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Bacteroides and Prevotella Species and Other Anaerobic Gram-Negative Bacilli
Clinically important anaerobic gram-negative bacilli (AGNB) include B. fragilis group (including Bacteroides and Parabacteroides ), Prevotella , Porphyromonas , and Fusobacterium . Fusobacterium are discussed in Chapter 193 . Infections due to Porphyromonas and Prevotella (previously named Bacteroides species) are not common in children, except for Prevotella melaninogenica and Prevotella intermedia. , Changing taxonomy has caused considerable clinical confusion in recategorization of Bacteroides species. Additionally, many new genera and several new species have been created to accommodate pathogens such as Bilophila wadsworthia , Sutterella wadsworthensis , Centipeda periodontii , and Anaerobiospirillum thomasii (see Chapter 187 ).
Bacteroides , Parabacteroides , and Prevotella are gram-negative and obligate anaerobic bacteria that constitute a major part of the normal flora of the oropharynx, gastrointestinal tract, and female genital tract. Bacteroides and Parabacteroides belong to the Bacteroidetes phylum occur with greatest frequency in clinical infections. These organisms are resistant to penicillins mostly through the production of β-lactamase. They include B. fragilis (the most commonly recovered member), B. ovatus , B. thetaiotaomicron , B. vulgatus , Parabacteroides distasonis (formerly Bacteroides distasonis , ), and B. fragilis group (including Parabacteroides ) reside primarily in the large intestine and predominate in infections that originate from gut flora (e.g., perirectal abscesses following perforation or surgical procedures, intra-abdominal abscesses, decubitus ulcers). A heterogeneous group of Bacteroides spp. ( B. ureolyticus , B. gracilis , B. forsythus ) have a less clear taxonomic relationship and may be more closely related genetically to Campylobacter. Pigmented Prevotella , previously called Bacteroides melaninogenicus group ( P. melaninogenica and P. intermedia ), Porphyromonas ( Porphyromonas asaccharolytica ), and nonpigmented Prevotella ( P. oralis , P. oris ) are part of the normal oral and vaginal flora and are the predominant gram-negative anaerobic species isolated from contiguous oral respiratory tract infections, such as aspiration pneumonia, lung abscess, chronic otitis media, chronic sinusitis, abscesses around the oral cavity, human bites, paronychia, brain abscesses, and osteomyelitis. Prevotella bivia and P. disiens (previously called Bacteroides ) are important in obstetric and gynecologic infections.
Bacteroides spp. possess virulence factors or special characteristics to compete successfully for microbial niches, including pili, fimbriae, adhesins (which enhance adherence), , hemagglutination, enzymes (collagenase, phospholipase A, hemolysin, peroxidase, protease, fibrolysin, heparinase, neuraminase, superoxide dismutase), toxins, lipopolysaccharide endotoxin, and capsular polysaccharide (which enhance invasion and evasion from host phagocytosis). , B. fragilis and P. melaninogenica are associated more frequently with infection than their relative density in normal flora would predict, and they are especially associated with abscess formation (see Chapter 187 ).
Bacteroides spp. possess important immunomodulatory effects and participate in the body’s energy balance. B. fragilis polysaccharide A enhances homeostatic immune function both in the gut and systemically and balances subset population size and function of T lymphocytes. ,
Pathophysiology
Most infections with AGNB originate from the endogenous mucosal flora. This feature allows for logical selection of antimicrobial agents for the treatment of infections predictably caused by anaerobic bacteria. AGNB infections generally are polymicrobial, and the number of unique isolates can reach 5–10. The types of other copathogens depend on the site and circumstances of the infection. AGNB promote infection through synergy with their aerobic and anaerobic counterparts. An indirect pathogenic role of AGNB is conferred by the ability of these organisms to produce a β-lactamase enzyme. Such organisms may protect not only themselves but also other penicillin-susceptible organisms from the activity of penicillins.
Clinical Manifestation And Prediction Of Pathogens
Central Nervous System Infections
Anaerobic bacteria, including AGNB, can cause a variety of intracranial infections: brain abscess, subdural empyema, epidural abscess, and meningitis. Infection generally is polymicrobial, mixed with microaerophilic streptococci. The main source of brain abscess is an adjacent, generally chronic infection in the middle ear, mastoid, sinus, oropharynx, teeth, or lungs. Middle ear or mastoid infections tend to spread to the temporal lobe or cerebellum, whereas sinusitis often causes abscess of the frontal lobe. Hematogenous spread often occurs after dental, oropharyngeal, or pulmonary infection. , Rarely, bloodstream infection (BSI) has another origin, or endocarditis can lead to central nervous system infection. Meningitis is rare and can follow respiratory tract infection, or it can occur as a complication of a cerebrospinal fluid shunt. Ventriculoperitoneal shunt infection with the B. fragilis group can have an enteric origin after perforation of the gut.
At the stage of encephalitis, antimicrobial therapy can prevent the formation of abscesses. Once an abscess has formed, excision or drainage generally are needed, combined with 4–8 weeks of antibiotics. Administration of antibiotics for an extended period is an alternative approach that can replace surgical drainage in selected patients. Depending on the organism or organisms isolated and β-lactamase production, metronidazole, penicillins, and chloramphenicol frequently are chosen because of their spectrum and favorable pharmacodynamic profile.
Head and Neck Infections
Anaerobic bacteria including AGNB are recovered from a variety of head and neck infections, especially chronic infections. These include acute and chronic otitis media, mastoiditis, sinusitis pharyngo-tonsillitis, peritonsillar, retropharyngeal and parapharyngeal abscesses, suppurative thyroiditis, cervical lymphadenitis, siladenitis (including parotitis), and deep neck infections including Lemierre syndrome.
Odontogenic Infections
Most dental infections involve anaerobic bacteria, including Prevotella and Porphyromonas of oral flora origin. , These infections include pulpitis and periodontal and endodontal (gingivitis and periodontitis) infections, periapical and dental abscesses, and perimandibular space infections. Vincent angina (trench mouth or acute necrotizing ulcerative gingivitis) is an acute, destructive ulcerative gingivitis that causes severe pain and putrid breath odor. Oral fusospirochetes and other oral flora are causative. ,
Peritonsillar, Lateral Pharyngeal, and Retropharyngeal Abscesses
The predominant anaerobes isolated are Prevotella , Porphyromonas , Fusobacterium , and Peptostreptococcus spp. Streptococcus pyogenes is isolated in approximately one-third of cases. , More than two-thirds of abscesses contain β-lactamase–producing organisms. Systemic antimicrobial therapy should be given in large doses; when pus is formed, adequate surgical drainage is needed. Untreated abscesses can rupture spontaneously into the pharynx and cause aspiration. Other complications are lateral extension or dissection into the posterior mediastinum and the prevertebral space.
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