Aetiological agents of acute pneumonia

Bacteria

1. Streptococcus Pneumoniae (i.e.: pneumococcal pneumonia) is the most common. More than 80 serotypes identified but 14 capsular types are most common to cause disease. Accounts for 80% of all pneumonia cases.

   1. Predisposing factors: impaired secretory clearance of URT.

   2. Pathogenesis: a) aspiration of URT secretions causes infection b) pneumococcal multiplication in alveolar spaces causes outpouring of oedema fluid, leukocytes, and RBCs c) bacterial + complement products stimulate chemotaxis of neutrophils but S. pneumoniae has capsular antiphagocytic properties + prevents complement lysis (i.e.: protective mechanism) d) consolidation occurs due to coagulation of exudates e) antibodies to capsular polysaccharide produced, bind to wall, activate complement  opsonisation  increases efficiency of phagocytosis by macrophages and neutrophils e) healing occurs after macrophages removed debris.

   3. Clinical features: rapid onset of fever, pleuritic chest pain, chills, cough, sputum (rust colour).

   4. Diagnosis: a) large numbers of lancet shaped gram +ve cocci arranged in pairs in conjunction with macrophages + neutrophils in sputum smears b) positive blood cultures confirmatory, but only 30% of pneumonia patients become bacteremic c) pleural fluid obtained by thoracocentesis, gram stain + culture for bacteria.

   5. Treatment: penicillin G but resistance is spreading so use 3^rd generation cephalosporin. Vancomycin if resistance spreads.

   6. Prophylaxis: polyvalent vaccin containing capsular polysaccharides from 14 most virulent strains.

2. Pyogenic cocci: Staph aureus, Strep pyogenes etc. Usually these are 2^nd to infections that depress immune system or introduction of infectious agents directly into blood stream by IVDU.

   1. Diagnosis: sputum, blood, pleural fluid culture + identification

   2. Treatment: antimicrobial therapy is targeted at cause of pneumonia gained from culture + identification.

3. Gram negative bacilli: Haemophilus influenzae (except type B), Klebsiella Penumoniae, Pseudomonas aeuroginosa (main pathogen in CF patients), E. coli, Leigionella Pneumophila. Gram –ve bacillary pneumonia accounts for about 10% of all cases of pneumonia.

   1. Predisposing factors: alcoholics, IVDU

   2. Diagnosis: culture + identification. If you want to differentiate between organisms that colonise the URT and organisms that have caused the pneumonia then do a aseptic: transtracheal or transthoracic aspirates & transbronchoscopic lung biopsy specimens.

   3. Treatment: Selection of antimicrobial agents are based on the results of susceptibility tests of isolated organisms. Cephalosporins + aminoglycosides used initially. Quinolones used to treat P. aeuroginosa + Proteus species.

4. Mycoplasma pneumoniae, Leigionella sp, Chl pneumoniae + psittaci + tracomatis, Coxiella Burnetti (Q fever), , Influenza: in order of incidence all cause atypical pneumonia syndrome. These organisms are hard to grow in laboratory agar media (unlike Strep, Haemophilus, Staph – which are easy to grow in such media). Diagnosis: antigen detection methods. Treatment: not susceptible to beta lactams (penicillin, cephalosporins) so use macrolides (erythromycin + roxithromycin) + tetracyclines.

Viruses

Most often viruses cause interstitial pneumonitis. Common aetiologic agents include:

1. RSV (most common in children: < id="rqo6130" face="Wingdings"> bronchiolitis, 4-5 yrs  bronchopneumonia), adenovirus (1, 2, 3, 5 - children), parainfluenza virus (types 1, 2, 3 - children) , influenza A (most common in adults) & B

Less common aetiological agents include:

2. Herpes virus, rhinoviruses, rubeola virus, echoviruses, corona viruses, coxsackie viruses.

3. Predisposing factors: age (elderly, very young) + immunological status (underlying disease affecting CMI, hospitalised patients)

4. Transmission: droplet inhalation (coughing + sneezing). Most prominent in winter + spring due to “population closeness”.

5. Pathogenesis: a) URT cells are infected by viruses b) spreads to other lung areas via infected secretions by URT, or by haematogenous + lymphatic spread.

6. Diagnosis:

   1. Influenza A virus: WBC count > 10, 000m³

   2. RSV: WBC are not specific, but CXR shows bilateral bronchopneumonia

   3. Adenovirus: WBC count > 30, 000m³

   4. Rubeola pneumonia (measles): CXR shows interstitial pneumonia

7. Treatment: most cases  supportive only. But sometimes: RSV  ribavirin

Fungi

1. Pneumocystis carinii: major cause in immunocompromised patients (esp. AIDS). Approximately half of AIDS patients develop Pneumonocystis carinii pneumonia.

   1. Diagnosis: Demonstration of P. carinii in lung. Bronchoalveolar lavage samples preferred but if –ve then use transbronchial biopsy specimens. Silver stains reveal multinucleated cysts.

   2. Treatment: trimethoprim-sulfamethoxazole is preferred, if intolerable then use pentamidine. Prophylaxis with medication is recommended in immunocompromised patients.

Category: Microbiology Notes