Pneumococcus
Respiratory Infections,Meningitis
- a member of the normal oropharyngeal flora of most people and an important human pathogen
- causes disease of the middle ear,paranasal sinuses,mastoids and the lung parenchyma but may spread to other sites such as the joints,peritoneum,endocardium and biliary tract and in particular,the meninges
- the major virulence factors are pneumolysin and the polysaccharide capsule
Description
- Str.pneumoniae is a member of the family Lactobacillaceae
- Gram positive
- Non-motile,encapsulated ovoid or lanceolate coccus
- 1 micrometre in diameter
- Tendency to occur in pairs
- Facultatively anaerobes,ocassionally isolates are strict anaerobes
- Complex nuritional requirements sych as enriched media
- Soluble in bile
- Can never survive outside the pH range 7-7.8
- Lacks a cytochrome system. Oxygen is utilized through a flavoprotein enzyme system with the formation of hydrogen peroxide,which is an inhibitor to growth. Pneumococci, lack catalase,which is necessary for the breakdown of hydrogen peroxide. The red blood cells in blood media provide a source of catalase and hence encourage growth in vitro
Pathogenesis
- within the lower respiratory tract,Str.pneumoniae is the commonest pathogen to cause pneumonia
- person to person contact is uncommon and infection usually results from aspiration of pneumococci within upper airway secretion into the lower respiratory tract. Aspiration occurs under a variety of circumstances,usually when the normal mechanisms of mucous entrapment and explusion by an intact glottic reflex and mucociliary escalator are impaired. This situation may arise when consciousness is disturbed in association with general anaesthesia,convulsions,cerebrovascular accidents,epilepsy,head trauma
- antecedent and pre-existing respiratory diseases such as acute lower respiratory tract viral infections,chronic bronchitis and other forms of chronic bronchial sepsis are well known to predispose to secondary bacterial infection,including pneumococcal disease
- other predisposing disease states include valvular and ischaemic heart disease,chronic renal failure,diabetes,mellitus,bronchogenic,metastatic malignancy as well as advancing age
a) Pneumococcal bacteraemia
- bacteraemia may complicate pneumococcal pneumonia in up to 15% of patients. This usually arises from pulmonary lymphatic spread and can result in metastatic involvement of the meninges,joints and rarely,the endocardium
b) Pneumococcal pneumonia
pneumococcal pneumonia follows aspiration with subsequent migration through the bronchial mucosa to involve the peribronchial lymphatics. The inflammatory reaction is primarily focused within the alveolus of a single lobe,although multilobar disease can also occur. The initial red blood cell diapedesis is followed by a primarily polymorphonuclear inflammatory reaction with a marked proteinaceous exudate,resulting in stiff,congested,consolidated lungs. This sequence of events gives rise to the classical descriptions of 'red and grey hepatization'. Blood is shunted through these areas of consolidation without oxygenation;hypoxaemia is therefore a frequent and variably severe accompaniment of pneumococcal pneumonia
c) meningitis
- assumed that invasion arises from the pharynx to the meninges via the bloodstream since bacteraemia usually coexists. Meningitis may occasionally complicate pneumococcal infection at other sites such as the lungs
d) Capsular polysaccharide
- capsular polysaccharide is the major pneumococcal virulence factor which is both antigenic and type-specific. The capsule is antiphagocytic,inhibiting entrapment and phagocytosis
e) Pneumolysin
- a membrane-damaging toxin which induces the features of lobar pneumonia and contributes to the mortality of this disease
Laboratory Diagnosis
- there may be difficulty in obtaining an expectorated specimen of pneumococcal,although postural drainage or inhaled aerosolized saline can encourage its production. A more invasive technique that is occasionally employed is the induction of expectoration by trantracheal saline injection. Sputum may also be obtained by bronchoscopy in patients who are ventilated
- previous antibiotic treatment substantially reduces the chance of isolating the pneumococcus from the sputum
- the adequacy of the sputum sample should be confirmed by microscopy,which should show a predominace of pus cells rather than squamous epithelial cells of buccal origin. The specimen is then homogenized with an agent such as N-acetylcysteine and the pneumococcus is readily isolated on 5% blood agar and the majority of isolates will grow in air although occasional isolates are strict anaerobes
- in the case of pneumococcal meningitis,the cerebrospinal fluid is often macroscopically cloudy. The cell count is usually markedly increased and shows a predominance of polymorphonuclear leucocytes
- blood cultures may also be positive in those with pneumococcal meningitis
Treatment
- sensitive to a wide range of antimicrobial agents. These include the penicillin,cephalosporins,erythromycin,tetracycline,clinamycin,vancomycin,teicoplanin,chloramphenicol and the sulphonamides with penicillin being the major impact on the morbidity and mortality of pneumococcal infections
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