National Committee for Clinical Laboratory Standards agar dilution susceptibility testing of anaerobic gram-negative bacteria

Unlocking the therapeutic potential of bioactive exopolysaccharide produced by marine actinobacterium Streptomyces vinaceusdrappus AMG31: A novel approach to drug development

Acidic exopolysaccharide (EPS) was produced by a marine actinobacterium Streptomyces vinaceusdrappus strain AMG31 with the highest yield of 10.6 g/l. The synthesized EPS has an average molecular weight of 5.1 × 104 g/mol and contains arabinose, glucose, galacturonic acid (0.5:2:2 M ratio), with 39.77 % uronic acid residues and 18.8 % sulfate detected. EPS exhibited antioxidant activities with 93.8 % DPPH radical scavenging and 344.7 μg/mg total antioxidant capacity. It displayed anti-inflammatory effects by inhibiting 5-LOX and COX-2. Regarding the cytotoxic activity, the IC50 values are 301.6 ± 11.8, 260.8 ± 12.2, 29.4 ± 13.5, 351.3 ± 11.2, 254.1 ± 9.8, and 266.5 ± 10.4 μg/ml for PC-3, HEP-2, MCF-7, HCT-116, A-549, HepG-2 respectively, which indicate that the produced EPS does not have strong cytotoxic activities. Moreover, the EPS showed anti-Alzheimer activity via inhibition of the Butyrylcholinesterase enzyme, with the highest percentage of 84.5 % at 100 μg/ml. Interestingly, the EPS showed superior anti-obesity activity by inhibiting lipase enzyme with a rate of 95.3 % compared to orlistat as a positive control (96.8 %) at a concentration of 1000 μg/ml. Additionally, the produced EPS displayed the highest anti-diabetic properties by inhibiting α-amylase (IC50 31.49 μg/ml) and α-glucosidase (IC50 6.48 μg/ml), suggesting antidiabetic potential analogous to acarbose. EPS exhibited promising antibacterial and antibiofilm activity against a wide range of Gram-positive and Gram-negative pathogenic bacteria.

Tapping the biosynthetic potential of marine Bacillus licheniformis LHG166, a prolific sulphated exopolysaccharide producer: structural insights, bio-prospecting its antioxidant, antifungal, antibacterial and anti-biofilm potency as a novel anti-infective lead

The escalating global threat of antimicrobial resistance necessitates prospecting uncharted microbial biodiversity for novel therapeutic leads. This study mines the promising chemical richness of Bacillus licheniformis LHG166, a prolific exopolysaccharide (EPSR2-7.22 g/L). It comprised 5 different monosaccharides with 48.11% uronic acid, 17.40% sulfate groups, and 6.09% N-acetyl glucosamine residues. EPSR2 displayed potent antioxidant activity in DPPH and ABTS+, TAC and FRAP assays. Of all the fungi tested, the yeast Candida albicans displayed the highest susceptibility and antibiofilm inhibition. The fungi Aspergillus niger and Penicillium glabrum showed moderate EPSR2 susceptibility. In contrast, the fungi Mucor circinelloides and Trichoderma harzianum were resistant. Among G+ve tested bacteria, Enterococcus faecalis was the most susceptible, while Salmonella typhi was the most sensitive to G-ve pathogens. Encouragingly, EPSR2 predominantly demonstrated bactericidal effects against both bacterial classes based on MBC/MIC of either 1 or 2 superior Gentamicin. At 75% of MBC, EPSR2 displayed the highest anti-biofilm activity of 88.30% against B. subtilis, while for G-ve antibiofilm inhibition, At 75% of MBC, EPSR2 displayed the highest anti-biofilm activity of 96.63% against Escherichia coli, Even at the lowest dose of 25% MBC, EPSR2 reduced biofilm formation by 84.13% in E. coli, 61.46% in B. subtilis. The microbial metabolite EPSR2 from Bacillus licheniformis LHG166 shows promise as an eco-friendly natural antibiotic alternative for treating infections and oxidative stress.

Antimicrobial susceptibility of western Canadian Brachyspira isolates: Development and standardization of an agar dilution susceptibility test method

The re-emergence of Brachyspira-associated disease in pigs since the late 2000s has illuminated some of the diagnostic challenges associated with this genus; notably, the lack of standardized antimicrobial susceptibility testing (AST) methods and interpretive criteria. Consequently, laboratories have relied heavily on highly variable in-house developed methods. There are currently no published investigations describing the antimicrobial susceptibility of Brachyspira isolates collected from pigs in Canada. The first objective of this study was therefore to develop a standardized protocol for conducting agar dilution susceptibility testing of Brachyspira spp., including determining the optimal standardized inoculum density, a key test variable that impacts test performance. The second objective was to determine the susceptibility of a collection of western Canadian Brachyspira isolates using the standardized methodology. After assessing multiple media, an agar dilution test was standardized in terms of starting inoculum (1-2 × 108 CFU/ml), incubation temperature and time, and assessed for repeatability. The antimicrobial susceptibility of a collection of clinical porcine Brachyspira isolates (n = 87) collected between 2009-2016 was then determined. This method was highly reproducible; repeat susceptibility testing yielded identical results 92% of the time. Although most of the isolates had very low MICs to the commonly used antimicrobials to treat Brachyspira-associated infections, several isolates with elevated MICs (>32 μg/ml) for tiamulin, valnemulin, tylosin, tylvalosin, and lincomycin were identified. Overall, this study underscores the importance of establishing CLSI approved clinical breakpoints for Brachyspira to facilitate the interpretation of test results and support the evidence-based selection of antimicrobials in swine industry.

Development of a robust microtiter plate-based assay method for assessment of bioactivity

A microtiter plate-based assay was developed for the quantitative monitoring of bioactive compound production in Streptomyces hygroscopicus fermentation samples. The method reported demonstrates the successful application of the theories of disk diffusion based methods of bioactivity assessment, to a microtiter assay for high throughput analysis. The assay method facilitates the generation of the dose-response curve of test organisms (Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae) to a bioactive compound. Using this dose-response curve, the method facilitates definition of three distinct Minimum Inhibitory Concentration (MIC) values for use in the characterisation of the bioactive attributes of a sample. The assay uses established standard procedures to facilitate adaptation of the assay for use with a wider range of test microorganisms. Errors due to the assumption of a linear relationship between turbidity and biomass concentration are also reduced, due to incorporation of a step to convert turbidity to biomass concentration, for use in the calculation of bioactivity.

Susceptibility testing: inoculum size dependency of inhibition using the Colworth MIC technique

The minimum inhibitory concentration, MIC, is an accepted and well used criterion for measuring the susceptibility of organisms to inhibitors. Many factors influence the MIC value obtained, including temperature, inoculum size and type of organism. A modification of the method developed in this laboratory to obtain inhibition profiles of antimicrobials was used to examine the effect of inoculum size on the degree of inhibition observed with respect to inhibitor concentration. The data obtained enabled the production of an empirical model of inhibition, based on a Gompertz function, relating the level of growth observed to both the inoculum size and concentration of the inhibitor. The inoculum size dependencies of phenethyl alcohol, phenoxyethanol, p-chloro-m-cresol, trichloro-phenol, thymol and dodecyltrimethylammmonium bromide against Staphylococcus aureus were obtained.

Comparison of the E-test and conventional agar dilution methods for susceptibility testing of gram-negative anaerobic rods

The E test was used to determine the susceptibility of 209 anaerobic Gram-negative rods to ampicillin, amoxicillin-clavulanate, ticarcillin, ticarcillin-clavulanate, piperacillin, piperacillin-tazobactam, clindamycin, chloramphenicol, and cefoxitin. Minimum inhibitory (MICs) were read where growth intersected the strips or where the upper indentation intersected the strips for beta-lactamase inhibitor-containing strips. Reference MICs were performed by the National Committee for Clinical Laboratory Standards agar dilution method. Organisms tested consisted of 117 Bacteroides fragilis group, 24 fusobacteria, and 68 other species (mainly Prevotella species). Agreement of MICs by both methods occurred in 33.4% of cases; 80.8% were within one, 99.0% were within two, and 99.95% were within three doubling dilutions apart. E-test results, expressed as MIC50/MIC90 values (microgram/ml), were as follows: ampicillin: 16/> 256; amoxicillin-clavulanate (2:1), 0.5/4; ticarcillin, 16/> 256; tircarcillin-clavulanate (2 micrograms/ml), 0.5/4; piperacillin, 16/> 256; piperacillin-tazobactam (4 micrograms/ml), 1/8; piperacillin-tazobactam (8:1), 2/8; cefoxitin, 8/64; clindamycin, 0.25/4; and chloramphenicol, 2/8. Conventional MIC methodology yielded MIC50s and MIC90s either identical to, or within one doubling dilution of, E-test results. Percentages of strains susceptible varied by < 3% between the two methods, except for cefoxitin (E test 5.2% lower). Based on these results, the E test is an accurate and practical method for use with the agents and organisms evaluated.

beta-Lactamase production and susceptibility of US and European anaerobic gram-negative bacilli to beta-lactams and other agents

The susceptibility of 1,476 US and European strains of anaerobic gram-negative bacilli to amoxicillin, amoxicillin/clavulanate, ticarcillin, ticarcillin/clavulanate, cefoxitin, imipenem and metronidazole was determined. All of the Bacteroides fragilis group and 51% of the non-Bacteroides fragilis group were beta-lactamase positive. Amongst the non-Bacteroides fragilis group, beta-lactamase positivity rates were higher for US strains (58%) than for European strains (39%). All strains were susceptible to imipenem and metronidazole. MIC90s of amoxicillin and ticarcillin for all beta-lactamase negative strains were 0.5 and 2 micrograms/ml, respectively. The addition of clavulanate reduced the MIC90s of amoxicillin (> or = 256 micrograms/ml) and ticarcillin (> or = 64 micrograms/ml) to 16 and 8 micrograms/ml, respectively, for the Bacteroides fragilis group, and to 4 micrograms/ml for both agents for the non-Bacteroides fragilis beta-lactamase producing group. Twenty-nine cefoxitin-resistant strains were found, mainly in the Bacteroides fragilis group, while 95 beta-lactamase producing strains (predominantly Bacteroides fragilis group and fusobacteria) did not show synergy between beta-lactams and clavulanate. Of the newe agents tested, meropenem and piperacillin-tazobactam were the most active (100% of strains susceptible), followed by amoxicillin-BRL 42715 (99% of strains susceptible); 94 to 98% of the strains were susceptible to cefoperazone-sulbactam, tosufloxacin, temafloxacin and clindamycin. Only 73% of the strains were susceptible to cefotetan, compared to 91% to cefoxitin; 88% of the strains were susceptible to trospectomycin. Overall, all of the beta-lactam/beta-lactamase inhibitor combinations, imipenem, meropenem, cefoxitin, tosufloxacin, temafloxacin and clindamycin had good activity against beta-lactamase producing strains, while all agents tested had good activity against beta-lactamase negative strains.

E-test: a new technique for antimicrobial susceptibility testing for periodontal microorganisms

The purpose of the study was to validate the Epsilometer test (E-test) method for antimicrobial susceptibility testing of selected periodontopathic microorganisms using the agar dilution method as a standard. The E-test has been developed to provide a direct quantification of antimicrobial susceptibility of microorganisms. The device consists of a predefined, continuous, and exponential gradient of antibiotic concentrations immobilized along a rectangular plastic test strip. After 48 hours incubation a drop-shaped inhibition zone intersects the graded test strip at the inhibitory concentration (IC) of the antibiotic. Twenty-two subgingival plaque samples from periodontitis sites were plated on trypticase soy agar supplemented with 5% rabbit blood or 5% sheep blood and trypticase soy agar supplemented with vancomycin and bacitracin. A total of 60 strains of key periodontal pathogens (Prevotella intermedia, Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, Eikenella corrodens, Campylobacter rectus, and Capnocytophaga) isolated from 22 sites of 12 patients were studied. The antibiotics tested were clindamycin, metronidazole, tetracycline, ciprofloxacin, cefoxitin, and ampicillin at concentrations above and below the achieved blood or gingival crevicular fluid levels. As a standard reference the minimal inhibitory concentrations (MICs) were determined using the agar dilution method. MICs were compared with ICs determined using the E-test method. The results showed an agreement ranging from 67% to 100%; sensitivity ranging from 75% to 100%; predictability ranging from 56% to 100% and specificity ranging from 33% to 96%. The E-test ICs for ampicillin, cefoxitin, and metronidazole against the Gram-negative capnophilic and microaerophilic rods and the black-pigmented anaerobic rods ICs for ampicillin, clindamycin, metronidazole, and tetracycline showed a high percentage of agreement with the agar dilution MICS.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the efficacy of three different treatments with imipenem versus the classical clindamycin plus tobramycin in experimental peritonitis

This study compares the efficacy of three different treatment modalities of imipenem/cilastatin and the conventional clindamycin plus tobramycin in an experimental model of intra-abdominal sepsis. 145 Wistar rats were used. 40 served as control and 105 as study groups. A capsule with 0.5 ml of inoculum was surgically implanted in the peritoneal cavity. The inoculum was prepared from human feces of healthy volunteers, with a composition of E. coli 10(6), E. faecalis 10(6), B. fragilis, Clostridium sp 10(5) to 10(6) and anaerobic streptococci 10(5) to 10(6). Eighty animals were treated with imipenem/cilastatin and divided in 3 subgroups: "short pretreatment"--29 animals treated 1 hour prior to surgery and 3 days after; "short"--26 animals starting treatment 2 hours post-surgery and continuing it for 3 days; and "long"--25 animals treated for 10 days, starting 2 hours post-surgery. 25 animals received clindamycin plus tobramycin for 10 days. Mortality and the presence of visceral and peritoneal abscesses were the endpoints of the study. The control group had 100% mortality. There were no statistically significant differences among the treated groups although lower mortality was obtained with "short pretreatment" and "long" treatment with imipenem. The presence of abscesses were statistically significant between the imipenem and the combination group. In the imipenem groups, the "short pretreatment" and the long treatment had fewer abscesses than the short one. We conclude that imipenem may be a good alternative monotherapy to conventional therapy with clindamycin plus tobramycin. The "short pretreatment" seemed as good as the long one and better than the short treatment.

Cephalosporin antimicrobial agents and related compounds

Cephalosporins are broad-spectrum antimicrobial agents that are often used empirically to treat suspected bacterial infections and also to treat culture-proven infections due to selected gram-positive and gram-negative microorganisms. Cephalosporins differ widely in their spectrum of activity, susceptibility to beta-lactamases, serum half-life, and penetration of the central nervous system. In general, the first-generation and second-generation agents are most active against staphylococci and streptococci, and the third-generation agents are most active against the Enterobacteriaceae and Pseudomonas. As a group, cephalosporins have a favorable profile of toxicity in comparison with other antimicrobial agents. The development of bacterial resistance has affected all steps of the cephalosporin mechanism of action, including production of beta-lactamases, alterations in penicillin-binding proteins, and modification of the cell wall. New cephalosporins are among the most expensive pharmaceutical agents in use today. Maintaining expertise in the choice and use of these agents will remain a challenge to physicians as additional investigational cephalosporins continue to be developed and introduced into clinical practice.

Antibiotic choices in surgical intensive care unit patients

Antibiotics have dramatically changed the care of the critically ill patient over the last 60 years. Patients with complex physiological conditions present with infectious processes requiring the effective use of antimicrobial drugs. In many situations, the inability to eradicate the infectious process is complicated by the progressive development of resistance among the causative organisms. Systemic antibiotic prophylaxis is warranted only for the prevention of wound infections. Regimens in these cases should use large doses of nontoxic antibiotics covering the spectrum of organisms likely to contaminate the wound. The duration of wound prophylaxis should be short, essentially covering only the period of active wound closure; this is usually less than 24 hours. Prevention of most other infections in the ICU depends on the recognition and correction of the various disturbances of host defenses. Topical antibiotic therapy may reduce the level of colonization for a few specific types of infection. Initial empiric antibiotic therapy should be started for clear indications. The antibiotics chosen should be those most likely to be effective against the probable organisms, those which have the lowest toxicity, and those with the smallest likelihood of inducing multiresistance. They must be adjusted promptly based on the microbiologic sensitivities observed. The realization that the physiology of critical illness may alter the normal relations between drug dosages and the tissue antibiotic levels obtained mandates a different approach to the treatment of these patients. The drug volumes of distribution are generally markedly expanded in these patients. Furthermore, these patients require high tissue antibiotic concentrations to improve the chances for successful therapy. Thus, the antibiotics selected must be capable of providing these levels without significant toxicity to the host. Therapy should be continued based on the clinical response observed. Premature cessation of effective therapy often results in relapse.

Susceptibilities of 394 Bacteroides fragilis, non-B. fragilis group Bacteroides species, and Fusobacterium species to newer antimicrobial agents

The susceptibilities of 374 selected beta-lactamase-producing gram-negative anaerobes (including 22 cefoxitin-resistant strains and 36 strains refractory to the enhancing effect of beta-lactamase inhibitors) and 20 beta-lactamase-negative strains were tested by agar dilution against selected new agents. The organisms included 217 Bacteroides fragilis group strains, 137 non-B. fragilis group Bacteroides spp., and 40 fusobacteria. All strains were susceptible to piperacillin-tazobactam, imipenem, and meropenem. For the B. fragilis group, 96% were susceptible to ampicillin-sulbactam, 95% were susceptible to amoxicillin-clavulanate and cefoperazone-sulbactam, 94% were susceptible to tosufloxacin, 91% were susceptible to cefoxitin, 88% were susceptible to trospectomycin, and 73% were susceptible to cefotetan. For the beta-lactamase-positive non-B. fragilis group Bacteroides spp., greater than or equal to 94% were susceptible to cefoxitin, amoxicillin-clavulanate, ampicillin-sulbactam, cefoperazone-sulbactam, and trospectomycin, 90% were susceptible to cefotetan, and 85% were susceptible to tosufloxacin (the most resistant strains were B. bivius and B. disiens). For the beta-lactamase-positive fusobacteria, greater than or equal to 97% were susceptible to amoxicillin-clavulanate, ampicillin-sulbactam, cefoperazone-sulbactam, trospectomycin, and cefoxitin, 90% were susceptible to cefotetan, and 89% were susceptible to tosufloxacin. All agents showed excellent activity against beta-lactamase-negative strains (for trospectomycin, 95% were susceptible; for all other drugs, 100% were susceptible). Overall, both carbapenems and piperacillin-tazobactam were most active. Amoxicillin-clavulanate, ampicillin-sulbactam, and cefoperazone-sulbactam lacked activity against some cefoxitin-resistant B. fragilis group strains but had excellent activity against other organisms. Tosufloxacin, a new quinolone, had very good activity against B. fragilis group strains (94% susceptible), good activity against other beta-lactamase-positive strains (less than or equal 85% susceptible), and excellent activity against beta-lactamase-negative strains (100% susceptible; MIC for 90% of strains, 0.5 microgram/ml). Trospectomycin was active against >90% of all strains except for B. fragilis group strains (88% susceptible; MIC for 90% of strains, 32 microgram/ml). Clinical studies are required to delineate the role of newer agents in the therapy of anaerobic infections.

Spiral gradient endpoint method compared to standard agar dilution for susceptibility testing of anaerobic gram-negative bacilli

More efficient and reproducible alternative methods of performing agar dilution susceptibility testing are desirable, particularly for anaerobic bacteria. Anaerobes generally grow more reliably on solid media than they do in broth microdilution wells. A new method, the revised spiral gradient endpoint (SGE) method, was evaluated against the standard agar dilution (SAD) method by using a wide variety of anaerobic gram-negative bacilli (161 strains) and eight antimicrobial agents. For the SGE method, a spiral plater was used to set up a concentration gradient of an antimicrobial agent within an agar plate across which bacterial strains were inoculated as radial streaks. After incubation, the MIC of the antimicrobial agent was calculated from the radial endpoint location where bacterial growth ceased along the streak. The MICs for 90% of strains tested (in micrograms per milliliter) and the cumulative percentages of susceptible strains at the breakpoints for the SGE and SAD methods, respectively, and for all 161 strains were as follows: for metronidazole, 2 and 100 versus 2 and 100; for imipenem, 1 and 99 versus 0.5 and 98; for ampicillin-sulbactam, 8 and 97 versus 8 and 98; for clindamycin, 4 and 90 versus 4 and 91; for cefoxitin, 32 and 95 versus 32 and 95; for mezlocillin, 256 and 88 versus greater than 128 and 86; for ampicillin, greater than or equal to 256 and 51 versus greater than 64 and 51; and for penicillin (in units per milliliter), greater than or equal to 512 and 71 versus greater than 64 and 65. The excellent agreement of these data and the greater sensitivity reproducibility, and efficiency of the revised SGE method warrant further evaluations. Assuming that these advantages are confirmed, the revised SGE method should be a useful alternative test method when detailed susceptibility data are desired.

Rapid antimicrobial resistance screening method for Bacteroides intermedius

The purpose of the study was to validate a rapid resistance screening (RRS) method for antimicrobial susceptibility testing of a selected periodontopathic microorganism using the standard broth dilution method as a control. Twenty-five subgingival plaque samples from gingivitis or periodontitis sites were plated on Trypticase soy agar supplemented with 5% rabbit blood with antibiotic discs (RRS method) and without (control). The antibiotics tested were: Augmentin, clindamycin, erythromycin, metronidazole, penicillin G and tetracycline hydrochloride. Bacteroides intermedius isolated from both groups of plates were placed onto antibiotic supplemented Trypticase soy broth. The antibiotic susceptibilities of B. intermedius isolated from the plates with antibiotic discs and the standard broth method were compared. The results showed high sensitivity and predictability for the RRS method compared with the control. The percentage of agreement was: 100% for Augmentin 30 micrograms, clindamycin 2 micrograms and tetracycline 30 micrograms; 96% for erythromycin 15 micrograms, metronidazole 80 micrograms and penicillin 10 IU; 92% for penicillin 2 IU; 88% for erythromycin 2 micrograms and 84% for tetracycline 5 micrograms. The results of this study document the feasibility of the RRS method for testing antimicrobial resistance of whole samples if its efficacy can be demonstrated for other bacteria. This method may be a quick and useful test for the periodontal practitioner in determining the antibiotic susceptibility of periodontal plaque pathogens.

Comparative in vitro activities of a new quinolone, WIN 57273, and piperacillin plus tazobactam against anaerobic bacteria

The in vitro activities of a new quinolone, WIN 57273, and the combination of piperacillin and tazobactam, a beta-lactamase inhibitor, were compared with those of cefoxitin, ceftizoxime, chloramphenicol, clindamycin, imipenem, metronidazole, and piperacillin for 123 clinical anaerobic isolates. Ceftizoxime and cefoxitin had equivalent activities, while metronidazole was active against gram-negative isolates. In the Bacteroides fragilis group, species other than B. fragilis were the most resistant. The combination of piperacillin with tazobactam in a ratio of 8 to 1 was more effective than piperacillin against B. fragilis group organisms when the MIC of piperacillin was greater than or equal to 64 micrograms/ml. Overall, WIN 57273 (i) and imipenem (ii) were the most active agents, with MICs for 50 and 90% of strains of (i) 0.25 and 0.5 and (ii) 0.125 and 2 microgram/ml, respectively.

Beta-lactamase production and susceptibilities to amoxicillin, amoxicillin-clavulanate, ticarcillin, ticarcillin-clavulanate, cefoxitin, imipenem, and metronidazole of 320 non-Bacteroides fragilis Bacteroides isolates and 129 fusobacteria from 28 U.S. centers

beta-Lactamase production (nitrocefin disk method) and agar dilution susceptibility of amoxicillin, amoxicillin-clavulanate, ticarcillin, ticarcillin-clavulanate, cefoxitin, imipenem, and metronidazole were determined for 320 Bacteroides species (not Bacteroides fragilis group) and 129 fusobacteria from 28 U.S. centers. Overall, 64.7% of Bacteroides species and 41.1% of fusobacteria were beta-lactamase positive. Among the Bacteroides species, positivity rates were highest for B. bivius (85.0%), followed by B. splanchnicus (83.3%), B. eggerthii (77.8%), and B. oralis (77.1%); 54.5% of black-pigmented Bacteroides species were beta-lactamase positive. Among the fusobacteria, Fusobacterium mortiferum showed the highest rate of beta-lactamase positivity (76.9%). MICs of amoxicillin (128 micrograms/ml) and ticarcillin (64 micrograms/ml) for 90% of all beta-lactamase-positive strains were reduced to 4 and 2 micrograms/ml, respectively, with the addition of clavulanate. MICs of amoxicillin and ticarcillin for 90% of all beta-lactamase-negative strains were 1 and 4 micrograms/ml, respectively, and greater than or equal to 98.4% of the strains were susceptible to the beta-lactams tested. Of the beta-lactamase-producing strains, 45.9% were susceptible to amoxicillin at less than or equal to 4 micrograms/ml and 93.4% were susceptible to ticarcillin at less than or equal to 64 micrograms/ml; the addition of clavulanate raised the rates to 90.4 and 100%, respectively. All strains were susceptible to cefoxitin, imipenem, and metronidazole. The activity of amoxicillin against 29 beta-lactamase-producing strains (10 Bacteroides species and 19 fusobacteria) was not enhanced by the addition of clavulanate; however, 82.7% of these strains were susceptible to amoxicillin, and all were susceptible to ticarcillin. Although beta-lactamase positivity is on the increase in non-B. fragilis group Bacteroides species and fusobacteria, amoxicillin-clavulanate, ticarcillin, cefoxitin, imipenem, and metronidazole should be suitable for the treatment of infections with these strains. The addition of clavulanate does not appreciably improve the efficacy of ticarcillin against these organisms.

Susceptibility of Bacteroides non-fragilis and fusobacteria to amoxicillin, amoxicillin/clavulanate, ticarcillin, ticarcillin/clavulanate, cefoxitin, imipenem and metronidazole

The susceptibility of 234 Bacteroides non-fragilis strains and 56 fusobacteria from 12 European centers to amoxicillin, amoxicillin/clavulanate, ticarcillin, ticarcillin/clavulanate, cefoxitin, imipenem and metronidazole was tested and related to beta-lactamase production. Beta-lactamase production was detected in 42.3 % of the Bacteroides strains and 26.8% of the fusobacteria. The MIC90 of amoxicillin for beta-lactamase-negative strains was 0.5 microgram/ml and the MIC90 of ticarcillin 2.0 micrograms/ml. In the case of beta-lactamase-positive strains the MIC90 of amoxicillin (32 micrograms/ml) and ticarcillin (16 micrograms/ml) dropped to less than or equal to 1.0 microgram/ml upon addition of clavulanate; 65.8% of these strains were susceptible to amoxicillin and 98.2% to ticarcillin, but all were susceptible when clavulanate was added. All strains were susceptible to imipenem and metronidazole, and 99.3% to cefoxitin.

Evaluation of two methods for rapid testing for beta-lactamase production in Bacteroides and Fusobacterium

A total of 978 strains of Bacteroides and Fusobacterium were tested for beta-lactamase production by a disk test (Cefinase) and a microtiter nitrocefin assay. In 83% of strains both tests were positive and in 14.8% both were negative. In 1.7% of strains the disk test was positive and the microtiter test negative, and in 0.4% the disk test negative but the microtiter test positive. The disk test was less discriminatory in detecting amoxicillin-resistant strains. The microtiter test was less sensitive than the disk test, but more discriminatory if results were read within 1 h for Fusobacterium spp., within 8 h for the Bacteroides fragilis group, and within 2 h for other Bacteroides spp. Neither test should be used clinically at present.

Selection of constitutively resistant mutants of inducible clindamycin-resistant Bacteroides vulgatus

The rate of spontaneous mutation to constitutive clindamycin resistance and the kinetics of selection of such mutants in preinduced and non-preinduced cells was evaluated in the Bacteroides vulgatus strain RYC18F6 (original MIC less than or equal to 0.25 micrograms/ml), which shows inducible resistance to clindamycin and erythromycin. Mutants demonstrating a high level of constitutive resistance to clindamycin (64 micrograms/ml) occurred at a frequency of 10(-7). Culture in broth containing either subinhibitory or inhibitory levels of clindamycin resulted in induction of clindamycin resistance. This permitted survival of a part of the population that seemed to facilitate the further growth of constitutively resistant mutants (up to a frequency of 10(-1)). At both low and high clindamycin concentrations mutants appeared to be selected over merely induced cells. Preincubation of cultures with erythromycin gave rise to an apparently higher level of induction. Under these circumstances, the selection of constitutively resistant mutants was less effective (10(-4). The use of erythromycin or clindamycin against populations of Bacteroides spp. exhibiting inducible resistance may contribute to selection of mutants showing constitutive clindamycin resistance.

Patterns of resistance to anaerobic organisms in Australia

This study investigated patterns of resistance in anaerobic organisms isolated at Westmead Hospital in Sydney, Australia, during the years 1987 to 1988. Minimal inhibitory concentrations (MICs) to ampicillin, sulbactam/ampicillin, metronidazole, clindamycin, and cefoxitin were determined by agar dilution for 200 anaerobes from clinically significant infections. Antibiotics active against nearly all of these anaerobes included metronidazole and sulbactam/ampicillin, which demonstrated good activity against beta-lactamase producing Bacteroides spp. with the exception of Bacteroides distasonis. Resistance in non-beta-lactamase producing anaerobes was similar to that seen with ampicillin. As expected, ampicillin resistance was common in the Bacteroides fragilis group where beta-lactamase production was frequent. In addition, beta-lactamase was detected in 33% of other Bacteroides spp. Ampicillin resistance was also seen in 5 to 15% of additional anaerobes that did not produce beta-lactamase. Clindamycin resistance occurred in 4 to 18% of the B. fragilis group. Clindamycin resistance was also seen in 7 to 8% of Clostridium spp. and anaerobic Gram-positive cocci. Resistance to cefoxitin was variable in the B. fragilis group with the highest levels of resistance occurring in the indole-negative subgroup. Resistance in other anaerobes was not commonly seen.

Comparative in vitro antimicrobial susceptibilities of a special panel of 74 Bacteroides fragilis group isolates in Wilkins-Chalgren agar with and without sheep blood

Seventy-four cefoxitin-resistant Bacteroides fragilis group isolates were tested by the serial twofold agar dilution method for susceptibility to imipenem and other agents in medium with and without 5% sheep blood. Imipenem (MIC for 90% of strains tested, 1 microgram/ml) and metronidazole (MIC for 90% of strains tested, 2 micrograms/ml) were the two most active agents. The addition of 5% sheep blood to the medium had little or no effect on the activity of the antibiotics tested.

Comparison of the in vitro action and interaction of cefotaxime and desacetylcefotaxime against clinical isolates of Bacteroides spp

Desacetylcefotaxime (dCTX), the in vivo metabolite of cefotaxime (CTX), possess significant in vitro antimicrobial activity similar to the parent compound against a variety of aerobic and anaerobic bacteria. In vitro susceptibility studies showed that CTX inhibited 86% of 473 strains of the Bacteroides fragilis at a concentration of 32 micrograms/ml while dCTX inhibited 91% of the test isolates at the same concentration. Strains of the B. fragilis species were significantly more susceptible to CTX than were the non-B. fragilis species. Susceptibility testing of CTX and dCTX in a 1:1 ratio produced significantly more inhibitory activity, especially against the non-B. fragilis strains. Synergy studies showed that the interaction of CTX and dCTX was either completely or partially synergistic against 85% of 92 test organisms. The presence of dCTX was also shown to lower the CTX MIC values four-fold or greater in 82% of the synergy studies. Synergy was noted against strains of the B. fragilis group, B. melaninogenicus group, B. bivius, B. disiens, and B. capillosus. Through the use of time-kill kinetics studies, the interaction of CTX and dCTX was shown to be additive at subinhibitory and inhibitory concentrations and suggestedly synergistic at suprainhibitory concentrations against strains of the B. fragilis group. These in vitro studies demonstrate that dCTX increases the inhibitory and bactericidal activity of CTX when tested in combination.