Anaerobic infection
Encyclopedia
Anaerobic infections are caused by anaerobic bacteria. Anaerobic bacteria do not grow on solid media in room air (10% carbon dioxide and 18% oxygen); facultative anaerobic bacteria can grow in the presence as well as in the 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 to 8% oxygen. 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:
1. Six genera of Gram-negative rods (Bacteroides
Bacteroides
Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids...

, Prevotella
Prevotella
Prevotella is a genus of bacteria.Bacteroides melaninogenicus has recently been reclassified and split into Prevotella melaninogenica and Prevotella intermedia....

, Porphyromonas, Fusobacterium
Fusobacterium
Fusobacterium is a genus of filamentous, anaerobic, Gram-negative bacteria, similar to Bacteroides.Fusobacterium contribute to several human diseases, including periodontal diseases, Lemierre's syndrome, and topical skin ulcers...

, Bilophila and Sutterella spp.);
2. Gram-positive cocci (primarily Peptostreptococcus
Peptostreptococcus
Peptostreptococcus is a genus of anaerobic, Gram-positive, non-spore forming bacteria. The cells are small, spherical, and can occur in short chains, in pairs or individually. Peptostreptococcus are slow-growing bacteria with increasing resistance to antimicrobial drugs.The most frequently...

 spp.);
3. Gram-positive spore-forming (Clostridium
Clostridium
Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...

 spp.) and nonspore-forming bacilli (Actinomyces
Actinomyces
Actinomyces from Greek "actino" that means mucus and fungus, is a genus of the actinobacteria class of bacteria. They are all Gram-positive and are characterized by contiguous spread, suppurative and granulomatous inflammation, and formation of multiple abscesses and sinus tracts that may...

, Propionibacterium
Propionibacterium
Propionibacterium is a genus of bacteria named for their unique metabolism: They are able to synthesize propionic acid by using unusual transcarboxylase enzymes....

, Eubacterium
Eubacterium
Eubacterium is a Gram positive bacteria genus in the family Eubacteriaceae....

, Lactobacillus
Lactobacillus
Lactobacillus is a genus of Gram-positive facultative anaerobic or microaerophilic rod-shaped bacteria. They are a major part of the lactic acid bacteria group, named as such because most of its members convert lactose and other sugars to lactic acid. They are common and usually benign...

 and Bifidobacterium spp.); and
4. Gram-negative cocci (mainly Veillonella spp.) .

The frequency of isolation of anaerobic bacterial strains varies in different infectious sites. 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 skin’s and mucous membranes normal flora, they are a common cause infections of endogenous origin. Because of their fastidious nature, anaerobes are hard to 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 antimicrobial resistance of anaerobic bacteria to antimicrobials.

Signs and symptoms

Anaerobes have been isolated in infections at all body locations. The frequency of their recovery depends on the employment of proper methods of collection of specimen, their transportation to the microbiology laboratory and cultivation. The recovery of organism 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 oropharynx, teeth or lungs. 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
Meningitis
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...

 due to anaerobic bacteria is infrequent and may follow respiratory tract infection or complicate a cerebrospinal fluid shunt. Nerological shunt infections are often caused by skin bacteria such as Propionibacterium acnes, or in instances of ventriculoperitoneal shunts that perforate the gut, by anaerobes of enteric origin (i.e. Bacteroides fragilis).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
Prevotella
Prevotella is a genus of bacteria.Bacteroides melaninogenicus has recently been reclassified and split into Prevotella melaninogenica and Prevotella intermedia....

, Porphyromonas, Bacteroides), Fusobacterium and Peptostreptococcus spp. Microaerophilic and other streptococci are also often isolated. Actinomyces
Actinomyces
Actinomyces from Greek "actino" that means mucus and fungus, is a genus of the actinobacteria class of bacteria. They are all Gram-positive and are characterized by contiguous spread, suppurative and granulomatous inflammation, and formation of multiple abscesses and sinus tracts that may...

 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 nessecery, 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., 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.

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
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....

, chloramphenicol
Chloramphenicol
Chloramphenicol is a bacteriostatic antimicrobial that became available in 1949. It is considered a prototypical broad-spectrum antibiotic, alongside the tetracyclines, and as it is both cheap and easy to manufacture it is frequently found as a drug of choice in the third world.Chloramphenicol is...

, penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

s, 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
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

 (CT) and other scans and utilization of metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....

 therapy.

Upper respiratory tract and head and neck infections

Anaerobes can be isolated from most types of upper respiratory tract and head and neck and infection and are especially common in chronic ones. These include tonsillar, peritonsillar and retropharyngeal abscesses, chronic otitis media
Otitis media
Otitis media is inflammation of the middle ear, or a middle ear infection.It occurs in the area between the tympanic membrane and the inner ear, including a duct known as the eustachian tube. It is one of the two categories of ear inflammation that can underlie what is commonly called an earache,...

, sinusitis
Sinusitis
Sinusitis is inflammation of the paranasal sinuses, which may be due to infection, allergy, or autoimmune issues. Most cases are due to a viral infection and resolve over the course of 10 days...

 and mastoiditis
Mastoiditis
Mastoiditis is an infection of mastoid process, the portion of the temporal bone of the skull that is behind the ear which contains open, air-containing spaces. It is usually caused by untreated acute otitis media and used to be a leading cause of child mortality. With the development of...

, eye ocular) infections, all deep neck space infections, parotitis
Parotitis
Parotitis is an inflammation of one or both parotid glands, the major salivary glands located on either side of the face, in humans. The parotid gland is the salivary gland most commonly affected by inflammation.-Infectious parotitis:...

, sialadenitis, thyroiditis, odontogenic infections, and postsurgical and nonsurgical head and neck wounds and abscesses., The predominant organisms are of oropharyngeal flora origin and include AGNB, Fusobacterium and Peptostreptococcus spp.

Anaerobes involve almost all dental infections. These include periapical and dental abscesses, endodontal pulpitis
Pulpitis
- Symptoms :Increased sensitivity to stimuli, specifically hot and cold, is a common symptom of pulpitis. A prolonged throbbing pain may be associated with the disease...

 and periodontal (gingivitis
Gingivitis
Gingivitis is a term used to describe non-destructive periodontal disease. The most common form of gingivitis is in response to bacterial biofilms adherent to tooth surfaces, termed plaque-induced gingivitis, and is the most common form of periodontal disease...

 and periodontitis) infections, and perimandibular space infection., Pulpitis
Pulpitis
- Symptoms :Increased sensitivity to stimuli, specifically hot and cold, is a common symptom of pulpitis. A prolonged throbbing pain may be associated with the disease...

 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.

Lung infections

In adults the most common source of aspiration pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

 is aspiration of oropharyngeal secretions or gastric contents. In children the most cause is aspiration of infected amniotic fluid, or vaginal secretions. Severe periodontal or gingival disease are the an 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., 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. Anaerobic bacteria can also be isolated in about 35% of individuals who suffer from nosocomial-acquired aspiration pneumonia and pneumonia associated with tracheostomy with and without mechanical ventilation, 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 micro flora.

Abdominal infections

Secondary peritonitis and intra-abdominal abscesses including splenic and hepatic abscesses generally occur because of the entry of enteric micro-organisms into the peritoneal cavity through a defect in the wall of the intestine or other viscus as a result of obstruction, infarction or direct trauma. Perforated appendicitis, diverticulitis, inflammatory bowel disease with perforation and gastrointestinal surgery are often associated with polymicrobial infections caused by aerobic and anaerobic bacteria, where the number of isolates can average 12 (two-thirds are generally anaerobes). The commonest aerobic and facultative bacteria are Escherichia coli, Streptococcus spp. (including Enterococcus spp.), and the most frequently isolated anaerobic bacteria are the B. fragilis group, Peptostreptococcus spp., and Clostridium spp.

Abdominal infections are characteristically biphasic: an initial stages of generalized peritonitis associated with Escherichia coli sepsis, and a later stages, in which intra abdominal abscesses harboring anaerobic bacteria ( including B. fragilis group ) emerge.

The clinical manifestations of secondary peritonitis are a reflection of the underlying disease process. Fever, diffuse abdominal pain, nausea and vomiting are common. Physical examination generally show signs of peritoneal inflammation, isuch as rebound tenderness, abdominal wall rigidity and decrease in bowel sounds. These early findings may be followed by signs and symptoms of shock.

Biliary tract infection is usually caused by E. coli, Klebsiella and Enterococcus spp. Anaerobes (mostly B. fragilis group, and rarely C. perfringens) can be recovered in complicated infections associated with carcinoma, recurrent infection, obstruction, bile tract surgery or manipulation.

Laboratory studies show elevated blood leukocyte count and predominance of polymorphonuclear forms. Radiographs studies may show free air in the peritoneal cavity, evidence of ileus or obstruction and obliteration of the psoas shadow. Diagnostic ultrasound, gallium and CT scanning may detect appendiceal or other intra-abdominal abscesses. Polymicrobial postoperative wound infections can occur.

Tretment of mixed aerobic and anaerobic abdominal infections requires the utilization of antimicrobials effective against both components of the infection as well as surgical correction and drainage of pus. Single and easily accessible abscesses can be drained percutaneously.

Female genital infections

Female genital tract infections caused by anaerobic bacteria can occur are polymicrobial and include: soft-tissue perineal, vulvar and Bartholin gland abscesses; bacterial vaginosis; endometritis; pyometra; salpingitis; adnexal abscess; tubo-ovarian abscesses; intrauterine contraceptive device-associated infection; pelvic inflammatory disease, which may include pelvic cellulitis and abscess; amnionitis; septic pelvic thrombophlebitis; septic abortion; and postsurgical obstetric and gynecologic infections., Getting adequate microbiological cultures is essential. It is important to avoid contaminating the culture with the normal genital flora. Methods that can ensure adequate cultures are laparoscopy, culdocentesis, or obtaining quantitative endometrial cultures employing a telescoping catheter.

The anaerobes often recovered include Prevotella bivia, Prevotella disiens, and Peptostreptococcus, Porphyromonas and Clostridium spp. Bacteroides fragilis group is rarely recovered in these infections compared to intra-abdominal infection. Actinomyces spp. and Eubacterium nodatum are often recovered in infections associated with intrauterine devices. Mobiluncus spp. can be associated with bacterial vaginosis. The aerobic bacteria also found mixed with these anaerobic bacteria include Enterobacteriaceae, Streptococcus spp. (including groups A and B), Neisseria gonnorhoeae and Chlamydia spp. and Mycoplasma hominis
Mycoplasma hominis
Mycoplasma hominis is a strain of bacteria present in the vagina. It may or may not belong to the normal vaginal flora. It is also believed to be a cause of pelvic inflammatory disease. If so, it would be only one of many known causes....

.
.
Free gas in the tissues, abscess formation and foul-smelling discharge is commonly associated with the presence of anaerobic bacteria. Treatment of these infections includes the use of antimicrobials active against all of the potential aerobic and anaerobic bacterial pathogens. Additional antimicrobial should be added against sexually transmissible pathogens should also be administered.

Skin and soft-tissue infections

The infections that frequently involve anaerobic bacteria include superficial infections, including infected paronychia, infected human or animal bites, cutaneous ulcers, cellulitis, pyoderma, and hidradenitis suppurativa Secondary infected sites include secondary infected diaper rash, gastrostomy or tracheostomy site wounds, scabies or kerion infections, eczema, psoriasis, poison ivy, atopic dermatitis, eczema herpeticum, infected subcutaneous sebaceous or inclusion cysts, and postsurgical wound infection.

Skin involvement in subcutaneous tissue infections includes: cutaneous and subcutaneous abscesses, breast abscess, decubitus ulcers, infected pilonidal cyst or sinus, Meleney’s ulcer infected diabetic (vascular or trophic) ulcers, bite wound, anaerobic cellulitis and gas gangrene, bacterial synergistic gangrene, and burn wound infection. Deeper anaerobic soft-tissue infections are necrotizing fasciitis, necrotizing synergistic cellulitis, gas gangrene and crepitus cellulitis. These can involve the fascia aas well as the muscle surrounded by the fascia, and may also induce myositis and myonecrosis.

The isolates found in soft-tissue infections can vary depending on the type of infection. The infection’s location and the circumstances causing the infection can also influence the nature of the microorganisms recovered. Bacteria that are members of the ‘normal flora’ of the region of the infection are often also isolated from lesions involving anaerobic bacteria.

Specimens obtained from wounds and subcutaneous tissue infections and abscesses in the rectal area (perirectal abscess, decubitus ulcer) or that are of gut flora origin(i.e. diabetic foot infection) often to yield colonic flora organisms. These are generally B. fragilis group, Clostridium spp., Enterobacteriaceae and Enterococcus spp. On the other hand, infections in and around the oropharynx, or infections that originate from that location, frequently contain oral flora organisms (i.e. paronychia, bites, breast abscess). These bacteria include pigmented Prevotella and Porphyromonas, Fusobacterium and Peptostreptococcus spp. Skin flora organisms such as S. aureus and Streptococcus spp., or nosocomially acquired microorganisms can be recovered at all body locations. Human bite infections often contain Eikenella spp. and animal bites harbor Pasteurella multocida in addition to oral flora,

Anaerobes infections are often polymicrobial in nature, and sometimes (i.e. decubitus ulcers, diabetic foot ulcer) they are complicated by bacteremia and or osteomyelitis . Infections which are in the deep tissues ( necrotizing cellulitis, fasciitis and myositis) often include Clostridium spp., S. pyogenes or polymicrobic combinations of both aerobic and anaerobic bacteria. Gas in the tissues and putrid-like pus with a gray thin quality are often found in these infections, and they are frequently associated with a bacteremia and high mortality rate.

Treatment of deep-seated soft-tissue infections includes: vigorous surgical management that includes surgical debridement and drainage. Even though there are no controlle studies that support this approach improvement of the involved tissues oxygenation by enhancement of blood supply and administration of hyperbaric oxygen, especially in clostridial infection, may be helpful.

Osteomyelitis and septic arthritis

Anaerobic bacteria are often found in oesteomyelitis of the long bones especially after trauma and fracture, osteomyelitis associated with peripheral vascular disease, and decubitus ulcers and osteomyelitis of the facial and cranial bones. Many of these bone infections are polymicrobial in nature.

Cranial and facial bones anaerobic osteomyelitis often originates by the spread of the infection from a contiguous soft-tissue source or from dental, sinus, or ear infection. The high concentration of anaerobic bacteria in the oral cavity explains their importance in cranial and facial bone infections. The high number of gut anaerobes in pelvic osteomyelitis is generally caused by their spread from decubitus ulcers sites. The anaerobic organisms in osteomyelitis associated with peripheral vascular disease generally reach the bone from adjacent soft-tissue ulcers. Long bones osteomyelitis is often caused by trauma, hematogenic spread, or the presence of a prosthetic device.

Peptostreptococcus and Bacteroides spp. are the most frequently recovered isolates at all bone infections, including those caused by bites and cranial infection. Pigmented Prevotella and Porphyromonas spp. are especially common in bite and skull bone infections, whereas members of the B. fragilis group are often found in vascular disease or neuropathy. Fusobacterium spp., which belongs to the oral microflora, are most often isolated from bites and from cranial and facial bone infections. Clostridium spp. are frequently recovered in long bones infections, mostly in association with traumatic wounds. Because Clostridium spp. colonize the lower gastrointestinal tract, they can contaminate compound lower extremities fractures.

Septic arthritis due to anaerobic bacteria is frequently associated with contiguous or hematogenous infection spread, prosthetic joints and trauma. Most septic arthritis cases caused by anaerobic bacteria are monomicrobial. The predominant anaerobic bacteria isolated are Peptostreptococcus spp. and P. acnes (frequently found in prosthetic joint infection), B. fragilis and Fusobacterium spp. (frequently found in infections of hematogenic origin), and Clostridium spp. (frequently found in infections after trauma).

Bacteremia

The incidence of anaerobic bacteria in bacteremia varies between 5% to15%, The incidence of anaerobic bacteremia in the 1990's declined to about 4% (0.5%-12%) of all cases of bacteremias. A resurgence in bacteremia due to anaerobic bacteria was observed recently. This is explained by a greater number of anaerobic bacteremia in patients with complex underlying disease or those that are immunosuppressed. The commonest isolates are B. fragilis group (over 75% of anaerobic isolates), Clostridium spp. (10–20%), Peptostreptococcus spp. (10–15%), Fusobacterium spp. (10–15%) and P. acnes (2–5%).

The type of bacteria involved in bacteremia is greatly influenced by the infection’s portal of entry and the underlying disease. The isolation of B. fragilis group and Clostridium spp. is often associated with a gastrointestinal source, pigmented Prevotella and Porphyromonas spp. and Fusobacterium spp.with oropharynx and pulmonary sites, Fusobacterium spp. with the female genital tract locations, P. acnes with a foreign body, and Peptostreptococcus spp. with all infection sources, but mostly with oropharyngeal, pulmonary and female genital tract locations. The association of these organisms is related to the origin of the initial infection and the endogenous bacterial flora at that site.

The main factors which predispose to anaerobic bacteremia are: hematologic disorders; organ transplant; recent gastrointestinal,; obstetric, or gynecologic surgery; malignant neoplasms intestinal obstruction; decubitus ulcers; dental extraction; sickle cell disease; diabetes mellitus; postsplenectomy; the newborn; and the administration of cytotoxic agents or corticosteroids.,

The clinical presentations of anaerobic bacteremia are not different from those observed in aerobic bacteremia, except for the infection’s signs observed at the portal of entry of the infection. It often includes fever, chills, hypotension, shock, leukocytosis, anemia and disseminated intravascular coagulation. Clinical features that are characteristic of anaerobicbacteremia include hyperbilirubinemia, metastatic lesions, and suppurative thrombophlebitis. The mortality rate varies between 15% and 30% and can be improved in those who are diagnosed early and receive appropriate antimicrobial therapy and their primary infection when present is resolved.

Neonatal infection

The newborn`s exposure to the maternal vaginal bacterial flora which contains aerobic and anaerobic bacterial flora can lead to the development of anaerobic bacterial infection. These infections include cellulitis of the site of fetal monitoring (caused by Bacterodes spp.), bacteremia, aspiration pneumonia (caused by Bacterodes spp.), conjunctivitis (caused by clostridia,) omphalitis (caused by mixed flora), and infant botulism. Clostridial species may play a role in necrotizing enterocolitis. Management of these infection necessitates treating of the underlying condition(s) when present, and administration of proper antimicrobial therapy.

Causes

Condition predisposing to anaerobic infections include: exposure of a sterile body location to a high inoculum of indigenous bacteria of mucous membrane flora origin, inadequate blood supply and tissue necrosis which lower the oxidation and reduction potential which support the growth of anaerobes. Condition which can lowers the blood supply and can predispose to anaerobic infection are: trauma, foreign body, malignancy, surgery, edema, shock, colitis and vascular disease. Other predisposing conditions include splenectomy, neutropenia, immunosuppression, hypogammaglobinemia, leukemia, collagen vascular disease and cytotoxic drugs and diabetes mellitus. A preexisting infection caused by aerobic or facultative organisms can alter the local tissue conditions and make them more favorable for the growth of anaerobes. Impairment in defense mechanisms due to anaerobic conditions can also favor anaerobic infection. These include production of leukotoxins (by Fusobacterium spp.), phagocytosis intracellular killing impairments (often caused by encapsulated anaerobes and by succinic acid ( produced by Bacteroides spp.), chemotaxis inhibition (by Fusobacterium, Prevotella and Porphyromonas spp.), and proteases degradation of serum proteins (by Bacteroides
Bacteroides
Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids...

 spp.) and production of leukotoxins (by Fusobacterium spp.).

The hallmarks of anaerobic infection include suppuration, establishment of an abscess, thrombophlebitis and gangrenous destruction of tissue with gas generation. Anaerobic bacteria are very commonly recovered in chronic infections, and are often found following the failure of therapy with antimicrobials that are ineffective against them, such as trimethoprim–sulfamethoxazole (co-trimoxazole), aminoglycosides, and the earlier quinolones.

Some infections are more likely to be caused by anaerobic bacteria, and they should be suspected in most instances. These infections include brain abscess
Brain abscess
Brain abscess is an abscess caused by inflammation and collection of infected material, coming from local or remote infectious sources, within the brain tissue...

, oral or dental infections, human or animal bites, aspiration pneumonia
Aspiration pneumonia
Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials into the bronchial tree, usually oral or gastric contents...

 and lung abscesses, amnionitis, endometritis, septic abortions, tubo-ovarian abscess, peritonitis
Peritonitis
Peritonitis is an inflammation of the peritoneum, the serous membrane that lines part of the abdominal cavity and viscera. Peritonitis may be localised or generalised, and may result from infection or from a non-infectious process.-Abdominal pain and tenderness:The main manifestations of...

 and abdominal abscesses following viscus perforation, abscesses in and around the oral and rectal areas, pus-forming necrotizing infections of soft tissue or muscle and postsurgical infections that emerge following procedures on the oral or gastrointestinal tract or female pelvic area. Some solid malignant tumors, ( colonic, uterine and bronchogenic, and head and neck necrotic tumors, are more likely to become secondarily infected with anaerobes. The lack of oxygen within the tumor that are proximal to the endogenous adjacent mucosal flora can predispose such infections.

Management

Recovery from an anaerobic infection depends on adequate and rapid management. The main principles of managing anaerobic infections are neutralizing the toxins produced by anaerobic bacteria, preventing the local proliferation of these organisms by altering the environment and preventing their dissemination and spread to healthy tissues.

Toxin can be neutralized by specific antitoxins, mainly in infections caused by Clostridia (tetanus and botulism). Controlling the environment can be attained by draining the pus, surgical debriding of necrotic tissue, improving blood circulation, alleviating any obstruction and by improving tissue oxygenation. Therapy with hyperbaric oxygen (HBO) may also be useful. The main goal of antimicrobials is in restricting the local and systemic spread of the microorganisms.

The available parenteral antimicrobials for most infections are metronidazole, clindamycin
Clindamycin
Clindamycin rINN is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria...

, chloramphenicol
Chloramphenicol
Chloramphenicol is a bacteriostatic antimicrobial that became available in 1949. It is considered a prototypical broad-spectrum antibiotic, alongside the tetracyclines, and as it is both cheap and easy to manufacture it is frequently found as a drug of choice in the third world.Chloramphenicol is...

, cefoxitin
Cefoxitin
Cefoxitin is a cephamycin antibiotic developed by Merck & Co., Inc., often grouped with the second−generation cephalosporins. It is sold under the brand name Mefoxin.- Microbiology :...

, a penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

 (i.e. ticarcillin, ampicillin, piperacillin) and a beta-lactamase inhibitor (i.e. clavulanic acid
Clavulanic acid
Clavulanic acid is a competitive β-lactamase inhibitor combined with penicillin group antibiotics to overcome certain types of antibiotic resistance. It is used to overcome resistance in bacteria that secrete β-lactamase, which otherwise inactivates most penicillins...

, sulbactam, tazobactam), and a carbapenem (imipenem
Imipenem
Imipenem is an intravenous β-lactam antibiotic developed in 1980. It has an extremely broad spectrum of activity.Imipenem belongs to the subgroup of carbapenems. It is derived from a compound called thienamycin, which is produced by the bacterium Streptomyces cattleya...

, meropenem
Meropenem
Meropenem is an ultra-broad spectrum injectable antibiotic used to treat a wide variety of infections, including meningitis and pneumonia. It is a beta-lactam and belongs to the subgroup of carbapenem, similar to imipenem and ertapenem. Meropenem was originally developed by Sumitomo Pharmaceuticals...

, doripenem
Doripenem
Doripenem is an ultra-broad spectrum injectable antibiotic. It is a beta-lactam and belongs to the subgroup of carbapenems. It was launched by Shionogi Co. of Japan under the brand name Finibax in 2005 and is being marketed outside Japan by Johnson & Johnson...

, ertapenem
Ertapenem
Ertapenem is a carbapenem antibiotic marketed by Merck as Invanz. It is structurally very similar to meropenem in that it possess a 1-β-methyl group.-Indications:...

). An antimicrobial effective against Gram-negative enteric bacilli (i.e. aminoglycoside) or an anti-pseudomonal cephalosporin (i.e. cefepime ) are generally added to metronidazole, and occasionally cefoxitin when treating intra-abdominal infections to provide coverage for these organisms. Clindamycin
Clindamycin
Clindamycin rINN is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria...

 should not be used as a single agent as empiric therapy for abdominal infections. Penicillin can be added to metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....

 in treating of intracranial, pulmonary and dental infections to provide coverage against microaerophilic streptococci, and Actinomyces.

Oral agents adequate for polymicrobial oral infections include the combinations of amoxicillin plus clavulanate, clindamycin and metronidazole plus a macrolide. Penicillin can be added to metronidazole in the treating dental and intracranial infections to cover Actinomyces spp., microaerophilic streptococci, and Arachnia spp. A macrolide can be added to metronidazole in treating upper respiratory infections to cover S. aureus and aerobic streptococci. Penicillin can be added to clindamycin to supplement its coverage against Peptostreptococcus spp. and other Gram-positive anaerobic organisms.

Doxycycline is added to most regimens in the treatment of pelvic infections to cover chlamydia and mycoplasma. Penicillin is effective for bacteremia caused by non-beta lactamase producing bacteria. However, other agents should be used for the therapy of bacteremia caused by beta-lactamase producing bacteria.

Because the length of therapy for anaerobic infections is generally longer than for infections due to aerobic and facultative anaerobic bacteria, oral therapy is often substituted for parenteral treatment. The agents available for oral therapy are limited and include amoxacillin plus clavulanate, clindamycin, chloramphenicol and metronidazole.

In 2010 the American Surgical Society and American Society of Infectious Diseases have updated their guidelines for the treatment of abdominal infections.
The recommendations suggest the following:

For mild-to-moderate community-acquired infections in adults, the agents recommended for empiric regimens are: ticarcillin- clavulanate, cefoxitin, ertapenem, moxifloxacin, or tigecycline as single-agent therapy or combinations of metronidazole with cefazolin, cefuroxime, ceftriaxone, cefotaxime, levofloxacin, or ciprofloxacin. Agents no longer recommended are: cefotetan and clindamycin ( Bacteroides fragilis group resistance) and ampicillin-sulbactam (E. coli resistance) and ainoglycosides (toxicity).

For high risk community-acquired infections in adults, the agents recommended for empiric regimens are: meropenem, imipenem-cilastatin, doripenem, piperacillin-tazobactam, ciprofloxacin or levofloxacin in combination with metronidazole, or ceftazidime or cefepime in combination with metronidazole. Quinolones should not be used unless hospital surveys indicate >90% susceptibility of E. coli to quinolones.

Aztreonam plus metronidazole is an alternative, but addition of an agent effective against gram-positive cocci is recommended. The routine use of an aminoglycoside or another second agent effective against gram-negative facultative and aerobic bacilli is not recommended in the absence of evidence that the infection is caused by resistant organisms that require such therapy.
Empiric use of agents effective against enterococci is recommended and agents effective against methicillin-resistant S. aureus (MRSA) or yeast is not recommended in the absence of evidence of infection due to such organisms.

Empiric antibiotic therapy for health care-associated intra-abdominal infection should be driven by local microbiologic results. Empiric coverage of likely pathogens may require multidrug regimens that include agents with expanded spectra of activity against gram-negative aerobic and facultative bacilli. These include meropenem, imipenem-cilastatin, doripenem, piperacillin-tazobactam, or ceftazidime or cefepime in combination with metronidazole. Aminoglycosides or colistin may be required.

Antimicrobial regimens for children include an aminoglycoside-based regimen, a carbapenem (imipenem, meropenem, or ertapenem), a beta-lactam/beta-lactamase-inhibitor combination (piperacillin-tazobactam or ticarcillin-clavulanate), or an advanced-generation cephalosporin (cefotaxime, ceftriaxone, ceftazidime, or cefepime) with metronidazole.

Clinical judgment, personal experience, safety and patient compliance should direct the physician in the choice of the appropriate antimicrobial agents. The length of therapy generally ranges between 2 and 4 weeks, but should be individualized depending on the response. In some instances treatment may be required for as long as 6–8 weeks, but can often be shortened with proper surgical drainage.

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