Other Anaerobic Infections

Bacteremia

Anaerobes account for approximately 5% of bloodstream bacterial isolates in adults, but this rate is lower in children. The most common blood isolates of anaerobic bacteria in children are Bacteroides fragilis group, Peptostreptococcus , Clostridium, and Fusobacterium spp.

Isolation of anaerobes from the blood is often an indication of a serious primary anaerobic infection. The lower gastrointestinal (GI) tract and wound infections are the 2 most common sources for bacteremia. Risk factors for anaerobic bacteremia include malignancy, hematologic disorders, solid-organ transplant, recent surgery (GI, obstetric, gynecologic), intestinal obstruction, decubiti, dental extraction, early infancy, sickle cell disease, diabetes mellitus, splenectomy, and chemotherapy or other immunosuppressive drug use.

As with certain aerobes, the cell wall of gram-negative anaerobes may contain endotoxins , which can be associated with the development of hypotension and shock when present in the circulatory system. Clostridia produce hemolysins , and the presence of these organisms in the blood can result in massive hemolysis and cardiovascular collapse.

Central Nervous System

Anaerobic meningitis is rare but can occur in neonates as a complication of ear or neck infections or from anatomic defects of meninges (dural sinus tracts). Anaerobic cerebrospinal fluid (CSF) shunt infections may occur when the distal end of the ventriculoperitoneal shunt perforates the intestinal tract.

Brain abscess and subdural empyema are usually polymicrobial, with anaerobes typically involved (see Chapter 622 ). Brain abscess usually occurs because of spread from infected sinuses, middle ear, or lung and rarely from endocarditis. Clostridium perfringens can cause brain abscess and meningitis after head injuries or after intracranial surgery. Brain abscesses may require surgical drainage combined with a prolonged course of antibiotic therapy.

Upper Respiratory Tract

The respiratory tract is colonized by both aerobes and anaerobes. Anaerobic bacteria are involved in chronic sinusitis, chronic otitis media, peritonsillar infections, parapharyngeal and retropharyngeal abscesses, and periodontal infections. The predominant organisms involved are Prevotella, Porphyromonas, Bacteroides, Fusobacterium, and Peptostreptococcus spp.

Anaerobic periodontal disease is most common in patients with poor dental hygiene or who are receiving drugs that induce gingival hypertrophy. Vincent angina, also known as acute necrotizing ulcerative gingivitis or trench mouth, is an acute, fulminating, mixed anaerobic bacterial-spirochetal infection of the gingival margin and floor of the mouth. It is characterized by gingival pain, foul breath, and pseudomembrane formation. Ludwig angina is an acute, life-threatening cellulitis of dental origin of the sublingual and submandibular spaces. Infection spreads rapidly in the neck and may cause sudden airway obstruction.

Lemierre syndrome, or postanginal sepsis, is a suppurative infection of the lateral pharyngeal space, of increasing prevalence, that often begins as pharyngitis (see Chapter 409 ). It may complicate Epstein-Barr virus or other viral and bacterial infections of the pharynx. It usually manifests as a unilateral septic thrombophlebitis of the jugular venous system with septic pulmonary embolization. Patients present with prolonged pharyngitis, neck pain and fever. Clinical signs include unilateral painful cervical swelling, trismus, and dysphagia, culminating with signs of sepsis and respiratory distress. Fusobacterium necrophorum is the most commonly isolated organism, although polymicrobial infection may occur. Metastatic infections involving muscles, bones, internal organs (often lungs), and the brain can occur as a complication of Lemierre syndrome.

Lower Respiratory Tract

Anaerobic lung abscess, empyema, and anaerobic pneumonia are most often encountered in children who have disordered swallowing or seizures or in whom an inhaled foreign body is occluding a bronchus. Infections are usually polymicrobial. Children and adults can aspirate oral or gastric contents during sleep, seizure, or periods of unconsciousness. In most cases, lung cilia and phagocytes clear particulate matter and microbes. If the aspiration is of increased volume or frequency or a foreign body blocks normal ciliary clearance, normal pulmonary clearance mechanisms are overcome and infection ensues. Appropriate cultures need to avoid specimen contamination with oral flora through use of bronchoalveolar lavage, lung biopsy, or thoracentesis.

In unusual cases, particularly in patients with poor dental hygiene, aspirated mouth contents may contain the anaerobe Actinomyces israelii , resulting in pulmonary actinomycosis (see Chapter 216 ). Anaerobic pneumonitis associated with this microorganism is remarkable for the ability of the infection to traverse tissue planes. Affected patients often develop fistulas on their chest walls overlying areas of intrathoracic infection. These may extrude distinctive, pathognomonic particles composed of bacterial colonies, called “sulfur granules.”

Intraabdominal Infection

The entire digestive tract is heavily colonized with anaerobes. The density of organisms is highest in the colon, where anaerobes outnumber aerobes 1,000 : 1. Perforation of the gut leads to leakage of intestinal flora into the peritoneum, resulting in peritonitis involving both aerobes and anaerobes. Secondary sepsis caused by aerobes often occurs early. As the peritoneal infection is walled off, an abscess containing both aerobes and anaerobes often evolves. The predominant aerobic organisms are Escherichia coli and Streptococcus spp. (including Enterococcus spp.), and the anaerobes are the B. fragilis group, Peptostreptococcus , Clostridium, and Fusobacterium spp.

Secondary hepatic abscesses may then develop as complications of appendicitis, intestinal perforation, inflammatory bowel disease, or biliary tract disease. In children with malignancy receiving chemotherapy, the intestinal mucosa is often damaged, leading to translocation of bacteria and focal invasion of bowel wall. Typhlitis is a mixed infection of the gut wall in neutropenic patients, usually located in the ileocecum and characterized by abdominal pain, diarrhea, fever, and abdominal distention. Similarly, a mixed aerobic-anaerobic infection of the intestinal wall and peritoneum may develop in a small infant as a complication of necrotizing enterocolitis , believed to be a result of the relative vascular insufficiency of the gut and hypoxia (see Chapter 123.2 ).