Tentative interpretive standards for agar disk diffusion antimicrobial susceptibility testing of cefoperazone.

Cefoperazone is a new cephalosporin with a very wide spectrum of activity, including activity against Pseudomonas aeruginosa. It has less activity on enterococci and Acinetobacter. Of the 459 selected bacterial strains tested in this study, only 1.5% (7 strains and 6 genera) had minimum inhibitory concentrations of greater than or equal to 128 micrograms/ml. For a minimum inhibitory concentration breakpoint of less than or equal to 32 micrograms/ml (susceptible), we recommend that the disk diffusion test be done with a 75-micrograms disk and breakpoints of greater than or equal to 18 mm for susceptible, 15 to 17 mm for intermediate, and less than or equal to 14 mm for resistant. Diffusion tests using these criteria yielded only 1.1% very major or major errors.     

Evaluation of proposed criteria of disk susceptibility testing for tosufloxacin and lomefloxacin in NCCLS guidelines and WHO standards]

Disk diffusion zone diameter breakpoint criteria for Tosufloxacin and Lomefloxacin were tentatively established by correlating MICs with 1-, 5- and 10 micrograms of Tosufloxacin disk inhibitory zone diameters and with 10 micrograms of Lomefloxacin disk of those to 418 clinical isolates representing 32 species. We recommend 5 micrograms disks for Tosufloxacin with the following breakpoints: Susceptible (MIC, less than or equal to 0.5 microgram/ml), greater than or equal to 22 mm; intermediate, 17 to 21 mm; and resistant (MIC, greater than or equal to 2.0 micrograms/ml), less than or equal to 16 mm. We recommend 10 micrograms disks for Lomefloxacin with the following breakpoints: Susceptible (MIC, less than or equal to 2.0 micrograms/ml), greater than or equal to 21 mm; intermediate, 16 to 20 mm; and resistant (MIC, greater than or equal to 8.0 micrograms/ml), less than or equal to 15 mm. Using these criteria for Tosufloxacin and Lomefloxacin, the occurrence rate of major errors in judging susceptibility and resistance was 0.48%.     

Assessment of metronidazole susceptibility in Helicobacter pylori: statistical validation and error rate analysis of breakpoints determined by the disk diffusion test

Metronidazole susceptibility of 100 Helicobacter pylori strains was assessed by determining the inhibition zone diameters by disk diffusion test and the MICs by agar dilution and PDM Epsilometer test (E test). Linear regression analysis was performed, allowing the definition of significant linear relations, and revealed correlations of disk diffusion results with both E-test and agar dilution results (r2 = 0.88 and 0.81, respectively). No significant differences (P = 0.84) were found between MICs defined by E test and those defined by agar dilution, taken as a standard. Reproducibility comparison between E-test and disk diffusion tests showed that they are equivalent and with good precision. Two interpretative susceptibility schemes (with or without an intermediate class) were compared by an interpretative error rate analysis method. The susceptibility classification scheme that included the intermediate category was retained, and breakpoints were assessed for diffusion assay with 5-microg metronidazole disks. Strains with inhibition zone diameters less than 16 mm were defined as resistant (MIC > 8 microg/ml), those with zone diameters equal to or greater than 16 mm but less than 21 mm were considered intermediate (4 microg/ml < MIC 相似文献   

Proposed disk diffusion susceptibility criteria for ofloxacin.

Disk diffusion zone diameter breakpoint criteria for ofloxacin were tentatively established by correlating MICs with 1-, 3-, and 5-micrograms disk inhibitory zone diameters for 638 bacterial isolates representing 36 species. We recommend use of 5-micrograms disks with the following breakpoints: susceptible (MIC, less than or equal to 2.0 micrograms/ml), greater than or equal to 16 mm; intermediate (MIC, 4.0 micrograms/ml), 13 to 15 mm; and resistant (MIC, greater than or equal to 8.0 micrograms/ml), less than or equal to 12 mm.     

Screening pneumococci for penicillin resistance

Eighty-four pneumococci with various MICs of penicillin (38 with MICs of less than or equal to 0.06 micrograms/ml [susceptible], 35 with MICs of 0.12 to 1.0 micrograms/ml [relatively resistant], and 11 with MICs of greater than 1.0 micrograms/ml [resistant] ) were screened by a disk diffusion test using oxacillin and methicillin to see how well they distinguished penicillin-susceptible strains from those with decreased susceptibility to penicillin. The effects of Mueller-Hinton agar plus 5% sheep blood and Trypticase soy agar plus 5% sheep blood and two atmospheres, ambient air and a candle extinction jar (increased CO2), were compared. There were no obvious differences between the effects of the two media, but zones were generally larger in ambient air than in increased CO2. Although the oxacillin test can separate penicillin-susceptible and -resistant strains, it cannot separate penicillin-resistant from relatively penicillin-resistant strains by using the breakpoint of less than 20 mm recommended by the National Committee for Clinical Laboratory Standards. When the 20-mm breakpoint was applied to methicillin, 12% of the relatively resistant strains tested were erroneously classified as susceptible. When different breakpoints were used for methicillin, there was better separation of the two classes of penicillin-resistant isolates, but a few relatively resistant strains were still classified as susceptible. We recommend that oxacillin, not methicillin, be used as the screening agent with Mueller-Hinton sheep blood agar and ambient air incubation and that the breakpoint be less than 20 mm to indicate resistance or relative resistance.     

Regression analysis, proposed interpretative zone size standards, and quality control guidelines for a new macrolide antimicrobial agent, A-56268 (TE-031).

A-56268 is a 6-O-methyl derivative of erythromycin A which has a spectrum of activity similar to that of erythromycin and is 1 log2 dilution more potent than erythromycin against most organisms that have been tested. The correlation of zone size diameters and MICs of A-56268 for 461 strains of bacteria isolated from clinical specimens was investigated. Based on anticipated levels in human serum of 2 micrograms/ml, 15-microgram disks have been recommended with zone size standards of greater than or equal to 15 mm for susceptibility (MIC correlate, less than or equal to 2.0 micrograms/ml) and less than or equal to 11 mm for resistance (MIC correlate, greater than or equal to 8 micrograms/ml). Selection of these tentative breakpoints resulted in no very major errors (false susceptible), a major error (false resistant) rate of 0.22%, and an acceptable minor error (intermediate) rate of 2.82%. MIC ranges and zone diameter limits for quality control organisms used in the standardized agar dilution and disk diffusion susceptibility tests with A-56268 are given.     

Relationship between in vitro susceptibility test results for chloramphenicol and production of chloramphenicol acetyltransferase by Haemophilus influenzae, Streptococcus pneumoniae, and Aerococcus species.

Haemophilus influenzae, Streptococcus pneumoniae, and Aerococcus species were tested for susceptibility to chloramphenicol by standard broth microdilution and disk-diffusion methods. MICs and zone diameter breakpoints were correlated with production of chloramphenicol acetyltransferase (CAT). A comparison of MICs and zone diameters indicated that the interpretative criteria for H. influenzae and S. pneumoniae should be an MIC of less than or equal to 4 micrograms/ml or a zone diameter greater than or equal to 25 mm for susceptible strains and an MIC of greater than or equal to 8 micrograms/ml or a zone diameter of less than or equal to 20 mm for resistant strains; for Aerococcus species, interpretative criteria should be an MIC of less than or equal to 8 micrograms/ml or a zone diameter of greater than or equal to 20 mm for susceptible strains and an MIC of greater than or equal to 32 micrograms/ml or a zone diameter of less than or equal to 12 mm for resistant strains. All but four strains of H. influenzae and one strain of S. pneumoniae that were resistant to chloramphenicol by these criteria produced CAT. For Aerococcus species, however, chloramphenicol-resistant strains were negative for CAT as determined by a commercially available disk test. When comparing susceptibility results with CAT production, thiamphenicol was a better indicator of the presence of the enzyme than chloramphenicol and may be useful in assaying resistance to chloramphenicol.     

Reevaluation of interpretive criteria for Haemophilus influenzae by using meropenem (10-microgram), imipenem (10-microgram), and ampicillin (2- and 10-microgram) disks.

A collection of 300 Haemophilus influenzae clinical strains was used to assess in vitro susceptibility to carbapenems (meropenem, imipenem) by MIC and disk diffusion methods and to compare disk diffusion test results with two potencies of ampicillin disks (2 and 10 micrograms). The isolates included ampicillin-susceptible or- intermediate (167 strains), beta-lactamase-positive (117 strains), and beta-lactamase-negative ampicillin-resistant (BLNAR; 16 strains) organisms. Disk diffusion testing was performed with 10-micrograms meropenem disks from two manufacturers. Meropenem was highly active against H. influenzae strains (MIC50, 0.06 microgram/ml; MIC90, 0.25 microgram/ml; MIC50 and MIC90, MICs at which 50 and 90%, respectively, of strains are inhibited) and was 8- to 16-fold more potent than imipenem (MIC50, 1 microgram/ml; MIC90, 2 micrograms/ml). Five non-imipenem-susceptible strains were identified (MIC, 8 micrograms/ml), but the disk diffusion test indicated susceptibility (zone diameters, 18 to 21 mm). MIC values of meropenem, doxycycline, ceftazidime, and ceftriaxone for BLNAR strains were two- to fourfold greater than those for other strains. The performance of both meropenem disks was comparable and considered acceptable. A single susceptible interpretive zone diameter of > or = 17 mm (MIC, < = or 4 micrograms/ml) was proposed for meropenem. Testing with the 2-micrograms ampicillin disk was preferred because of an excellent correlation between MIC values and zone diameters (r = 0.94) and superior interpretive accuracy with the susceptible criteria at > or = 17 mm (MIC, < or = 1 microgram/ml) and the resistant criteria at < or = 13 mm (MIC, > or = 4 micrograms/ml). Among the BLNAR strains tested, 81.3% were miscategorized as susceptible or intermediate when the 10-micrograms ampicillin disk was used, while the 2-micrograms disk produced only minor interpretive errors (12.5%). Use of these criteria for testing H. influenzae against meropenem and ampicillin should maximize reference test and standardized disk diffusion test performance with the Haemophilus Test Medium. The imipenem disk diffusion test appears compromised and should be used with caution for detecting strains for which imipenem MICs are elevated.     

New recommendations for disk diffusion antimicrobial susceptibility tests for methicillin-resistant (heteroresistant) staphylococci

The agar disk diffusion susceptibility test was reevaluated for its ability to discriminate between susceptible and resistant Staphylococcus aureus (128 strains) and coagulase-negative staphylococci (19 strains) when tested with methicillin, oxacillin, and nafcillin. The results show that the current recommendations for disk potencies and interpretive zone diameters do not fit well with MIC correlates that we now recommend. Based on data from this study, we suggest that these parameters of the test be changed. For methicillin, we recommend a 10-micrograms disk with breakpoints of less than or equal to 11 mm (greater than or equal to 16 micrograms/ml) to indicate resistance and greater than or equal to 15 mm (less than or equal to 4 micrograms/ml) to indicate susceptibility. For oxacillin and nafcillin, we recommend 4-micrograms disks with breakpoints of less than or equal to 12 mm (greater than or equal to 8 micrograms/ml) to indicate resistance and greater than or equal to 16 mm (less than or equal to 2 micrograms/ml) to indicate susceptibility. MIC breakpoints were from a broth microdilution system which used a medium containing salt. If one of these three penicillins were to be selected for routine tests, we would recommend oxacillin, based on our data, but we recognize that this may depend upon the population of staphylococci within a particular hospital.     

Suggested modifications for disk diffusion susceptibility testing criteria for levofloxacin and sparfloxacin following tests with a predictor panel of ciprofloxacin-resistant clinical isolates.

A predictor panel of 300 clinical bacterial isolates (200 resistant to ciprofloxacin) was used to compare 5-micrograms disk diffusion test results with the MICs of ofloxacin (control), levofloxacin, and sparfloxacin. Regression analysis demonstrated high correlations between the methods for all three fluoroquinolones (r > or = 0.95). In order to minimize disk diffusion testing errors among the fluoroquinolone-resistant strains, the following modifications to previously proposed or published interpretive criteria were suggested: for levofloxacin, susceptible at > or = 17 mm (< or = 2 micrograms/ml) and resistant at < or = 13 mm (> or = 8 micrograms/ml); for sparfloxacin, susceptible at > or = 20 mm (< or = 1 microgram/ml) and resistant at < or = 16 mm (> or = 4 micrograms/ml). The study control drug, ofloxacin, did not appear to possess a significant error rate (5% minor error) when fluoroquinolone-resistant strains were tested, and no modifications were proposed. Under these proposed interpretive criteria, the absolute categorical agreements between standardized susceptibility testing methods for levofloxacin and sparfloxacin results were 91.3 and 94.0%, respectively (< or = 0.3% major errors and nil very major errors).     

Ciprofloxacin disk susceptibility tests: interpretive zone size standards for 5-microgram disks.

Evaluations of 5-microgram ciprofloxacin disk diffusion susceptibility tests were performed independently by seven different investigators. The results of the separate tests were combined to increase the number of resistant strains in the challenge set of microorganisms. Based on data with 2,652 isolates, the following interpretive breakpoints are tentatively proposed for use in ongoing clinical trials of ciprofloxacin: less than or equal to 15 mm, resistant (MIC greater than 2.0 micrograms/ml); 16 to 20 mm, intermediate (1.0 less than MIC less than or equal to 2.0 micrograms/ml); and greater than or equal to 21 mm, susceptible (MIC less than or equal to 1.0 micrograms/ml). Disk tests with Streptococcus spp. and with Pseudomonas maltophilia were not reliable; other microorganisms were accurately categorized by the disk diffusion test.     

Interpretive criteria for susceptibility testing of CI-960 (PD127391, AM-1091), fleroxacin, lomefloxacin, and temafloxacin against Neisseria gonorrhoeae, including drug stability in GC agar medium.

CI-960, fleroxacin, lomefloxacin, and temafloxacin were tested against over 100 strains of Neisseria gonorrhoeae. Each organism was tested in triplicate by using agar dilution and disk diffusion methods recommended by the National Committee for Clinical Laboratory Standards. CI-960 was the most potent compound, with a MIC against 90% of the strains tested of 0.008 microgram/ml, and the least active was fleroxacin (MIC against 90% of strains, 0.12 microgram/ml). Only the susceptible interpretive category was recommended for the CI-960 tests as follows: 5-micrograms disk, greater than or equal to 39 mm (MIC correlate, less than or equal to 0.12 microgram/ml). Three interpretive categories were proposed for the other fluoroquinolones as follows: fleroxacin, 5-micrograms disk susceptible at greater than or equal to 33 mm (MIC correlate, less than or equal to 0.25 microgram/ml), intermediate at 28 to 32 mm (MIC correlate, 0.5 microgram/ml), and resistant at less than or equal to 27 mm (MIC correlate, greater than 0.5 microgram/ml); lomefloxacin, 10-micrograms disk susceptible at greater than or equal to 35 mm (MIC correlate, less than or equal to 0.12 microgram/ml), intermediate at 28 to 34 mm (MIC correlates, 0.25 to 0.5 microgram/ml), and resistant at less than or equal to 27 mm (MIC correlate, greater than 0.5 microgram/ml); and temafloxacin, 5-micrograms disk susceptible at greater than or equal to 36 mm (MIC correlate, less than or equal to 0.06 microgram/ml), intermediate at 28 to 35 mm (MIC correlates 0.12 to 0.25 microgram/ml), and resistant at less than or equal to 27 mm (greater than 0.25 microgram/ml). Interpretive agreement between disk diffusion results and the MICs was 100% for each agent, with the exception of lomefloxacin, which had a 0.9% minor error. All drugs were stable in GC agar medium for at least 21 days when stored at 2 to 5 degrees C.     

In vitro activity of vancomycin and teicoplanin against gram-positive cocci]

The activities of vancomycin and teicoplanin against 148 strains of Gram-positive cocci were tested using agar diffusion and liquid microdilution MIC determination. Tested strains included 84 staphylococci, 32 S. aureus, 52 coagulase-negative staphylococci (CNS), 52 enterococci, and 12 streptococci. Most strains (136) were susceptible to both agents, with inhibition diameters of 17 mm or more. MRSA strains exhibited lower geometric MIC means with teicoplanin (0.90 micrograms/ml) than with vancomycin (1.79 micrograms/ml); this difference was found for methicillin-susceptible S. aureus strains (1.07 and 1.38 micrograms/ml for teicoplanin and vancomycin, respectively). In contrast, methicillin-susceptible and methicillin-resistant strains of CNS exhibited similar MICs (1.60 micrograms/ml approximately). Enterococci were more susceptible to teicoplanin (MIC 0.25 micrograms/ml) than to vancomycin (MIC 1.35 micrograms/ml). Both vancomycin and teicoplanin were thus found to be consistently effective against Gram-positive cocci; however, teicoplanin proved more effective than vancomycin against enterococci and methicillin-resistant S. aureus strains and may therefore be a valuable therapeutic alternative for these multiresistant organisms.     

Reliability of the E test for detection of ampicillin, vancomycin, and high-level aminoglycoside resistance in Enterococcus spp.

By comparison with agar dilution results, the E test was investigated for the ability to detect high-level aminoglycoside (gentamicin and streptomycin), ampicillin, and vancomycin resistance among strains representing six enterococcal species. For ampicillin and vancomycin, disk diffusion results also were obtained. No false high-level aminoglycoside resistance occurred, and no false gentamicin susceptibility was noted. With the high-range streptomycin E test (2,048 micrograms), 24% of the 38 resistant strains were falsely susceptible. However, these discordances could likely be reconciled by adjustments in incubation duration and by using broth microdilution rather than agar screen breakpoint criteria, or by using the lower-range (1,024-micrograms) strip. For ampicillin, category results obtained by E test and disk diffusion showed good agreement with agar dilution; E test MICs were generally comparable to agar dilution MICs. The E test was more sensitive than disk diffusion for detecting vancomycin-intermediate strains, but for these strains and those exhibiting low-level vancomycin resistance (MIC, 32 to 128 micrograms/ml), disk diffusion and E test inhibition zones must be interpreted with caution. Given the reliability of E test for detecting resistance to anti-enterococcal agents, the decision to use this method should be based on convenience, cost, testing frequency, and satisfaction with currently used methods.     

In vitro activity of teicoplanin and vancomycin against gram-positive bacteria from human clinical and veterinary sources.

The minimum inhibitory concentration (MIC) of teicoplanin and vancomycin was determined by the agar dilution method for 186 Gram-positive bacteria from human clinical and veterinary sources. Teicoplanin MIC values were less than or equal to 4 micrograms/ml for 94% of staphylococci (group A, n = 52) and less than or equal to 2 micrograms/ml for all streptococci, enterococci, aerococci and pediococci (group B, n = 75). Seventy-eight percent of Gram-positive rods, Rhodococcus and Leuconostoc spp. (group C, n = 59) were inhibited by 4 micrograms/ml. Teicoplanin resistance (MIC greater than or equal to 16 micrograms/ml) was demonstrated for all Nocardia strains and for some strains of Lactobacillus, E. rhusiopathiae, Leuconostoc, and S. haemolyticus. Cross-resistance between teicoplanin and vancomycin was observed for all Nocardia strains and for some strains of Lactobacillus, E. rhusiopathiae, and Leuconostoc. Three methicillin-resistant S. haemolyticus strains were either resistant or intermediately susceptible to teicoplanin and susceptible to vancomycin. Eight strains (motile enterococci four, E. rhusiopathiae three and Leuconostoc sp. one) were susceptible to teicoplanin and resistant to vancomycin. Teicoplanin disc diffusion on Danish Blood Agar with NeoSensitabs (Rosco), PDM AB Biodisc and locally prepared discs revealed a wide range of zone diameters in groups B and C. The relation between MIC values and zone diameters for teicoplanin was analysed by the error-rate bounded method. Zone size interpretive criteria as suggested by the manufacturers (greater than or equal to 15 mm) produced 2.7% (95% confidence limits 0.9-6.2%) and 1.6% (95% confidence limits 0.3-4.6%) very major errors for NeoSensitabs and PDM-disc, respectively. Using a zone size breakpoint for susceptibility of greater than or equal to 25 mm for NeoSensitabs and greater than or equal to 20 mm for PDM-disc, the proportions of very major errors were 0.5% (95% confidence limits 0.0-3.0%) at the expense of 5.9% (95% confidence limits 3.0-10.3%) indeterminate strains that belonged to E. rhusiopathiae, Leuconostoc, Lactobacillus and S. haemolyticus. However, using these zone size breakpoints five major errors (beta-haemolytic streptococci, group B three, S. aureus one, Leuconostoc sp. one) were observed for NeoSensitabs and two major errors (beta-haemolytic streptococcus, group B one, Leuconostoc sp. one) were observed for PDM-disc. Susceptibility testing against teicoplanin among these taxa should therefore include a determination of MIC.     

Modification of interpretive breakpoints for netilmicin disk susceptibility tests with Pseudomonas aeruginosa.

Regression analysis of data correlating 30-micrograms netilmicin disk zone diameters with microdilution MICs, obtained by testing close-interval dilution steps, was performed with 77 selected strains of Pseudomonas aeruginosa, each tested in three independent laboratories. A zone of greater than or equal to 15 mm correlated with an MIC of less than or equal to 12 micrograms/ml (susceptible), and a zone of less than or equal to 12 mm correlated with an MIC of greater than 16 micrograms/ml (resistant). Additional disk tests were performed with 256 strains having known resistance mechanisms and 280 susceptible strains: the majority were appropriately categorized by these interpretive zone standards. The previously recommended standards of greater than or equal to 17 mm (MIC, less than or equal to 8.0 micrograms/ml) for the susceptible category inappropriately placed a significant number of truly susceptible P. aeruginosa strains in the intermediate category.     

Tentative disk diffusion susceptibility interpretive criteria for pefloxacin.

Standardized broth microdilution and disk diffusion susceptibility tests for pefloxacin were performed on 585 clinical isolates. The 5-micrograms pefloxacin disk is recommended, and the following breakpoints are proposed: susceptible, greater than or equal to 19 mm (MIC, less than or equal to 2.0 micrograms/ml); resistant, less than or equal to 15 mm (MIC, greater than 4.0 micrograms/ml); and intermediate, 16 to 18 mm.     

Online-Only Abstract

With the emergence of reduced susceptibility of Clostridium difficile to metronidazole and vancomycin the value of antimicrobial susceptibility testing has increased. The aim of our study was to evaluate disk diffusion for susceptibility testing of C. difficile by comparing disk diffusion results with MICs from gradient tests and to propose zone diameter breakpoint correlates for the EUCAST epidemiological cut-off values (ECOFFs) recently published. We tested 211 clinical isolates of C. difficile, from patients with diarrhoea hospitalized at Aarhus and Odense University Hospitals, Denmark. Furthermore, ten clinical isolates of C. difficile from the Anaerobe Reference Laboratory, University Hospital of Wales, with known reduced susceptibility to either metronidazole or vancomycin, were included. Isolates were tested with Etest gradient strips and disk diffusion towards metronidazole, vancomycin and moxifloxacin on Brucella Blood Agar supplemented with hemin and vitamin K. We found an excellent agreement between inhibition zone diameter and MICs. For each MIC value, the inhibition zones varied from 0 to 8 mm, with 93% of values within 6 mm for metronidazole, 95% of values within 4 mm for vancomycin, and 98% of values within 4 mm for moxifloxacin. With proposed zone diameter breakpoints for metronidazole, vancomycin and moxifloxacin of WT ≥ 23 mm, WT ≥ 19 and WT ≥ 20 mm, respectively, we found no very major errors and only major errors below 2%. In conclusion, we suggest that disk diffusion is an option for antimicrobial susceptibility testing of C. difficile.     

Antimicrobial susceptibility testing of Clostridium difficile using EUCAST epidemiological cut-off values and disk diffusion correlates

With the emergence of reduced susceptibility of Clostridium difficile to metronidazole and vancomycin the value of antimicrobial susceptibility testing has increased. The aim of our study was to evaluate disk diffusion for susceptibility testing of C. difficile by comparing disk diffusion results with MICs from gradient tests and to propose zone diameter breakpoint correlates for the EUCAST epidemiological cut-off values (ECOFFs) recently published. We tested 211 clinical isolates of C. difficile, from patients with diarrhoea hospitalized at Aarhus and Odense University Hospitals, Denmark. Furthermore, ten clinical isolates of C. difficile from the Anaerobe Reference Laboratory, University Hospital of Wales, with known reduced susceptibility to either metronidazole or vancomycin, were included. Isolates were tested with Etest gradient strips and disk diffusion towards metronidazole, vancomycin and moxifloxacin on Brucella Blood Agar supplemented with hemin and vitamin K. We found an excellent agreement between inhibition zone diameter and MICs. For each MIC value, the inhibition zones varied from 0 to 8 mm, with 93% of values within 6 mm for metronidazole, 95% of values within 4 mm for vancomycin, and 98% of values within 4 mm for moxifloxacin. With proposed zone diameter breakpoints for metronidazole, vancomycin and moxifloxacin of WT ≥ 23 mm, WT ≥ 19 and WT ≥ 20 mm, respectively, we found no very major errors and only major errors below 2%. In conclusion, we suggest that disk diffusion is an option for antimicrobial susceptibility testing of C. difficile.