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 + coccus0.5 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

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

Figure 23-10 - The incidence of carriage &associated disease is highest during the cool months.

CLINICAL DISEASES

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.

- Pneumonia

CLINICAL DISEASES

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

- Sinusitis and Otitis Media

CLINICAL DISEASES

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.

- Meningitis

CLINICAL DISEASES

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.

- Bacteremia

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