Important Properties
Anaerobes are characterized by their ability to grow only in an atmosphere containing less than 20% oxygen (i.e., they grow poorly if at all in room air). They are a heterogeneous group composed of a variety of bacteria, from those that can barely grow in 20% oxygen to those that can grow only in less than 0.02% oxygen. Table 1 describes the optimal oxygen requirements for several representative groups of organisms. The obligate aerobes, such as Pseudomonas aeruginosa, grow best in the 20% oxygen of room air and not at all under anaerobic conditions. Facultative anaerobes such as Escherichia coli can grow well under either circumstance. Aerotolerant organ isms such as Clostridium histolyticum can grow to some extent in air but multiply much more rapidly in a lower oxygen con centration. Microaerophilic organisms such as Campylobacter jejuni require a reduced oxygen concentration (approximately 5%) to grow optimally. The obligate anaerobes such as Bacteroides fragilis and Clostridium perfringens require an almost total absence of oxygen. Many anaerobes use nitrogen rather than oxygen as the terminal electron acceptor.
Table1. Optimal Oxygen Requirements of Representative Bacteria
The main reason why the growth of anaerobes is inhibited by oxygen is the reduced amount (or absence) of catalase and superoxide dismutase (SOD) in anaerobes. Catalase and SOD eliminate the toxic compounds hydrogen peroxide and super oxide, which are formed during the production of energy by the organism. Another reason is the oxidation of essential sulfhydryl groups in enzymes without sufficient reducing power to regenerate them.
In addition to oxygen concentration, the oxidation–reduction potential (Eh) of a tissue is an important determinant of the growth of anaerobes. Areas with low Eh, such as the periodontal pocket, dental plaque, and colon, support the growth of anaerobes well. Crushing injuries that result in devitalized tissue caused by impaired blood supply produce a low Eh, allowing anaerobes to grow and cause disease.
Anaerobes of Medical Interest
The anaerobes of medical interest are presented in Table2. It can be seen that they include both rods and cocci and both gram-positive and gram-negative organisms. The rods are divided into the spore formers (e.g., Clostridium) and the non spore formers (e.g., Bacteroides). In this book, three genera of anaerobes are described as major bacterial pathogens, namely, Clostridium, Actinomyces, and Bacteroides. Streptococcus is a genus of major pathogens consisting of both anaerobic and facultative organisms.
Table2. Anaerobic Bacteria of Medical Interest
Clinical Infections
Many of the medically important anaerobes are part of the normal human flora. As such, they are nonpathogens in their normal habitat and cause disease only when they leave those sites. The two prominent exceptions to this are Clostridium botulinum and Clostridium tetani, the agents of botulism and tetanus, respectively, which are soil organisms. C. perfringens, another important human pathogen, is found in the colon and in the soil.
Diseases caused by members of the anaerobic normal flora are often characterized by abscesses, which are most frequently located in the brain, lungs, female genital tract, biliary tract, and other intra-abdominal sites. Most abscesses contain more than one organism, either multiple anaerobes or a mixture of anaerobes plus facultative anaerobes. It is thought that the facultative anaerobes consume sufficient oxygen to allow the anaerobes to flourish.
Three important findings on physical examination that arouse suspicion of an anaerobic infection are a foul-smelling discharge, gas in the tissue, and necrotic tissue. In addition, infections in the setting of pulmonary aspiration, bowel surgery, abortion, cancer, or human and animal bites frequently involve anaerobes.
Laboratory Diagnosis
Two aspects of microbiologic diagnosis of an anaerobic infection are important even before the specimen is cultured: (1) obtaining the appropriate specimen and (2) rapidly trans porting the specimen under anaerobic conditions to the laboratory. An appropriate specimen is one that does not contain members of the normal flora to confuse the interpretation. For example, such specimens as blood, pleural fluid, pus, and trans tracheal aspirates are appropriate, but sputum and feces are not. In the laboratory, the cultures are handled and incubated under anaerobic conditions.
Treatment
In general, surgical drainage of the abscess and administration of antimicrobial drugs are indicated. Drugs most effective for the treatment of anaerobic infections are beta lactams plus a beta lactam inhibitor, carbapenems, clindamycin, and metronidazole. Many other antibiotics are not reliably active.
