Pseudomonas and opportunistic pathogens, Legionella, and Bordetella

Question 1

What are the morphologic and physiologic features of Pseudomonas?

Answer 1

• Motile, straight or slightly curved, Gram-negative rods
• Typically arranged in pairs
• Capable of using many organic compounds as sources of carbon and nitrogen
• P. aeruginosa produces the blue–green pigment pyocyanin and the yellow pigment fluorescein
• Have a distinctive smell
• Their production of cytochrome oxidase differentiates them from Enterobacteriaceae

Question 2

What are the virulence factors of Psuedomonas?

Answer 2

• Adhesins: flagella, pili, LPS, and alginate (a mucoid exopolysaccharide that forms a prominent capsule)
• Toxins and enzyme: exotoxin A (disrupts protein synthesis by blocking peptide chain elongation)
• Phospholipase C: a heat-stable hemolysin
• P. aeruginosa is intrinsically resistant to many antibiotics (due to low rate of entry of antibiotics through the outer membrane pores and the use of efflux pumps). Acquired and adaptive resistance are also present

Question 3

What diseases do Pseudomonas cause?

Answer 3

• Pulmonary infections: asymptomatic colonization (in patients with cystic fibrosis), benign inflammation of the bronchials (tracheobronchitis), or severe necrotizing bronchopneumonia. Previous therapy with broad-spectrum antibiotics and use of mechanical ventilation equipment are predisposing factors
• Skin and soft-tissue infections: folliculitis and burn wounds
• UTIs: seen in patients with long-term indwelling urinary catheters
• Ear infection: external otitis (swimmer’s ear)
• Eye infections: occur after trauma to the cornea
• Bacteremia and endocarditis, with P. aeruginosa having a higher mortality rate

Question 4

What are the risk factor for infection with Pseudomonas?

Answer 4

• Presence of the organism in a moist reservoir
• Compromised host defenses (e.g. cutaneous trauma, elimination of normal flora due to antibiotics, neutropenia)

Question 5

How are Pseudomonas treated?

Answer 5

A combination of antibiotics. Therapy is difficult as the bacteria are typically resistant to most antibiotics and typical hosts are already immunocompromised

Question 6

What are the morphologic features of Moraxella catarrhalis?

Answer 6

• Gram-negative diplococci
• Obligate anaerobic, oxidase-positive

Question 7

What is the epidemiology and clinical features of Moraxella catarrhalis?

Answer 7

• The peak rate of colonization is around 2 years of age, with very little colonization in adults
• The bacterium has emerged as an important cause of upper respiratory tract infections in healthy children and elderly people
• Causes lower respiratory tract infections, particularly in adults with COPD
• Considered a nosocomial pathogen

Question 8

What are the morphologic features of Legionella?

Answer 8

• Slender, pleomorphic Gram-negative rods
• Obligate aerobes
• Nutritionally fastidious—require cysteine for growth

Question 9

What is the epidemiology of Legionella?

Answer 9

• Infections are associated with exposure to contaminated aerosols (e.g. AC cooling towers, whirlpool spas, showerheads)
• The organisms can survive in moist environments for long periods of time

Question 10

What are the clinical features of Legionella?

Answer 10

• Legionellae are facultative intracellular bacteria that replicate in macrophages and amoebae
• Cytokines released by the infected macrophages stimulate a robust inflammatory response that is characteristic of infections with Legionella
• L. pneumophila is the cause of 90% of all infections, and affects the lungs in one of two forms:
  
  • an influenza-like illness (Pontiac fever)—a self-limited, febrile illness
  • a severe form of pneumonia (Legionnaire’s disease), which causes considerable morbidity

Question 11

Which medium is most commonly used for isolation of legionellae?

Answer 11

Buffered charcoal yeast extract (BCYE) agar

Question 12

What are the morphologic features of Bordetella?

Answer 12

• Extremely small Gram-negative coccobacillus
• Very fastidious obligate aerobe

Question 13

How is pertussis transmitted?

Answer 13

Mainly by respiratory droplets

Question 14

What type of vaccine is used for pertussis?

Answer 14

Subunit vaccine consisting of inactivated pertussis toxin, filamentous hemagglutinin, and pertactin

Question 15

What is the pathogenesis of pertussis?

Answer 15

• Pertussis toxin (an A-B toxin) inactivates the protein that controls adenylate cyclase, leading to increased cAMP levels and an increase in respiratory secretions and mucus production
• The bacteria attach to the cilia of the respiratory epithelial cells, produce toxins that paralyze cilia, and cause inflammation of the respiratory tract that interferes with the clearing of the increased secretions

Question 16

What is the course of pertussis?

Answer 16

• Incubation for 7–10 days
• Catarrhal phase (1–2 weeks): malaise, fever, sneezing, anorexia
• Paroxysmal phase (2–4 weeks): whooping cough, vomiting, leukocytosis
• Convalescent phase (3–4 weeks or longer): symptoms begin to resolve. Complications may occur at this stage

Question 17

What are the morphologic features of Haemophilus?

Answer 17

• Small, sometimes pleomorphic, Gram-negative rods
• Many strains of H. influenzae are covered with a polysaccharide capusle and six antigenic serotypes (A–F)
• Require supplementation of media with hemin (X factor) and/or NAD (V factor)

Question 18

What is the pathogenesis of Haemophilus influenzae?

Answer 18

• The major virulence factor in type b is the antiphagocytic polysaccharide capsule, which contains ribose, ribitol, and phosphate (polyribitol phosphate, PRP)
• Antibodies directed against the capusle greatly stimualte phagocytosis and the complement
• The antibodies develop due to natural infection, vaccination with purified PRP, or transfer of maternal antibodies

Question 19

What is the epidemiology of Haemophilus influenzae?

Answer 19

• Vaccines with purified PRP antigens greatly reduced incidence in children under age 5 in the US
• Most of type B infections now occur in children who are not immune and in elderly adults with waning immunity
• Serotype B was responsible for more than 95% of all invasive infections, but now the distribution is shifting in favor of nonencapsulated (nontypeable) strains

Question 20

What are the clinical features of Haemophilus influenzae infection?

Answer 20

• Type B strains cause meningitis, epiglottitis (obstructive laryngitis) and cellulitis in unvaccinated children
• Nonencapsulated strains are opportunistic pathogens that can cause infections of the upper and lower airways
• H. influenzae and S. pneumoniae are two of the most common causes of acute and chronic otitis and sinusitis

Question 21

How is H. influenzae diagnosed and treated?

Answer 21

• Diagnosed by Gram stain morphology and demonstration of a requirement of both X and V factors in culture
• Treated with prompt antimicrobial therapy

Question 22

What are the diseases caused by Haemophilus spp. other than H. influenzae?

Answer 22

• H. aegyptius (Koch–Weeks bacillus): purulent conjunctivitis
• H. ducreyi: chancroids, an STD most commonly diagnosed in men. Approximately 5–7 days after exposure, a tender papule with an erythematous base develops on the genitalia or perianal area

Question 23

Which members of Aggregatibacter are important human pathogens?

Answer 23

• A. actinomycetemcomitans
• A. aphrophilus

Question 24

What are the morphologic and clinical features of Aggregatibacter?

Answer 24

• Facultatively anaerobic, non-motile Gram-negative
• Found in association with localized aggressive periodontitis
• Both species colonzie the human mouth and can spread from there into the blood and stick to a previously damaged heart valve or artificial valve,causing endocarditis