- Amoxicillin-clavulanate is effective against β-lactamase producing strains.
Chemoprophylaxis for Close Contacts:
- Rifampin is used to eliminate nasopharyngeal carriage in unvaccinated contacts.
Vaccination (The Ultimate Victory):
- Hib conjugate vaccine has dramatically ↓ incidence.
- Contains capsular polysaccharide (PRP) conjugated to a carrier protein (e.g., tetanus toxoid).
- Routinely given between 2 and 18 months of age.

⭐ For invasive H. influenzae meningitis, ceftriaxone is the primary antibiotic. Dexamethasone is often co-administered to reduce the risk of neurological sequelae, such as sensorineural hearing loss.

High‑Yield Points - ⚡ Biggest Takeaways

Gram-negative coccobacillus requiring chocolate agar with growth factors V (NAD+) and X (hematin).

Causes Meningitis, Epiglottitis, Pneumonia, and Otitis Media, mainly in unvaccinated children.

The primary virulence factor for invasive disease is the type b (Hib) capsule (a polyribosylribitol phosphate polymer).

The Hib conjugate vaccine (PRP-protein) has nearly eliminated invasive Hib disease.

Treat systemic disease with ceftriaxone; use rifampin for prophylaxis in close contacts.

Non-typeable (unencapsulated) strains commonly cause mucosal infections (otitis, sinusitis).

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play