Infections caused by anaerobic bacteria are common, and may be serious and life-threatening. Anaerobes predominant in the bacterial flora of normal human skin and mucous membranes, and are a common cause of bacterial infections of endogenous origin. Infections due to anaerobes can evolve all body systems and sites. The predominate ones include: abdominal, pelvic, respiratory, and skin and soft tissues infections. Because of their fastidious nature, they are difficult to isolate and are often overlooked. Failure to direct therapy against these organisms often leads to clinical failures. Their isolation requires appropriate methods of collection, transportation and cultivation of specimens. Treatment of anaerobic bacterial infection is complicated by the slow growth of these organisms, which makes diagnosis in the laboratory only possible after several days, by their often polymicrobial nature and by the growing resistance of anaerobic bacteria to antimicrobial agents.

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Pulmonary Infections caused by anaerobes

Pulmonary infections due to anaerobic bacteria usually occur in those prone to oral secretions or gastric contents aspiration because of impaired cough reflex.  This tendency can be due to tracheoesophageal malformations, central nervous system disorders, alcoholism, drug addiction, debilitation, and temporary or permanent altered consciousness.  Poor oral hygiene, gingivitis, and periodontitis, as well as therapy with diphenylhydantoin contribute to poor oral hygiene and promote the development of pneumonia. 1   Anaerobes may also play a role in pulmonary infections in cystic fibrosis.1 a


Microbiology 
Aspiration pneumonia and lung abscess in adults and childrenThe most frequently isolated genera of anaerobes 2,3 are pigmented Prevotella and Porphyromonas, Bacteroides, Fusobacterium , and Peptostreptococcus spp. ( Table 1 )   The predominant aerobic and facultative bacteria are Pseudomonas aeruginosa, Streptococcus pneumoniae, Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus.  Many od these organisms can produce the enzyme beta- lactamase. Prompt identification of these organisms can assist in the initiation of appropriate antimicrobial therapy.
Empyema:  The organisms responsible for empyema in adults and children are S. aureus, S. pneumoniae, Haemophilus influenzae type b, Streptococcus pyogenes, K. pneumoniae, Mycoplasma pneumoniae, and anaerobic bacteria.4,5
                                          
Infections Following Tracheostomy, Intubation and the Use of Ventilatory Tubes: The patient are at risk for developing tracheobronchitis or nosocomial pneumonia generally involving aerobic Gram-negative or Gram-positive bacteria. aerobic Gram-negative or-positive bacteria.5 The main aerobes are Pseudomonas aeruginosa, Staphylococcus aureus, Enterobacter spp, Haemophilus influenzae, Serratia marcescens, Escherichia coli, and Acinetobacter spp .6  The predominant anaerobic bacteria were Peptostreptococcus spp. and pigmented Prevotella and Porphyromonas spp. Most of these infections are polymicrobial aerobic-anaerobic bacterial flora. Similar organisms are also isolated from Tracheostomy site wound infection.






Pathogenesis

Aspiration pneumonia and lung abscess:  The aspiration of saliva and oropharyngeal secretions that contain many aerobic and anaerobic bacteria introduces them into the lower respiratory tract.
The initial lesion following aspiration is pneumonitis that is insidious, and involve the dependent segments of the lung.  If left untreated, tissue necrosis may lead to abscess formation or empyema after 7  to 14 days.  Excavation may lead to solitary lung abscess or multiple small areas of necrosis of the lung, with or without air-fluid levels (necrotizing pneumonia). 

Empyema:  Acute empyema is generally secondary to infection at another site and generally is an extension of pneumonia or lung abscess; oral, retropharyngeal or skin abscess;4 mediastinal lymph nodes or paravertebral abscess, or external introduction of organisms related to trauma or surgery. 

Infections Following Tracheostomy, Intubation and the Use of Ventilatory Tubes: Colonization of the tracheobronchial tree with micro-organisms almost always follows tracheal intubation, tracheostomy, and the use of ventilatory tubes. Tacheobronchitis, and pneumonia can also develop following tracheostomy and long-term intubation caused by aerobic and anaerobic bacteria.  Infection of the tracheostomy wound site frequently occurs following prolonged use of the tracheostomy. 7  

Diagnosis
Appropriate materials for anaerobic cultures should be obtained using a technique that bypasses the normal oropharyngeal flora.  The most appropriate lower respiratory specimen is a percutaneous transtracheal aspirates (TTA), tracheal aspirates obtained through protected double lumen catheter or by lung puncture. Collection of materials from the pleural fluid and from closed abscesses is also acceptable.


Right Lower Lobe Aspiration pneumonia





Aspiration pneumonia and lung abscess: A sudden onset of fever, chills, rapid respiration, cough, vomiting and diarrhea, abdominal distention, and elevated peripheral white blood cells are the commonest manifestations.  Infection onset is sometimes insidious.  Weeks to months of malaise, low-grade fever, and cough, with significant weight loss and anemia, may precede consolidation and abscess formation.  Examination may reveal dyspnea with frequent expiratory grunts, dilated nostrils, flushed cheeks, cyanosis, rales, diminished breath sounds, dullness, and prolonged expiration.  A pneumonic process in a posterior upper lobe or superior lower lobe suggests aspiration and  lung abscess. Cavitation with an air-fluid level establishes the diagnosis.






Right lobe abscess and infiltrate 



Empyema.  Fever, sweating, chest pain, anemia, leukocytosis and weight loss are the presenting signs.  Pleural effusion can be diagnosed on physical and radiological examination.  Lateral decubitus x‑rays and fluoroscopy may be helpful.  The pleural fluid obtained through thoracentesis is studied for volume, specific gravity, color, pH, consistency, odor, lactic acid, total protein content, glucose, red and white cell count, differential Gram stain and acid-fast stain, wet mount for fungi, mycobacteria, and aerobic and anaerobic cultures. Foul smell may suggest the presence of anaerobes.




Right lobe empyema 




Infections Following Tracheostomy, Intubation and the Use of Ventilatory Tubes: The isolation of a potential pathogen from tracheal cultures for at least four weeks, in the absence of purulent tracheobronchial secretions or clinical evidence of infection is defined as colonization. When expectorated secretions are purulent, and no evidence of pneumonia is evident, tracheobronchitis should be considered. When unequivocal clinical and radiographic evidence of pulmonary parenchymal involvement is present, the patient has leukocytosis and develops fever, the diagnosis of pneumonia can be made.

Antimicrobials are of utmost importance and their selection is based on age, history, physical examination and radiographic findings, and by the organism recovered from reliable sources ( i.e. blood, properly collected sputum, or pleural space ). Antimicrobials are directed at the major pathogens.  Antimicrobials that are effective against penicillin-resistant anaerobic organisms are clindamycin, cefoxitin, chloramphenicol, metronidazole, a carbapenem ( i.e.  imipenem ), or the combination of a penicillin plus a beta-lactamase inhibitor.  Penicillin should be added to metronidazole to cover microaerophilic and anaerobic streptococci.  Coverage against Enterobacteriaceae or P. aeruginosa may require the addition of an aminoglycoside, a quinolone ( not approved for children) or a wide spectrum cephalosporin ( i.e. cefepime ). When antistaphylococcal coverage is needed a beta-lactam resistant penicillin (i.e. oxacillin), vancomycin, or linezolid are used.
Aspiration pneumonia and lung abscess: The duration of treatment is up to 6 weeks and depends on the type of pulmonary involvement, and. In uncomplicated instances, therapy is continued until clinical improvement,  the patient has been afebrile for 5 to 7 days, and has shown improvement on chest roentgenogram.  Of note is that radiographic changes can lag up to 10 days behind clinical improvement.  Necrotizing pneumonia and lung abscess may require a longer courses of therapy than pneumonitis.2  In addition to  antimicrobials, drainage may  be require and bronchoscopy and postoral drainage may be needed. 

Empyema: Surgical drainage and administration of appropriate antimicrobial agents are the mainstay of therapy.  Drainage can help lung re-expansion, reduction of respiratory distress, and prevention of the formation of a thick peel that restricts lung expansion. 
Delay in instituting drainage may result in fluid loculations and thickening. Closed intercostal drainage with suction may be effective, but open drainage with rib resection is often required. Immediate infusion through the inserted tube of tissue plasminogen activator (TPA) or urokinase may be employed. Thoracoscopy with removal of adhesions, decortication, removal of the entire empyema, plus drainage are also options. 

Infections Following Tracheostomy, Intubation and the Use of Ventilatory Tubes: Treatment includes postural drainage and frequent suctioning and cleaning of tracheostomy tubes, which should be changed once a week, and the administration antibiotics if pneumonia is suspected. Prophylactic antimicrobial therapy to prevent  pneumonia is not recommended, and the use of selective decontamination of the oral and gut flora is controversial. 

References 
1. Finegold SM.  Anaerobic bacteria in human diseaseNew York:  Academic Press.  1977.
 
2. Bartlett J G,  Anaerobic bacterial infections of the lung and pleural space.


3. Brook I, Finegold SM.  Bacteriology of aspiration pneumonia in children.  Pediatrics.  65:1115-1120.  1980. 
5. Bryant RE, Salmon CJ. Pleural empyema. Cl Infect  Dis. 22: 747-64, 1996.
7.  Brook, I.: Microbiological studies of tracheostomy site wounds. Eur. J. Respir. Dis. 71:380-3, 1987.




       Table 1. Predominate aerobic and anaerobic organisms recovered in aspiration pneumonia, lung abscess, and empyema


                                                                          
                                                                          

Anaerobic bacteria                                                  

Anaerobic cocci
   Peptostreptococcus spp.                                         
   Veillonella spp.                                                       
   Microaerophilic streptococci                                    
Gram-positive bacilli
   Bifidobacterium spp.                                               
                                                                                   
Gram-negative bacilli
   Fusobacterium nucleatum                                      
   Fusobacterium spp.                                                
   pigmented Prevotella and Porphyromonas spp.     
   Bacteroides  ureolyticus                                         
   Prevotella oris-buccae                                            
   Prevotella oralis                                                      
   Bacteroides spp.
   Bacteroides fragilis group                                       
  

                                                                         

Aerobic and facultative bacteria                      

Gram-positive cocci
   Streptococcus. pneumoniae                             
   Alpha-hemolytic streptococci                             
   Group A beta-hemolytic streptococci                
   Staphylococcus  aureus                                    
   Staphylococcu epidermidis                              
Gram-negative bacilli
   Proteus spp.                                                      
   Pseudomonas  aeruginosa                               
   Klebsiella  pneumoniae                                    
   Escerichia  coli                                                 
   Serratia  marcescens                                        
   Citrobacter  spp.
   Enterobacter  spp.                                             
   Haemophilus influenzae                                   
   Haemophilus parainfluenzae                            
   Eikenella  corrodens                                         


Clin Infect Dis. 1993 :16 Suppl 4:S248-55.