Anaerobic infections are caused by anaerobic bacteria. Obligately anaerobic bacteria do not grow on solid media in room air (0.04% carbon dioxide and 21% oxygen); facultatively anaerobic bacteria can grow in the presence or absence of air. Microaerophilic bacteria do not grow at all aerobically or grow poorly, but grow better under 10% carbon dioxide or anaerobically. Anaerobic bacteria can be divided into strict anaerobes that can not grow in the presence of more than 0.5% oxygen and moderate anaerobic bacteria that are able of growing between 2 and 8% oxygen.Jousimies-Somer HR, Summanen P, Baron EJ, Citron DM, Wexler HM, Finegold SM. Wadsworth-KTL anaerobic bacteriology manual. 6th ed. Belmont, CA: Star Publishing, 2002. Anaerobic bacteria usually do not possess catalase, but some can generate superoxide dismutase which protects them from oxygen.
The clinically important anaerobes in decreasing frequency are:Brook, I.: "Anaerobic Infections Diagnosis and Management". A Textbook. Informa Healthcare USA, Inc. New York. 2007. 1. Six genera of Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium, Bilophila and Sutterella spp.); 2. Gram-positive cocci (primarily Peptostreptococcus spp.); 3. Gram-positive spore-forming (Clostridium spp.) and non-spore-forming bacilli (Actinomyces, Propionibacterium, Eubacterium, Lactobacillus and Bifidobacterium spp.); and 4. Gram-negative cocci (mainly Veillonella spp.).
The frequency of isolation of anaerobic bacterial strains varies in different infectious sites.Nagy E. Anaerobic infections: update on treatment considerations. Drugs. 2010; 70:841–58 Mixed infections caused by numerous aerobic and anaerobic bacteria are often observed in clinical situations.
Anaerobic bacteria are a common cause of infections, some of which can be serious and life-threatening. Because anaerobes are the predominant components of the normal flora of the skin and mucous membranes, they are a common cause of infections of endogenous origin.Hentges DJ. The anaerobic microflora of the human body . Clin Infect Dis 1993; 164:S175–80. Because of their fastidious nature, anaerobes are hard to culture and isolate and are often not recovered from infected sites. The administration of delayed or inappropriate therapy against these organisms may lead to failures in eradication of these infections. The isolation of anaerobic bacteria requires adequate methods for collection, transportation and cultivation of clinical specimens. The management of anaerobic infection is often difficult because of the slow growth of anaerobic organisms, which can delay their identification by the frequent polymicrobial nature of these infections and by the increasing resistance of anaerobic bacteria to antimicrobials.
Signs and symptoms
Anaerobes have been found in infections throughout the human body.Nagy E. Anaerobic infections: update on treatment considerations.Drugs. 2010; 70:841–58 The frequency of the host or patient's recovery depends on the employment of proper methods of collection of specimen, their transportation to the microbiology laboratory and cultivation. The recovery of organisms depends on the site of infection and is related to the adjacent mucous membranes' microbial flora.
Central nervous system
Anaerobes are able to cause all types of intracranial infections. These often cause subdural empyema, and brain abscess, and rarely cause epidural abscess and meningitis. The origin of brain abscess is generally an adjacent chronic ear, mastoid, or sinus infection Brook I. Microbiology and antimicrobial treatment of orbital and intracranial complications of sinusitis in children and their management. Int J Pediatr Otorhinolaryngol. 2009; 73:1183–6. oropharynx, teeth Brook I, Microbiology of intracranial abscesses associated with sinusitis of odontogenic origin. Ann Otol Rhinol Laryngol. 2006; 115:917–20. or lungs.Le Moal G, Landron C, Grollier G, Bataille B, Roblot F, Nassans P, Becq-Giraudon B. Characteristics of brain abscess with isolation of anaerobic bacteria. Scand J Infect Dis. 2003; 35:318–211. Mastoid and ear or infections generally progress to the temporal lobe or cerebellum, while facial sinusitis commonly causes frontal lobe abscess. Hematogenous spread of the infection into the CNS often occurs after oropharyngeal, dental, or pulmonary infection. Infrequently bacteremia originating of another location or endocarditis can also cause intracranial infection.
Meningitis due to anaerobic bacteria is infrequent and may follow respiratory tract infection or complicate a cerebrospinal fluid shunt.Brook I. Meningitis and shunt infection caused by anaerobic bacteria in children. Pediatr Neurol.; 26:99–105. 2002. Neurological shunt infections are often caused by skin bacteria such as Cutibacterium acnes,Brook I, Infection caused by Propionibacterium in children. Clin Pediatr (Phila). 1994; 33:485–90. or in instances of ventriculoperitoneal shunts that perforate the gut, by anaerobes of enteric origin (i.e. Bacteroides fragilis).Brook I, Johnson N, Overturf GD, Wilkins J. Mixed bacterial meningitis: a complication of ventriculo- and lumbo-peritoneal shunts. J Neurosurg 1977; 47:961–4.Clostridium perfringens can cause of brain abscesses and meningitis following intracranial surgery or head trauma.
The anaerobes often isolated from brain abscesses complicating respiratory and dental infections are anaerobic Gram-negative bacilli (AGNB, including Prevotella, Porphyromonas, Bacteroides), Fusobacterium and Peptostreptococcus spp. Microaerophilic and other streptococci are also often isolated. Actinomyces are rarely isolated.
At the stage of encephalitis, antimicrobial therapy and utilization of measures to lower the increase in the intracranial pressure can prevent the formation of an intracranial abscess However, after an abscess has emerged, surgical removal or drainage may be necessary, along with an extended course of antimicrobial therapy (4–8 weeks). Some advocate complete drainage of intracranial abscess, while others use repeated aspirations of the abscess.Bernardini GL. Diagnosis and management of brain abscess and subdural empyema. Curr Neurol Neurosci Rep. 2004; 4:448–56. Repeated aspirations of an abscess are preferable in those with multiple abscesses or when the abscess is located in a predominate brain site. Administration of antimicrobials in a high-dose for an extended period of time can offer an alternative treatment strategy in this type of patients and may substitute for surgical evacuation of an abscess.Bernardini GL. Diagnosis and management of brain abscess, and subdural empyema. Curr Neurol Neurosci Rep. 2004; 4:448–56.
Because of the poor penetration of many antimicrobial agents through the blood–brain barrier, there are few agents available for the treatment of intracranial infections. The antimicrobials with good intracranial penetration are metronidazole, chloramphenicol, penicillins, and meropenem. Optimally, the selection of antimicrobial is done according to the recovered isolates and their antimicrobial susceptibilities. A substantial improvement in patients' survival rate has occurred after the introduction of computed tomography (CT) and other scans and utilization of metronidazole therapy.
Upper respiratory tract and head and neck infections
Anaerobes can be isolated from most types of upper respiratory tract and head and neck infection, and are especially common in chronic ones. These include tonsillar,Brook I/ The role of anaerobic bacteria in tonsillitis. Int J Pediatr Otorhinolaryngol. 2005; 69:9–1. peritonsillar and retropharyngeal abscesses, chronic otitis media, sinusitis and mastoiditis, eye ocular) infections,Brook I. Ocular infections due to anaerobic bacteria in children. J Pediatr Ophthalmol Strabismus. 2008; 45:78–84. all deep neck space infections, parotitis, sialadenitis, thyroiditis, odontogenic infections, and postsurgical and nonsurgical head and neck wounds and abscesses.Brook I. The role of anaerobic bacteria in upper respiratory tract and other head and neck infections. Curr Infect Dis Rep. 2007; 9:208–17. The predominant organisms are of oropharyngeal flora origin and include AGNB, FusobacteriumLe Monnier A, Jamet A, Carbonnelle E, Barthod G, Moumile K, Lesage F, Zahar JR, Mannach Y, Berche P, Couloigner V. Fusobacterium necrophorum middle ear infections in children and related complications: report of 25 cases and literature review. Pediatr Infect Dis J. 2008; 27:613–7. and Peptostreptococcus spp.
Anaerobes involve almost all dental infections. These include dental abscesses,Robertson D, Smith AJ. The microbiology of the acute dental abscess. J Med Microbiol. 2009; 58:155-62. endodontal pulpitis and periodontal (gingivitis and periodontitis) infections, and perimandibular space infection.Brook I, Frazier EH, Gher ME. Aerobic nd anaerobic microbiology of periodontal abscess. Oral Microbiol Immunol 1991; 6:123–5.Tatakis DN, Kumar PS. Etiology and pathogenesis of periodontal diseases. Dent Clin North Am. 2005; 49:491–516,. Pulpitis can lead to abscess formation and eventually spread to the mandible and other neck spaces. In addition to strict anaerobic bacteria, microaerophilic streptococci and Streptococcus salivarius can also be present.
Fusobacterium spp. and anaerobic spirochetes are often the cause of acute necrotizing ulcerative gingivitis (or Vincent's angina) which is a distinct form of ulcerative gingivitis.
Deep neck infections that develop as a consequence of oral, dental and pharyngeal infections are generally polymicrobial in nature. These include extension of retropharyngeal cellulitis or abscess, mediastinitis following esophagus perforation, and dental or periodontal abscess.Brook I, Frazier EH. Microbiology of mediastinitis. Arch Intern Med 1996; 156:333–6.
Lung infections
In adults the most common source of aspiration pneumonia is aspiration of oropharyngeal secretions or gastric contents. In children the most common cause is aspiration of infected amniotic fluid, or vaginal secretions. Severe periodontal or gingival diseases are important risk factors for establishment of an anaerobic pleuropulmonary infection. Progression of the infection from pneumonitis into necrotizing pneumonia and pulmonary abscess can occur, with or without the development of empyema.Brook I, Microbiology of empyema in children and adolescents. Pediatrics. 1990; 85:722–6.Brook, I.: Frazier, E.H. Aerobic and anaerobic microbiology of empyema. A retrospective review in two military hospitals. Chest. 1993; 103:1502–7. The infection is often polymicrobial in nature and isolates of community-acquired infection (in 60–80% of cases) are aerobic and anaerobic belonging to the individual's oropharyngeal flora. The anaerobic bacteria commonly recovered are Prevotella, Porphyromonas, Fusobacterium and Peptostreptococcus spp., and the aerobic bacteria are beta-hemolytic and microaerophilic streptococci.Bartlett JG. Anaerobic bacterial infections of the lung and pleural space.Clin Infect Dis. 1993 Suppl 4:S248–55. Anaerobic bacteria can also be isolated in about 35% of individuals who have nosocomial-acquired aspiration pneumonia Brook I, Finegold SM. Bacteriology of aspiration pneumonia in children. Pediatrics. 1980; 65:1115–20. and pneumonia associated with tracheostomy with and without mechanical ventilation,Brook I. Bacterial colonization tracheobronchitis and pneumonia, following tracheostomy and long-term intubation in pediatric patients. Chest 1979; 70:420–4. where they are often isolated along with Enterobacteriaceae, Pseudomonas spp. and Staphylococcus aureus. It is important that specimens are obtained in a method that avoids their contamination by the oral microflora.
Abdominal infections