3.18 Bartonella Henselae

Microbiology review for the USMLE Step 1 exam

• Bartonella henselae is an intracellular gram negative rod that causes several different diseases
• Bartonella targets and lives inside specialized immune cells called CD34+ cells
• Bartonella creates a protective vacuole that protects it and helps it evade immune detection
• Bartonella henselae is a facultative intracellular bacteria
• Three different clinical syndromes associated with bartonella henselae: cat scratch disease, bacillary angiomatosis in immunocompromised patients, and bacterial endocarditis
• Cat scratch disease presents with cutaneous manifestations at the site of inoculation (warm, red, swollen, with a vesicle) and swollen lymph nodes near the site
• Bacillary angiomatosis symptoms include fever, multiple clustered red or violaceous papules or nodules on the skin and mucosa, and bone pain
• Bacterial endocarditis: Bartonella makes its way into the bloodstream and infects the inner surface of the heart
• Bartonella henselae is hard to culture from blood, usually diagnosed via serology
• Treatment depends on the clinical syndrome presented, cat scratch disease is usually self-limited and doesn’t necessarily require antibiotics, bacillary angiomatosis treated with a long 4-month course of either erythromycin or doxycycline.

3.17 Brucella Species

Microbiology review for the USMLE Step 1 exam

• Brucella species are gram-negative intracellular coccobacilli that are spread from animals to humans, causing a zoonotic infection called Brucellosis.
• B. melitensis is the most common species that causes infection in humans and is also the most virulent.
• Brucella only requires a very small dose (10-100 bacteria) to cause full-blown disease, often spread through contaminated unpasteurized milk or contact with animal carcasses.
• "Bruce Wayne's goat, Brucella Wayne, helps him SLAUGHTER bad guys"
• Symptoms include cyclical fever and chills, general malaise, headaches, joint pain, and nausea and vomiting.
• Diagnosis can be confirmed through blood cultures and PCR.
• Treatment involves antibiotics, typically doxycycline for a course of 6 weeks.
• People most at risk for infection include those who drink unpasteurized goat milk and those who work with animal carcasses, such as slaughterhouse workers.

3.16 Pasteurella Multocida

Microbiology review for the USMLE Step 1 exam

• Pasteurella multocida is a gram-negative coccobacillus that commonly causes infection following animal bites or scratches
• Classic presentation is a young male with swollen, red, and tender skin at the site of the bite or scratch
• Symptoms develop rapidly, usually within 3-48 hours after injury
• In rare cases, infection can progress to necrotizing fasciitis or bacteremia
• Diagnosis is made clinically using patient history and symptoms
• Treatment of choice is amoxicillin-clavulanate, and infections usually resolve with little complications when treated with antibiotic

3.15 Bordetella Pertussis

Microbiology review for the USMLE Step 1 exam

• Bordetella pertussis is a gram-negative bacterium that causes whooping cough
• Spread through respiratory droplets, most people are vaccinated against it
• Adheres to ciliated respiratory epithelial cells and produces toxins that cause inflammation and systemic effects
• Pertussis toxin and tracheal cytotoxin are important toxins secreted by the bacteria
• Symptoms include four stages: incubation, catarrhal, paroxysmal, and convalescent
• Infants and unvaccinated people are most commonly affected
• Treatment is mostly supportive, including oxygen, suctioning, hydration, and avoidance of respiratory irritants.

3.14 Haemophilus Influenzae

Microbiology review for the USMLE Step 1 exam

• Haemophilus influenzae is a gram negative coccobacilli that is often encapsulated, encapsulated strains are broken up into subgroups a through f, non-encapsulated strains are referred to as non-typable
• The most common pathogenic strain is type b, which causes disease in children and immunocompromised individuals, it is the strain that we vaccinate against
• The group B H. influenzae vaccine is a conjugated polysaccharide vaccine recommended as a routine childhood vaccination in the United States, provides protection against the group B strain of the bug
• H. influenzae mainly affects the extremes of age, very young children (younger than 5) and elderly adults (>65)
• In young children, it most commonly causes epiglottitis, otitis media, and meningitis, in elderly adults it is a cause of lower respiratory tract infections, especially in patients with COPD
• H flu infections in young children can be a medical emergency and requires treatment with antibiotics and intubation in some cases
• Otitis media is the most common H flu infection in children, caused by non-group B H flu, it is still the number one cause of otitis media in children

3.13 Moraxella Catarrhalis

Microbiology review for the USMLE Step 1 exam

• Moraxella Catarrhalis is a gram negative diplococcus that is part of normal upper respiratory tract flora
• Mainly causes three different types of infections: otitis media in children, lower respiratory tract infections in COPD patients, and pneumonia in elderly patients
• Otitis media is the most common bacterial infectious disease in childhood and the most common reason for which children receive antibiotics. M. catarrhalis causes about 15-20% of otitis media cases.
• Symptoms of otitis media in children include pulling or tugging at the ears, irritability, headache, disturbed or restless sleep, poor feeding, anorexia, vomiting, or diarrhea.
• In COPD patients, exacerbations are often due to bacterial and viral infections. M. catarrhalis is a bug to keep in mind and symptoms of a COPD exacerbation caused by this bug are very similar to those caused by other bacteria, including increased sputum production, sputum purulence, and dyspnea.
• Pneumonia due to M. catarrhalis is a cause of pneumonia in elderly patients, especially in elderly patients with an underlying cardiopulmonary disease like COPD.
• These infections are usually treated empirically with broad spectrum antibiotics such as 3rd generation cephalosporins and amoxicillin-clavulonic acid.

3.12 Neisseria Species

Microbiology review for the USMLE Step 1 exam

• Neisseria species are gram negative diplococci that infect and live inside neutrophils
• Two main species that cause infections in humans: Neisseria gonorrhoeae and Neisseria meningitidis
• Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, which is the second most common STI
• Risk factor for gonorrhea is unprotected sex with an infected person
• N. gonorrhoeae uses pilli to adhere to epithelial cells and invade, causing a local skin infection at the site of inoculation
• Females: cervix is the most common site of infection, causes cervicitis, symptoms include thick yellow or green vaginal discharge, dysuria, and pelvic pain. If left untreated, it can cause PID, ectopic pregnancy, infertility, and chronic pelvic pain
• Males: urethra is the most common site of infection, symptoms include yellow-green discharge, dysuria, and painful swollen testes (less common)
• If left untreated, it can infect more proximal structures and cause prostatitis
• Diagnosis is done by swabbing the urethra or cervix and sending it for nucleic acid amplification testing
• Can also cause septic arthritis (in contrast, Chlamydia causes reactive arthritis)
• "Gonorrhea gon go to the joints"
• Treatment typically involves a cephalosporin (like ceftriaxone) and azithromycin to also cover chlamydial infection
• Conjunctival gonorrhea can occur in newborns during the birthing process, causing conjunctivitis in the eye.

3.11 Nocardia and Actinomyces (Branching Filamentous Species)

Microbiology review for the USMLE Step 1 exam

• Nocardia and Actinomyces are two gram positive bacteria that grow in filaments, resembling fungal hyphae.
• Both are associated with chronic pulmonary lesions, subcutaneous abscesses, and multiple draining sinuses.
• Both are capable of producing sulfur granules.
• Actinomyces is anaerobic and colonizes the mouth, GI, and urinary tracts. It can cause "lumpy jaw syndrome" and requires a long course of high dose penicillin.
• Nocardia is aerobic and stains weakly acid fast. It is found in the environment and typically causes cutaneous disease in immunocompetent individuals and pulmonary disease in immunocompromised individuals.
• Treatment for Nocardia typically involves a combination of antibiotics such as sulfonamides and trimethoprim-sulfamethoxazole.
• SNAP mnemonic: Sulfa for Nocardia; Actinomyces gets Penicillin.

3.10 Clostridium Species

Microbiology review for the USMLE Step 1 exam

• Clostridium species are spore-forming anaerobic gram-positive rods that secrete toxins
• Four main species: Clostridium difficile, Clostridium perfringens, Clostridium botulinum, Clostridium tetani
• Clostridium difficile: common cause of antibiotic-associated diarrhea, caused by overgrowth of C. diff in gut after taking antibiotics that do not kill it, treatment involves stopping offending antibiotic and changing to one that can kill C. diff
• Clostridium perfringens: associated with acute GI infections, usually acquired through consuming contaminated food, symptoms include abdominal pain, watery diarrhea and vomiting
• Clostridium botulinum: acquired through eating contaminated food, causes flaccid paralysis through production of botulinum toxin which disrupts synaptic signaling
• Clostridium tetani: causes tetanus, acquired through introduction of spores through breaks in skin, produces tetanus toxin which causes muscle stiffness and spasms.

3.09 Corynebacterium Diptheriae

Microbiology review for the USMLE Step 1 Exam

• Corynebacterium diphtheriae is an aerobic gram positive rod that causes diphtheria, commonly spreads through respiratory droplets
• C. diphtheriae produces an exotoxin called diphtheria toxin which can cause systemic effects by inactivating elongation factor (EF-2) via ADP-ribosylation and shutting down protein synthesis
• Diphtheria causes fever, sore throat, croup-like cough, pseudomembranous pharyngitis, severe cervical lymphadenopathy, myocarditis and cardiac arrhythmias
• Cutaneous diphtheria involves ulcerative lesions or cellulitis on the skin that can occur independently of respiratory diphtheria
• Vaccine available and there are not many cases of it in the US, tends to have an outsized effect on developing countries
• Risk factors for C. diphtheriae infection include IV drug use, homelessness, and crowded living conditions
• Treatment involves diphtheria antitoxin and penicillin or erythromycin