The standardization of testing methods by the Clinical and Laboratory Standards Institute (CLSI) (Wayne, PA) allows for comparison of resistance trends among various laboratories (15 The MIC is read as the concentration where the elliptical zone of inhibition intersects the strip. Anaerobes colonize the skin, oral cavity, and genitourinary and lower gastrointestinal tracts and generally do not cause infection. The newer tetracycline analogs doxycycline and minocycline are more active than the parent compound. Thus, it has significant in vitro activity against streptococci, methicillin-susceptible staphylococci, Enterobacteriaceae (including extended-spectrum-beta-lactamase-producing strains), P. aeruginosa, Acinetobacter spp., and the B. fragilis group. The side effect of most concern is C. difficile-associated colitis (125, 126). The most commonly isolated antibiotic-resistant anaerobes are species within the Bacteroides fragilis group (18). Anaerobic bacteria make up a large part of the normal resident flora on mucous membranes, especially in the mouth, lower gastrointestinal tract, and vagina. Resistance among some anaerobes has increased significantly over the past 3 decades. Numbers of reports of broadly multidrug-resistant B. fragilis strains as well as numbers of reports of resistance arising during treatment have increased (6, 11, 14, 22, 28). Clindamycin has lost some of its activity against anaerobic Gram-positive cocci (Finegoldia magna [30% resistant] and Peptoniphilus spp., etc.) bacteria that survive without oxygen, which are often seen when blood flow is cut off to an area of the body. The plate for which the growth is no longer present should be considered the MIC. Tigecycline is the first antibiotic approved in a new class called glycylcyclines. (64) and could be helpful to predict likely resistance patterns (Fig. Microaerophilic streptococci, P. acnes, and Actinomyces spp. Amoxicillin-clavulanate remains the agent of choice for human and animal bite wound infections (77), especially when anaerobes may be involved. All of these factors affect the clinician's cognitive choices in the selection of an antimicrobial agent to treat anaerobic infections. The risk of fatal aplastic anemia with chloramphenicol is estimated to be approximately 1 per 25,000 to 40,000 patients treated. Detection of Resistance by Using Molecular MethodsMolecular methods are currently limited to research laboratories studying resistance mechanisms of anaerobic bacteria. •Facultative anaerobes: Can grow in “room air” or under anaerobic conditions. Each individual cephalosporin may have either a class or specific inhibitor enzyme that is able to inactivate it. TetracyclinesTetracycline, once the drug of choice for anaerobic infections, is presently of limited usefulness because of the development of resistance to it by virtually all types of anaerobes, including Bacteroides and Prevotella spp. Other adverse reactions include reversible neutropenia, phlebitis at intravenous infusion sites, and drug fever. If these tests are done in-house, they might yield faster results (results can typically be read after 48 h, while most test results from a commercial laboratory take up to 7 days) and would be clinically useful in therapeutic decisions. B. fragilis group resistance rates for piperacillin-tazobactam are generally <1% (27). Emergence of erythromycin-resistant organisms during therapy has been documented (119, 120). On the other hand, changes in susceptibility that are known to be due to specific mechanisms (such as the presence of the nitroimidazole reductase gene [nim], which causes metronidazole resistance, or cfiA genes, which can result in carbapenem resistance) may begin to appear as relatively modest changes in resistance rates and then quickly increase as the resistance determinant becomes disseminated. Over 90% of obligate anaerobes are susceptible to less than 2 μg/ml metronidazole (79). While aerobes are able to thrive in habitats that have abundant oxygen, anaerobes may die in the presence of oxygen. The antimicrobial spectrum of the narrow-spectrum cephalosporins against anaerobes is similar to that of penicillin G, although on a weight basis, they are less active. There is a certain margin of error (usually ±1 2-fold dilution) for any of these techniques. Recommend to perform this kind of study on a regular basis. Anaerobes from outside the body sometimes cause disease when they enter breaks in the skin or are consumed. The most appropriate susceptibility test method will differ depending on whether the test is being done for a specific isolate in a hospital laboratory (or a commercial laboratory used by the hospital) or whether surveillance testing is being performed at a hospital or reference laboratory. Unexpected metronidazole resistance was observed for Finegoldia magna isolates, Peptoniphilus isolates and Eggerthella lenta isolates. – Resistant B. fragilis group isolates carry one of nine known nim genes (nimA-I) on either the chromosome or a mobilizable plasmid that seems to encode a nitroimidazole reductase, which converts 4- or 5-Ni to 4- or 5-aminoimidazole, preventing the formation of toxic nitroso residues necessary for the agent's activity. (P. bivia [70% resistant], P. oralis, and P. melaninogenica [both 40% resistant]), although its activity against Fusobacterium and Porphyromonas spp. Some studies refer to other method documents; a recent German multicenter study referred to a specific German document for testing methodology (24). fragilis group species. Other classes of drugs, most notably the sulfonamides , may be effective antibacterials.Similarly, some antibiotics may have secondary uses, such as the use of demeclocycline (Declomycin, a tetracycline derivative) to treat the syndrome of inappropriate antidiuretic hormone (SIADH) secretion. Other side effects include the production of the potentially fatal “gray baby syndrome” when given to neonates, hemolytic anemia in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, and optic neuritis in those who take the drug for a prolonged time. The tolerance of metronidazole in patients is generally very good. are almost uniformly resistant (128). Bacteroides species resistance to fluoroquinolones has been attributed to either an alteration in efflux of the antibiotic or a mutation in the quinolone resistance-determining region (QRDR) of the gyrase A gene (gyrA) from single or multiple mutations (144). Agents to test should include those on the hospital formulary, and the agent that is being considered or used for therapy should be included if at all possible. In cases where other factors that can cause variability are also involved (e.g., differences in techniques or media or different people reading results), it becomes clear that minor changes (e.g., less than 15%) in percent susceptibility may not be significant in reports of large groups of strains; still, survey studies provide useful information on trends and patterns in antimicrobial susceptibility of anaerobes. Some broad-spectrum antibacterial agents possess such qualities, while for some organisms, additional agents should be added to the therapeutic regimen. However, the drug has potential significant toxicity. Overall, tigecycline has a low rate (5.5%) of resistance against B. fragilis group species (22). Susceptibility testing should be performed on organisms recovered from sterile body sites, those that are isolated in pure culture, or those that are clinically important and have variable or unique susceptibility patterns. Resistance patterns have been monitored through national and local surveys, but susceptibility testing of anaerobic bacteria at individual hospitals is rarely done (20). It is used for dental infections, especially in patients who are allergic to penicillin, and for aspiration pneumonia. Hence, the clinician is likely to consider only the in vitro activity of specific agents against B. fragilis and assume that all the other anaerobes will likely be susceptible as well. (Courtesy of bioMérieux, reproduced with permission.). Should rapid diagnostics (such as MALDI-TOF MS) become available for anaerobes in the future, it is likely that this paradigm would change. These anaerobes can cause disease when mucous membranes are broken. Azithromycin is slightly less active than erythromycin against these species (117). Aerobic and facultative anaerobes, such as coliforms, are usually highly resistant. (140) found that 90% of 831 B. fragilis group isolates were susceptible to ≤8 μg/ml of tigecycline and that P. distasonis isolates were the most resistant. Susceptibility testing should be performed on isolates recovered from sterile body sites, those that are isolated in pure culture, or those that are clinically important and have a variable or unique susceptibility. Resistance among Gram-positive organisms that are not strict anaerobes is frequent, especially for P. acnes and Actinomyces spp. Details of the procedure are described in the manufacturer's guidelines. In the cell, this compound is acted upon by the enzyme Pyruvate ferredoxin oxidoreductase (PFOR) which converts it into a nitro radical (nitro radical anion). The choice will be based on published literature and surveys and local or specific patient data. These findings suggested incomplete mobilization of nim gene-associated resistance. and microaerophilic streptococci, Gram-positive non-spore-forming anaerobic bacilli, and certain clostridia. (150). In this survey, most hospital laboratories used the Etest (62%; 13/21) for susceptibility testing, while only 17% of reference laboratories used it. At present, approximately 85% of B. fragilis isolates are susceptible to cefoxitin, but the other B. fragilis group species are more resistant (17). Resistance rates differed remarkably between the different countries. Clarithromycin is the most active macrolide against Gram-positive oral cavity anaerobes, including Actinomyces spp., Propionibacterium spp., Lactobacillus spp., and Bifidobacterium dentium. This has become the most popular test for testing of individual isolates (Fig. After the antimicrobial agents are allowed to diffuse, the isolates are deposited onto the plate with an automated inoculator or manually streaked from the center to the edge of the plate. – Broth microdilution. Wexler et al. Other experiments (130) have shown that administration of metronidazole to rats and hamsters does not induce any pathology. Metronidazole resistance is generally attributed to the nim gene. The CLSI reference standard is not intended for testing of single isolates; rather, it provides a standard against which other methods may be measured. Ampicillin/Sulbactam. This agent is an effective single agent for the therapy of mixed aerobic-anaerobic infections. Clustering around the breakpoint is a characteristic of the organism-drug interaction and is seen, to some degree, in all of the testing methods. (27) observed that 5% of B. fragilis, 3.6% of B. thetaiotaomicron, 3.3% of B. ovatus, and 7.2% of the unusual B. fragilis group species isolates showed resistance to tigecycline. Anaerobes (including the B. fragilis group) are conspicuous in their absence from the microbiological reports of most hospitals (17, 27, 78). Isolates from patients likely to undergo long-term therapy should be tested so that any development of resistance can be recognized. It may be an alternative for highly penicillin-allergic patients. 91). 15). In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenated environment. A multiplex PCR test that could detect multiple resistance determinants in B. fragilis isolates was recently described by Pumbwe et al. (100) The cephalosporinases are most often of the 2e class type and can be inhibited by three beta-lactamase inhibitors, clavulanic acid, sulbactam, and tazobactam. At this point, the most commonly used method by far is M11-A8 of the CLSI (16). Treatment for Anaerobic Bacteria - Antibiotics for Anaerobic Bacterial Infection. It is active against the Streptococcus anginosus group (including S. anginosus, S. intermedius, and S. constellatus), B. fragilis, B. thetaiotaomicron, Bacteroides uniformis, Bacteroides vulgatus, C. perfringens, C. difficile, and Parvimonas micra (Peptostreptococcus micros) (137). Clindamycin hydrochloride is rapidly and virtually completely absorbed from the gastrointestinal tract (121 In the past, he has served as an Associate Editor for Clinical Infectious Diseases and the Journal of Medical Microbiology. All this suggests that clinicians rely on “FDA indications, information from the manufacturers supplied by drug reps, published study/survey data or just make an educated guess at the appropriate empirical or directed therapy” (20). These methods include the use of agar dilution, broth microdilution, Etest, and the spiral gradient endpoint system. Obviously, any isolate from a therapy failure or in a case in which the therapeutic decisions will be influenced by the results should be tested. The results of the surveillance study should be maintained and recorded so that local trends in emerging resistance may be recognized and documented. This gene codes for an enzyme that converts 4- or 5-nitroimidazole (4- or 5-Ni, respectively) to 4- or 5-aminoimidazole (thus avoiding the formation of toxic nitroso radicals that are essential for antimicrobial activity). At least in aerobes, much of the multidrug resistance seen in the last several years is due to the action of multidrug efflux pumps, and we have indications that a similar phenomenon may be operative in anaerobes as well. Several tetracycline resistance genes have been described among several anaerobes, which encode protective proteins leading to protection of the ribosomes. Antimicrobial susceptibility of anaerobic bacteria in New Zealand: 1999-2003. However, with the reemergence of anaerobic bacteremia in the 1990s, many centers resumed processing of blood cultures for anaerobes. At the time of writing of this publication, the only ready-made commercially available broth microdilution panel identified was produced by Sensititre (Trek Diagnostic Systems). However, up to 30% of strains of the B. fragilis group are resistant to these agents (78). He has done extensive research on anaerobic and respiratory tract infections, anthrax, and infections following exposure to ionizing radiation. Studies on the antimicrobial susceptibility profile of anaerobic bacteria are underrepresented in the literature. Infections caused by anaerobic bacteria include. In the kidney, imipenem is metabolized by breakage of the beta-lactamase bond in the proximal tubular cells. Available data on the safety of the drug during pregnancy are contradictory, and valid data on the safety of metronidazole in pregnancy are still needed. 3). Also, metronidazole resistance could be induced in nim-negative strains by exposure to sub-MICs of metronidazole; the mechanisms behind the increased MICs are not clear (54, 55). Antibiotics sometimes work even when some of the bacterial species in a mixed infection are resistant to the antibiotic (eg, because of loss of support from other bacterial species or the necrotic anaerobic environment), especially if surgical debridement and drainage are adequate. When testing was performed, blood isolates were always tested. ; Bilophila wadsworthia; and Sutterella wadsworthensis. The overall outcomes of all patients were favorable and the resistance rate to commonly used antibiotics remained low. – (Right) Plate after growth of strains. Bottom Line. 4, middle). The European Committee on Antimicrobial Susceptibility Testing (EUCAST) publishes its own breakpoints; these are not always equivalent to those of the CLSI (29). Roberts SA(1), Shore KP, Paviour SD, Holland D, Morris AJ. The CLSI method specifies the use of control strains including B. fragilis ATCC 25285, Bacteroides thetaiotaomicron ATCC 29741, and Clostridium difficile ATCC 700057. The standardization of testing methods by the CLSI allows comparisons of resistance trends among various laboratories. Obligate anaerobes do not carry out oxidative phosphorylation. Spiral gradient endpoint technique. – Nov. 18, 2016 — A new report shows that in 2015, antibiotic resistance continued to increase for most bacteria and antibiotics under surveillance. 17). – Treatment of anaerobic bacterial infections is complicated by the relatively slow growth of these organisms (which makes diagnosis in the laboratory possible only after several days), by the frequent polymicrobial nature of the infection, and by the growing resistance of anaerobic bacteria to antimicrobial agents. are markedly elevated. The standardization of testing methods by the CLSI allows for comparison of resistance trends among various laboratories (15 At this time, most commercial laboratories use Etest methodology for performing anaerobic susceptibility testing on isolates sent to them for testing. All the reference laboratories used broth microdilution for susceptibility testing, as did 40% (8/20) of hospital laboratories (all commercially prepared). Despite all of these factors, a correlation between the antimicrobial resistance of the anaerobic pathogens and poor clinical outcome has been reported in several retrospective studies (7 123). Chloramphenicol is widely distributed in body fluids and tissue, with a mean volume of distribution of 1.4 liters/kg of body weight (115). Listeria bacteria are anaerobic, while Corynebacterium are aerobic. A test kit for identification of anaerobic bacteria-API-has been compared for accuracy in individual tests and for identification on the genus or species level with pre-reduced anaerobically sterilized media methods on 241 anaerobic strains. Anaerobic Bacteria: Nitrate, methane, sulfide, and acetate-like substances are produced by anaerobic bacteria. Cefoxitin is therefore the most effective cephalosporin against the B. fragilis group. •Aerobic: Grow in 18% O210% CO2. For example, metronidazole is active against only anaerobic bacteria and therefore cannot be administered as a single agent for the therapy of mixed infections. Recently, the IDSA has removed cefotetan from the recommended list of therapies for intra-abdominal infections due to poor B. fragilis group activity and resultant clinical failures (89 Patient characteristics and clinical outcomes were analyzed. 14). The CLSI has standardized many laboratory procedures, including anaerobic susceptibility testing, and has published documents for anaerobic susceptibility testing (commonly called M11) (16). Ellie J. C. Goldstein, M.D., F.I.D.S.A., F.S.H.E.A., is Clinical Professor of Medicine at the David Geffen School of Medicine, UCLA; Director of the R. M. Alden Research Laboratory; and in private practice in Santa Monica, CA. 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