Anaerobic infection.

Many of the infections are polymicrobial. 

Anaerobic bacteria are found in 60% to 85% of the cases.

50% of patients have only anaerobic organisms, while the other 50% have a combination of aerobic and anaerobic organisms with and anaerobic predominance.

Anaerobes that are frequently found include:

• Peptostreptococcus sp.
• Bacteroides sp. (B. melanogenicus, B. intermedius)
• Fusobacterium sp.

Aerobes that my be found in predominant anaerobic infection include:

• Microaerophilic streptococci
• Eikenella corrodens
• Pseudomonas aeruginosa
• Staphylococcus aureus
• Enterobacteriaceae

Normal person 

50% of healthy persons aspirate during sleep, usually goes unrecognized, and has few sequelae.

Determinants who gets infection?

The frequency, volume, and character of the aspirated material will determine whether or not a pulmonary infection will develop.

• Most anaerobic lung infections are initiated by the aspiration of the oropharyngeal secretions that contain high inoculum of bacteria, with the ratio of the anaerobes to aerobes being approximately 3:1 to 5:1. 
• As mention earlier, aspiration is a common occurrence in healthy person with usually no sequelae, unless there is high frequency of aspiration, and the volume of the material aspirated is high. 

Predisposing factors for aspiration

Factors associated with increased frequency of aspiration include.

• Mechanical disruption of the glottic closure or the cardiac sphincter, e.g. (endotracheal tubes, nasogastric feeding tubes, tracheostomy, and pharyngeal anesthesia.
• Reduced levels of consciousness that result in dysfunction of the glottic closure and cough reflex: e.g. ethanol abuse, seizures, drug overdose, general anesthesia, CVA, and metabolic encephalopathy
• Dysphagia from neurologic disease or esophageal disease.
• Periodontal disease and gingivitis are associated with increased anaerobic inoculum, and are frequently encountered in patients with aspiration pneumonia.
• In about 10% of lung abscess cases, no risk factors are identified.

His alcoholism and seizures probably are the predisposing factors for him. Gingivitis and caries provided a bigger bacterial inoculum for aspiration.

Pathogenesis of this pneumonia.

• Aerobes provide the necessary milieu for the aerobes to thrive.
• Initially there is dense consolidation of localized area .
• Liquefactive necrosis of the lung then leads to lung abscess.. 

Common sites for aspiration lung abscess 

The superior segments of RLL, LLL and axillary subsegments of anterior and posterior segments of RUL are common sites for aspiration and will account for 85% of all Lung abscesses. 

Gravitational forces determine the site of aspiration. Position of the patient at the time of aspiration determines the segment the aspiration is most likely to occur. 

Clinical picture of lung abscess

Most of the patients present with sub acute onset of illness and do not seek medical attention for three to four weeks since the onset of illness. 
Patients complain of cough, low grade fever, anorexia and weight loss of few weeks duration . 
Patients often have cough with large amounts of foul smelling sputum. 
Lack of foul smell does not exclude lung abscess, as 50% of anaerobic infections do not produce a foul smell. 

Other routes besides aspiration by which anaerobes can reach lungs

Less common mechanisms by which anaerobic bacteria reach the lung include:

• Transdiaphragmatic spread from subphrenic collections,
• Hematogenous spread from septic thrombophlebitis, and
• Tissue necrosis caused by pulmonary infarction, bronchiectasis, and bronchogenic carcinoma may serve as a nidus for anaerobic infection.

Complications are associated with this infection

If aspiration pneumonia goes untreated, progressive tissue necrosis may ensue. This results in

• Abscess formation
• Necrotizing pneumonia
• Bronchopleural fistula
• Empyema

Therapeutic strategy

• Antibiotic therapy is the key treatment for anaerobic pulmonary infections.
  • Drug of choice: Clindamycin 600 – 900 mg IV q8H
  • Alternative agents: Penicillin, Ampicillin/sulbactam, or Amoxicillin/clavulanic acid

  • Metronidazole should not be used as a single agent (because of its high failure rate). If metronidazole is used, penicillin also should be used to cover aerobic and microaerophilic streptococci. 

  • Duration of therapy will depend on radiographic clearance, and may range as long as 2-4 months. 

• Postural drainage is an important component of therapy.

• Rarely surgical therapy (lobectomy) may be required in non-resolving cases, and in patients with hemorrhage. 

• If an empyema develops treat with chest tube drainage in addition to antibiotic therapy.

Hospital acquired aspiration pneumonia

Patients with nosocomial aspiration pneumonia are more likely to have a mixed aerobic-anaerobic infection, in which the aerobic component (gram-negative bacilli) predominates.

Aerobic organisms:

• Klebsiella
• Enterobacter
• Serratia
• E. coli
• Pseudomonas aeruginosa
• Staphylococcus aureus

Anaerobic organisms:

• Peptostreptococcus sp.
• Bacteroides sp. (B. melanogenicus, B. intermedius)
• Fusobacterium sp.

Antibiotics

• Drug of choice: Clindamycin + aminoglycoside
• Alternative agents: Ticarcillin/clavulanate, or Imipenem, or Piperacillin, or Mezlocillin