Interpretive criteria and quality control limits for ceftibuten disk susceptibility tests. Collaborative Antimicrobial Susceptibility Testing Group.

In vitro studies were undertaken to evaluate susceptibility tests with 30-micrograms ceftibuten disks. The following interpretive criteria were proposed: less than or equal to 17 mm for resistance (MIC, greater than or equal to 32 micrograms/ml) and greater than or equal to 21 mm for susceptibility (MIC, less than or equal to 8.0 micrograms/ml). A multilaboratory quality control study led to the conclusion that Escherichia coli ATCC 25922 should provide zones 29 to 35 mm in diameter.     

Interpretive criteria and quality control parameters for testing bacterial susceptibility to the fluoroquinolone PD131628.

For testing bacterial susceptibility to PD131628, a 5-micrograms disk and the following tentative interpretive criteria may be used: > or = 19 mm for susceptible (MIC, < or = 1.0 micrograms/ml), 16 to 18 mm for intermediate (MIC, 2.0 micrograms/ml), and < or = 15 mm for resistant (MIC, > or = 4.0 micrograms/ml). For standard quality control strains, the following limits are proposed: for Escherichia coli ATCC 25922, zones of 31 to 41 mm or a MIC of 0.002 to 0.016 micrograms/ml; for Pseudomonas aeruginosa ATCC 27853, zones of 26 to 34 mm or a MIC of 0.12 to 0.5 micrograms/ml; for Staphylococcus aureus ATCC 25923, zones of 27 to 33 mm; for Staphylococcus aureus ATCC 29213, a MIC of 0.03 to 0.12 micrograms/ml; and for Enterococcus faecalis ATCC 29212, a MIC of 0.12 to 0.5 micrograms/ml.     

Criteria for disk susceptibility tests and quality control guidelines for the cefoperazone-sulbactam combination.

For in vitro susceptibility tests with cefoperazone and sulbactam (a beta-lactamase inhibitor), 75/30-micrograms disks may be used with the interpretive zone size breakpoints that are currently used for 75-micrograms cefoperazone disks. For dilution tests, a 2:1 ratio of cefoperazone to sulbactam is recommended. For quality control purposes, MIC limits that are used to monitor cefoperazone tests were also applied to tests with the combination of drugs. For gram-negative control strains, zone size limits were calculated to be 1 mm smaller than those used for cefoperazone disks. To monitor the sulbactam portion of the combination, Acinetobacter calcoaceticus subsp. anitratus ATCC 43498 was selected; zones with 75/30-micrograms disks were 26 to 32 mm in diameter, and broth microdilution MICs ranged from 1.0/0.5 to 8.0/4.0 micrograms/ml. With cefoperazone alone, MICs for Acinetobacter calcoaceticus subsp. anitratus were 16 to 64 micrograms/ml and zones ranged from 14 to 18 mm in diameter. For anaerobic dilution tests, only Bacteroides thetaiotaomicron ATCC 29741 is recommended for cefoperazone-sulbactam; MICs ranged from 8.0/4.0 to 32/16 micrograms/ml.     

Ofloxacin susceptibility testing quality control parameters for microdilution and disk diffusion, and confirmation of disk diffusion interpretive criteria.

The susceptibilities of 221 clinical isolates to ofloxacin were tested simultaneously by broth microdilution and disk diffusion methods with commercially prepared 5-micrograms ofloxacin disks. The acceptability of the following previously proposed zone diameter breakpoints was confirmed: greater than or equal to 16 mm, susceptible; 13 to 15 mm, intermediate; less than or equal to 12 mm, resistant. On the basis of a multilaboratory collaborative study, the following are proposed as acceptable ofloxacin MIC ranges for quality control organisms: Escherichia coli ATCC 25922, 0.03 to 0.06 micrograms/ml; Staphylococcus aureus ATCC 29213, 0.12 to 0.5 micrograms/ml; Pseudomonas aeruginosa ATCC 27853 and Enterococcus faecalis ATCC 29212, 1.0 to 4.0 micrograms/ml. Ofloxacin quality control zone diameter ranges for the disk diffusion test are tentatively proposed, but variations in the performance of different lots of Mueller-Hinton agar may prove to be a serious problem for users.     

Interpretive criteria for disk diffusion susceptibility testing of mupirocin, a topical antibiotic.

Five hundred gram-positive clinical bacterial isolates were tested for susceptibility to mupirocin by broth microdilution and disk diffusion methods (5- and 10-micrograms disks). All but 1 of 330 staphylococci (including 100 oxacillin-resistant Staphylococcus aureus) were susceptible to less than or equal to 1.0 micrograms of mupirocin per ml. With a susceptible MIC breakpoint of less than or equal to 2.0 micrograms/ml, the corresponding zone diameter breakpoints were greater than or equal to 14 mm for 5-micrograms disks and greater than or equal to 16 mm for 10-micrograms disks. With either disk potency, susceptible staphylococci were effectively separated from more resistant gram-positive species such as the enterococci and gram-positive bacilli.     

Proposed interpretive criteria and quality control parameters for testing susceptibility of Neisseria gonorrhoeae to beta-lactam-clavulanate combinations.

To support future clinical studies, in vitro susceptibility tests were examined to determine whether Neisseria gonorrhoeae could be tested reliably against two beta-lactam-clavulanate combinations. All isolates that were tested appeared to be susceptible to amoxicillin and ticarcillin in combination with clavulanic acid. In the absence of resistant isolates, only a breakpoint for a susceptible category could be defined for agar dilution tests with amoxicillin-clavulanic acid (MIC of less than or equal to 2.0/1.0 micrograms/ml is tentatively proposed). For disk diffusion tests, a corresponding breakpoint zone diameter of greater than or equal to 28 mm is suggested. The validity of the breakpoints for penicillinase-negative penicillin-resistant strains awaits clinical data. Proposed quality control limits for testing amoxicillin-clavulanic acid by agar dilution and disk diffusion methods are a MIC of 0.25/0.125 to 1.0/0.5 micrograms/ml and zones of 30 to 40 mm in diameter for N. gonorrhoeae ATCC 49226, a MIC of 0.125/0.06 to 0.5/0.25 micrograms/ml for Staphylococcus aureus ATCC 29213, and zones of 30 to 38 mm for S. aureus ATCC 25923. Ticarcillin-clavulanate is currently tested against other species by preparing doubling dilutions of ticarcillin with a constant 2 micrograms of clavulanate per ml. By that method, all gonococci were susceptible to low concentrations. However, the amount of clavulanic acid that is included (2 micrograms/ml) will, by itself, inhibit many strains of N. gonorrhoeae. Consequently, the role of ticarcillin in the combination cannot be determined, and such tests are not recommended.     

Interpretive standards and quality control guidelines for cefpiramide disk susceptibility tests.

Disk susceptibility tests with 30- and 75-micrograms cefpiramide disks were evaluated with 614 bacterial isolates. Quality control parameters were also evaluated, and control limits for disk tests are recommended. Tests with 75-micrograms disks are recommended, with zone size standards of greater than or equal to 19 mm for susceptible (MIC, less than or equal to 32 micrograms/ml) and less than or equal to 15 mm for resistant (MIC, greater than or equal to 128 micrograms/ml).     

Evaluation of teicoplanin and vancomycin disk susceptibility tests.

Disk tests with two glycopeptide antibiotics, teicoplanin and vancomycin, were evaluated, and MICs were compared with those of fusidic acid and coumermycin. For tests with 30-micrograms vancomycin disks, we recommend modification of the current zone size standards to less than or equal to 10 mm for resistant and greater than or equal 15 mm for susceptible. For teicoplanin disk tests, 30-micrograms disks are recommended, with zone size interpretive standards of less than or equal to 10 and greater than or equal 14 mm. Since no resistant clinical isolates are available at this time, susceptibility testing of either drug is rarely necessary, and zone size standards are tentative.     

Proposed interpretive criteria and quality control parameters for testing in vitro susceptibility of Neisseria gonorrhoeae to ciprofloxacin.

Ciprofloxacin was subjected to a multilaboratory study designed to determine its in vitro susceptibility criteria for Neisseria gonorrhoeae and its quality control parameters for both agar dilution and disk diffusion susceptibility testing for this species. All clinical isolates were susceptible, i.e., MICs were less than or equal to 0.06 microgram/ml and zones of inhibition were greater than or equal to 36 mm. A resistant category could not be defined, but in vitro-selected mutants gave zones of less than or equal to 35 mm, and MICs for these strains were greater than or equal to 0.12 microgram/ml. For quality control of ciprofloxacin agar dilution tests on supplemented GC agar, MICs for Staphylococcus aureus ATCC 29213 ranged from 0.12 to 0.5 microgram/ml. For quality control of 5-micrograms ciprofloxacin disk tests, N. gonorrhoeae ATCC 49226 and S. aureus ATCC 25923 produced acceptable zone diameter ranges of 48 to 58 mm and 22 to 26 mm, respectively.     

Quality control of susceptibility tests with 5-micrograms trimethoprim disks

In 1981 and again in 1984, we performed multi-laboratory studies to develop quality control limits for susceptibility tests with 5-micrograms trimethoprim disks. Zone size limits of 21 to 28 mm are recommended for tests with Escherichia coli ATCC 25922, and limits of 19 to 26 mm are recommended for tests with Staphylococcus aureus ATCC 25923. For screening Mueller-Hinton agar, tests with Streptococcus faecalis ATCC 33186 or ATCC 29212 are recommended: zones should be fairly clear and greater than or equal to 22 mm (trimethoprim disks) or greater than or equal to 24 mm (trimethoprim-sulfamethoxazole disks).     

Disk agar diffusion susceptibility testing with 30-micrograms ceftazidime disks: confirmation of interpretive breakpoints and quality control guidelines

Ceftazidime is a wide-spectrum, beta-lactamase-stable cephalosporin with remarkable potency against Pseudomonas spp., Enterobacteriaceae, and some gram-positive species. The reevaluation of the 30-micrograms ceftazidime disk diffusion tests with commercially prepared disks confirms the proposed susceptibility breakpoint zone of greater than or equal to 17 mm (minimal inhibitory concentration correlate, less than or equal to 8.0 micrograms/ml) and the resistance breakpoint zone of less than or equal to 13 mm (minimal inhibitory concentration correlate, greater than or equal to 32 micrograms/ml). Major and minor interpretive errors were only 4.4%, and these errors could be further reduced to 1.1% by not testing gram-positive organisms, particularly enterococci and Staphylococcus spp. On the basis of the results from a multilaboratory quality control study, the following zone diameter quality control guidelines are suggested: Escherichia coli ATCC 25922, 27 to 31 mm; Staphylococcus aureus, ATCC 25923, 16 to 20 mm; Pseudomonas aeruginosa ATCC 27853, 24 to 28 mm.     

Cefotiam susceptibility testing criteria.

Interpretive zone size breakpoints for diffusion tests with 30-micrograms cefotiam disks are diameters of greater than or equal to 18 mm for susceptible and less than or equal to 14 mm for resistant strains. The standard control strains Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 should both give zones 27 to 33 mm in diameter. Tests with 579 clinical isolates yielded an unacceptably high rate of very major discrepancies between disk tests and microdilution tests; such discrepancies were especially common among Enterobacter spp. Additional studies support the concept that standard microdilution tests and standard disk diffusion tests may fail to detect a potential for resistance among some microorganisms.     

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.     

Tentative criteria for confirming the in vitro susceptibilities of Haemophilus influenzae and Neisseria gonorrhoeae to two fluoroquinolones (sparfloxacin and levofloxacin), including quality control parameters.

Sparfloxacin and levofloxacin were evaluated against 150 Haemophilus influenzae isolates and 149 Neisseria gonorrhoeae isolates in order to define susceptibility testing parameters. Sparfloxacin-susceptible H. influenzae strains were defined as those for which the MICs were < or = 0.25 microgram/ml and the zones were > or = 30 mm, and N. gonorrhoeae susceptible strains were those for which the MICs were < or = 0.03 microgram/ml and the zones were > or = 39 mm (5-micrograms disks). Levofloxacin-susceptible strains of H. influenzae included those for which the MICs were < or = 0.12 microgram/ml and the zones were > or = 32 mm and N. gonorrhoeae susceptible strains were those for which the MICs were < or = 0.12 microgram/ml and the zones were > or = 37 mm (5-micrograms disks). Criteria for a resistant category cannot yet be defined for either quinolone. In multilaboratory studies with different lots of Haemophilus Test Medium, replicate tests with the standard control strain of H. influenzae (ATCC 49247) were evaluated. For sparfloxacin disk tests, the proposed zone size limits were 33 to 42 mm and broth microdilution MIC limits were 0.004 to 0.016 microgram/ml, whereas for levofloxacin tests, zone size limits were 32 to 41 mm and broth microdilution MIC limits were 0.008 to 0.03 microgram/ml. Other multilaboratory studies evaluated tests with supplemented GC agar and N. gonorrhoeae ATCC 49226; for both drugs, zone size limits were 44 to 52 mm and agar dilution MIC limits were 0.004 to 0.016 microgram/ml.     

Interpretive criteria and quality control guidelines for Neisseria gonorrhoeae susceptibility test standardization for cefotetan.

Cefotetan was tested in a multilaboratory study to standardize susceptibility testing criteria and quality control guidelines for Neisseria gonorrhoeae. Cefotetan was most active against penicillinase-producing and penicillin-susceptible strains (MIC for 50% of strains tested, 0.5 micrograms/ml) and was least active against the chromosomally resistant isolates (MIC for 50% of strains tested, 2 micrograms/ml). The recommended 30-micrograms disk cefotetan interpretive criteria were as follows: susceptible at greater than or equal to 26 mm (less than or equal to 2 micrograms/ml), intermediate at 20 to 25 mm (4 micrograms/ml), and resistant at less than or equal to 19 mm (greater than or equal to 8 micrograms/ml). Quality control guidelines for agar dilution and disk diffusion tests were established by using numerous GC agar lots, three cefotetan 30-micrograms disk lots, two quality control organisms, and a volume of tests consistent with National Committee for Clinical Laboratory Standards M23-T guidelines.     

Ampicillin disk diffusion susceptibility testing of Haemophilus influenzae.

A total of 114 strains of Haemophilus influenza were characterized with respect to beta-lactamase production and ampicillin MIC. Of this total, 41 strains produced a TEM-type beta-lactamase, and ampicillin MICs for these strains were greater than or equal to 2.0 microgram/ml. It was found that 54 strains lacked TEM-type beta-lactamase activity, and ampicillin MICs for them were less than or equal to 0.5 microgram/ml. The remaining 19 strains were beta-lactamase negative, but ampicillin MICs were greater than or equal to 2.0 micrograms/ml. Disk diffusion susceptibility tests were performed with two media, i.e., Mueller-Hinton agar containing 1.0% hemoglobin and 1.0% IsoVitaleX supplement (CHOC-MHA) and enriched chocolate agar (CHOC), by using disks containing 10 and 2 micrograms of ampicillin. If strains of H. influenzae for which ampicillin MICs were greater than or equal to 2.0 micrograms/ml were considered resistant, while strains for which MICs were less than or equal to 0.5 microgram/ml were considered susceptible, the following zone diameter interpretive criteria were identified as indicating ampicillin susceptibility: CHOC-MHA (10-micrograms disks), greater than or equal to 20 mm; CHOC-MHA (2-micrograms disks), greater than or equal to 17 mm; CHOC (10-micrograms disks), greater than or equal to 25 mm; and CHOC (2-micrograms disks), greater than or equal to 20 mm. In all cases, zones of inhibition less than those listed above would be interpreted as indicating resistance. Each of these four combinations was found to be essentially equivalent in identifying susceptible and resistant strains of H. influenzae, irrespective of beta-lactamase production.     

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.     

Tentative disk diffusion susceptibility interpretive criteria for BMY-28142, a new cephalosporin.

A total of 750 bacterial isolates, including 37 different species, were tested for susceptibility to BMY-28142 by standardized broth microdilution and disk diffusion tests. The zone diameter interpretive criteria tentatively suggested for the 30-micrograms BMY-28142 disk are as follows: susceptible = greater than or equal to 18 mm (MIC less than or equal to 8.0 micrograms/ml), resistant = less than or equal to 14 mm (MIC greater than or equal to 32 micrograms/ml), and indeterminate = 15 to 17 mm.     

Preliminary disk diffusion susceptibility testing criteria for cefdaloxime (RU29246, HR-916 metabolite), a new orally administered cephalosporin.

Cefdaloxime (formerly RU29246; Hoechst-Roussel Pharmaceuticals Inc., Somerville, N.J.) a new active component of the HR-916 ester, was tested by dilution and two disk (10- and 30-micrograms) diffusion susceptibility tests against 391 clinical isolates. Interpretive criteria were proposed for three potential MIC breakpoints of less than or equal to 1, less than or equal to 2, and less than or equal to 4 micrograms/ml. Analyses by regression line and error rate bounding methods minimized false-susceptible (very major) errors and produced a greater than or equal to 90% absolute interpretive agreement between susceptibility test methods. The less than or equal to 2-micrograms/ml breakpoint seemed optimal when 10-micrograms disks and the available human pharmacokinetics were used. The following inhibition zone diameter criteria were proposed: susceptible, greater than or equal to 19 mm; resistant, less than or equal to 15 mm. These recommendations for clinical trials should remain tentative until additional information about cefdaloxime formulations, pharmacokinetics, and patient outcomes can be correlated with in vitro susceptibility test results.     

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.