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 14–2 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 organisms such as Clostridium histolyticum can grow to some extent in air but multiply much more rapidly in a lower oxygen concentration. 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.
Table 14–2Optimal Oxygen Requirements of Representative Bacteria ||Download (.pdf) Table 14–2 Optimal Oxygen Requirements of Representative Bacteria
|Bacterial Type ||Representative Organism ||Growth Under Following Conditions |
|Aerobic ||Anaerobic |
|Obligate aerobes ||Pseudomonas aeruginosa ||3+ ||0 |
|Facultative anaerobes ||Escherichia coli ||4+ ||3+ |
|Aerotolerant organisms ||Clostridium histolyticum ||1+ ||4+ |
|Microaerophiles ||Campylobacter jejuni ||0 ||1+1 |
|Obligate anaerobes ||Bacteroides fragilis ||0 ||4+ |
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 superoxide, which are formed during production of energy by the organism (see Chapter 3). 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 Table 14–3. 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 nonspore 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. The remaining anaerobes are less important and are discussed in Chapter 27.
Table 14–3Anaerobic Bacteria of Medical Interest ||Download (.pdf) Table 14–3 Anaerobic Bacteria of Medical Interest
|Morphology ||Gram Stain ||Genus |
|Spore-forming rods ||+ ||Clostridium |
|– ||None |
|Non–spore-forming rods ||+ ||Actinomyces, Bifidobacterium, Eubacterium, Lactobacillus, Propionibacterium |
|– ||Bacteroides, Fusobacterium |
|Non–spore-forming cocci ||+ ||Peptococcus, Peptostreptococcus, Streptococcus |
|– ||Veillonella |
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 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.
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 transporting 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 transtracheal aspirates are appropriate, but sputum and feces are not.
In the laboratory, the cultures are handled and incubated under anaerobic conditions. In addition to the usual diagnostic criteria of Gram stain, morphology, and biochemical reactions, the special technique of gas chromatography is important. In this procedure, organic acids such as formic, acetic, and propionic acids are measured.
In general, surgical drainage of the abscess and administration of antimicrobial drugs are indicated. Drugs commonly used to treat anaerobic infections are penicillin G, cefoxitin, chloramphenicol, clindamycin, and metronidazole. Note, however, that many isolates of the important pathogen B. fragilis produce β-lactamase and are thus resistant to penicillin. Note also that aminoglycosides such as gentamicin are not effective against anaerobes because they require an oxygen-dependent process for uptake into the bacterial cell.