Streptococcus pneumoniae Chapter 23
Streptococcus pneumoniae • S. pneumoniae was isolated independently by Pasteur and Steinberg more than 100 years ago. • Also called “pneumococcus” • Pneumococcal disease is still a leading cause of morbidity and mortality.
PHYSIOLOGY AND STRUCTURE • Gram + coccus • 0.5 to 1.2 μm in diameter, oval or lancet shaped, and arranged in pairs and short chains (Figure 23-9). • Older cells decolorize readily and appear gram-negative.
PHYSIOLOGY AND STRUCTURE • S. pneumoniae has fastidious nutritional requirements and can grow only on enriched media supplemented with blood products. • α-hemolytic on blood agar if incubated aerobically and may be β-hemolytic if grown anaerobically. • The α-hemolytic appearance results from production of an enzyme that degrades hemoglobin, pneumolysin, which produces a green product. Figure 23-9 Gram stain of Streptococcus pneumoniae.
PHYSIOLOGY AND STRUCTURE • S. pneumoniae, like all streptococci, lacks catalase. • S. pneumoniae grows poorly in media with high glucose concentrations. • Fermentation → lactic acid rapidly reaches toxic levels in such preparations.
PHYSIOLOGY AND STRUCTURE • Virulent strains of S. pneumoniae are covered with a complex polysaccharide capsule. • It is these polysaccharides that are used for the serologic classification of strains; currently, 90 serotypes are recognized. • Purified capsular polysaccharides from the most commonly isolated serotypes are used in the pneumococcal vaccine.
Pathogenesis • Not very well understood • Primary damage and disease come from our immune response and not toxins, etc. • Virulence factors – Table 23-6
Epidemiology • 5-75% of people are colonized • Most infections are caused by endogenous spread from the colonized nasopharynx or oropharynx to distal site (e.g., lungs, sinuses, ears, blood, meninges) • Person-to-person spread through infectious droplets is rare • Typically a secondary infection (after the flu, etc.) • Young children and the elderly are at greatest risk for meningitis • Although the organism is ubiquitous, disease is more common in cool months
Figure 23-10 - The incidence of carriage & associated disease is highest during the cool months. Epidemiology • Most infections are caused by endogenous spread from the colonized nasopharynx or oropharynx to distal site (e.g., lungs, sinuses, ears, blood, meninges) • Person-to-person spread through infectious droplets is rare • Individuals with antecedent viral respiratory tract disease or other conditions that interfere with bacterial clearance from respiratory tract are at increased risk for pulmonary disease • Young children and the elderly are at greatest risk for meningitis • Although the organism is ubiquitous, disease is more common in cool months
CLINICAL DISEASES - Pneumonia • 500,000 cases per year • Acute onset, consisting of a severe shaking chill and sustained fever • Symptoms of a viral respiratory tract infection 1 to 3 days prior. • Cough with blood-tinged sputum • Chest pain (pleurisy). • Lobar pneumonia • Rapid recovery following the initiation of appropriate antimicrobial therapy, with complete resolution in 2 to 3 weeks.
CLINICAL DISEASES - Sinusitis and Otitis Media • Over 7 million cases per year • Acute infections of the paranasal sinuses and middle ear. • Usually preceded by a viral infection of the upper respiratory tract, • polymorphonuclear leukocytes (PMN) infiltrate and obstruct the sinuses and ear canal. • Middle ear infection (otitis media) is primarily seen in young children, but bacterial sinusitis can occur in patients of all ages. • Figure from other text
CLINICAL DISEASES - Meningitis • 6000 cases per year • Infection of the central nervous system following • bacteremia • infections of the ear or sinuses • head trauma that causes a communication between the subarachnoid space and the nasopharynx. • Bacterial meningitis can occur in patients of all ages but is primarily a pediatric disease. • Mortality and severe neurologic deficits are 4 to 20 times more common in patients with meningitis caused by S. pneumoniae than in those with meningitis resulting from other organisms.
CLINICAL DISEASES - Bacteremia • 55,000 cases per year • Occurs in 25% to 30% of patients with pneumococcal pneumonia and in more than 80% of patients with meningitis. • In contrast, bacteria are generally not present in the blood of patients with sinusitis or otitis media. • Endocarditis can occur in patients with normal or previously damaged heart valves.
Treatment, Prevention, and Control • Penicillin is the drug of choice for susceptible strains • Antibiotic resistance is increasingly common • In cases of allergy to penicillin or penicillin-resistnats other drugs are used; • Cephalosporins • Erythromycin • Chloramphenicol • Vancomycin • Immunization is recommended for all children younger than 2 years of age and for adults at risk for disease