Quality control limits for teicoplanin susceptibility tests and confirmation of disk diffusion interpretive criteria.

For monitoring the performance of teicoplanin susceptibility tests, the following quality control limits are recommended: Staphylococcus aureus ATCC 29213, MIC of 0.12 to 0.5 micrograms/ml; Enterococcus faecalis ATCC 29212, MIC of 0.06 to 0.25 micrograms/ml; and S. aureus ATCC 25923, zones 15 to 19 mm in diameter (30-micrograms disks). However, some lots of Mueller-Hinton agar provided unusually large zones of inhibition with both vancomycin and teicoplanin disks, and these lots should be excluded before routine use. Teicoplanin and vancomycin differed only in their activity against oxacillin-resistant strains of Staphylococcus haemolyticus, which had decreased susceptibility to teicoplanin but were fully susceptible to vancomycin. For tests with 30-micrograms teicoplanin disks, zones less than or equal to 10 and greater than or equal to 14 mm in diameter represent resistant and susceptible breakpoints, respectively.     

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.     

Molecular Epidemiology of Oxacillin-resistant Staphylococcus aureus and in vitro Activity of Glycopeptides against Staphylococcus species

Oxacillin resistant Staphylococcus aureus and coagulase negative Staphylococcus species (MRSA and MRSCoN)have become major pathogens in nosocomial infections.The first MRSA isolate in the world was identified in En gland in 1961 [1]. Since that tim…     

Emergence of teicoplanin-resistant coagulase-negative staphylococci.

Over a period of 5 years we have recovered 32 clinical isolates of coagulase-negative staphylococci (CoNS) exhibiting either decreased levels of susceptibility or true resistance to teicoplanin (MICs, 16 to 128 micrograms/ml); these isolates make up 0.55% of the total CoNS isolated by us. Twenty-nine of the strains were also methicillin resistant, and all were susceptible to vancomycin. Fourteen of the strains were Staphylococcus epidermidis, fourteen were Staphylococcus haemolyticus, and four were Staphylococcus hominis. In one case, a strain of S. haemolyticus was isolated with a vancomycin-resistant, teicoplanin-resistant Enterococcus faecalis strain. All strains were nosocomially acquired and were isolated from 17 different wards. Teicoplanin resistance occurred as a sporadic phenomenon, and none of the isolates were epidemiologically related. The isolates were from 30 patients, 13 of whom presented with true infections (43%). Five (38%) of the 13 patients with true infections had been previously treated with vancomycin. None of the infected patients were previously treated with teicoplanin. The in vivo development of resistance to teicoplanin among CoNS strains limits the therapy of infections by these microorganisms. There is a need for surveillance of nosocomial isolates of CoNS to determine resistance to glycopeptides.     

Resistance to vancomycin and teicoplanin: an emerging clinical problem.

Vancomycin and teicoplanin are glycopeptides active against a wide range of gram-positive bacteria. For 30 years following the discovery of vancomycin in 1956, vancomycin resistance was not detected among normally susceptible bacteria recovered from human specimens. Since 1986, however, bacteria resistant to vancomycin or teicoplanin or both have been described. Strains of the genera Leuconostoc, Lactobacillus, Pediococcus, and Erysipelothrix seem inherently resistant to glycopeptides. Species and strains of enterococci and coagulase-negative staphylococci appear to have acquired or developed resistance. There are at least two categories of glycopeptide resistance among enterococci, characterized by either high-level resistance to vancomycin (MIC, greater than or equal to 64 mg/liter) and teicoplanin (MIC, greater than or equal to 8 mg/liter) or lower-level vancomycin resistance (MIC, 32 to 64 mg/liter) and teicoplanin susceptibility (MIC, less than or equal to 1 mg/liter). The two categories appear to have similar resistance mechanisms, although genetic and biochemical studies indicate that they have arisen independently. Among coagulase-negative staphylococci, strains for which vancomycin MICs are up to 20 mg/liter or teicoplanin MICs are 16 to 32 mg/liter have been reported, but cross-resistance between these glycopeptides varies. The selective advantage accorded to glycopeptide-resistant bacteria and the observation that high-level resistance in enterococci is transferable suggest that such resistance may be expected to increase in incidence. Clinicians and microbiologists need to be aware of this emerging problem.     

Problems with the disk diffusion test for detection of vancomycin resistance in enterococci.

A total of 53 strains of enterococci, including recently isolated strains with high-level resistance to vancomycin, were tested for vancomycin susceptibility by broth microdilution and disk diffusion using Mueller-Hinton media with and without supplementation with 5% blood. By using currently published parameters of the National Committee for Clinical Laboratory Standards for the disk diffusion test, we found that strains for which MICs were 8 to 32 micrograms/ml were incorrectly placed in the susceptible or intermediate category, which caused both very major (1.9%) and minor (11.5%) errors. When we used newer, recently proposed breakpoints for vancomycin, we found 13.5% minor errors but no very major errors. Changing disk diffusion breakpoints to less than or equal to 14 mm for resistant [corrected] and greater than or equal to 15 mm for susceptible [corrected] would eliminate the problem for the strains with MICs of 32 micrograms/ml but not for those with MICs of 8 micrograms/ml. For those strains, it is necessary to perform an MIC test to differentiate them from strains with MICs of less than or equal to 4 micrograms/ml.     

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.     

Vancomycin resistance in Staphylococcus haemolyticus causing colonization and bloodstream infection.

The increase in the incidence of infections due to beta-lactam-resistant coagulase-negative staphylococci has resulted in expanded use of vancomycin for such infections. Despite this, coagulase-negative staphylococci have remained susceptible to vancomycin in recent years. This report describes a strain of Staphylococcus haemolyticus with increased resistance to vancomycin (MIC, 8.0 to 16 micrograms/ml). S. haemolyticus was initially isolated from a patient with acute leukemia and neutropenia in surveillance throat and stool cultures. The microdilution vancomycin MICs for these isolates were 1.0 to 2.0 micrograms/ml. Subsequent S. haemolyticus isolates from the bloodstream and tracheal aspirate occurred in the setting of prolonged empirical vancomycin therapy. MICs for these isolates were 8.0 to 16 micrograms/ml. Further vancomycin resistance (MIC, 32 micrograms/ml) could be selected for in vitro in all four isolates. Restriction endonuclease analysis of plasmid DNA indicated that the isolates were very closely related and likely to be of the same strain. We conclude that colonization with a vancomycin-susceptible strain of S. haemolyticus was subsequently linked to a nosocomial bloodstream infection with an apparently identical strain with intermediate levels of vancomycin resistance. Prolonged empirical vancomycin therapy was temporally associated with this episode.     

Clinical and microbiologic characteristics of pediococci.

Over a 43-month period, 23 separate isolates of nonenterococcal alpha- and nonhemolytic streptococci were reported by our clinical microbiology laboratory to be resistant to vancomycin. This constituted 0.32% of nonenterococcal alpha- and nonhemolytic streptococci reported and 4.4% of such streptococci upon which susceptibility testing was performed. Of 13 isolates which were available for further study, all were highly resistant to vancomycin (MIC greater than or equal to 1,024 micrograms/ml), but none were actually streptococci. Three were clearly gram-positive rods by Gram stain and were found to be homofermentative lactobacilli. Two strains with elongated gram-positive cocci from colonies on agar showed small gram-positive rods when grown in thioglycolate broth and were physiologically identified as Lactobacillus confusus. Two isolates with lenticular gram-positive cocci appeared to be Leuconostoc mesenteroides subsp. mesenteroides. Six gram-positive isolates with round cells from growth on agar and from broth were arranged in tetrads in broth and closely resembled Pediococcus acidilactici. Twelve additional strains of pediococci that were not of human origin were also found to be highly resistant to vancomycin. These findings confirm published reports of clinical isolation of organisms resembling pediococci and suggest that clinically isolated, vancomycin-resistant bacteria which superficially resemble streptococci are probably other lactic acid bacteria.     

Comparative in vitro activity of glycopeptides against coagulase negative staphylococci isolated in pediatric hospital units]

The minimum inhibitory concentrations (MICs) of vancomycin and teicoplanin were determined for 32 coagulase-negative staphylococci strains recovered from blood specimens from pediatric intensive care patients. All the strains were susceptible to vancomycin (MIC less than 4 mg/l). Sixteen strains were susceptible to teicoplanin (MIC less than 4 mg/l) and the sixteen remaining strains exhibited intermediate susceptibility, with MICs of 8 mg/l (10 strains) or 16 mg/l (6 strains). Inhibition zone diameters seen with vancomycin during agar diffusion susceptibility testing were consistently greater than 17 mm. Inhibition zones obtained with teicoplanin were 17 mm or more in diameter for 30 strains and under 17 mm in diameter for 2 strains. For 14 strains (44%), agar diffusion testing failed to detect the decreased susceptibility to teicoplanin revealed by MIC determinations. The agar diffusion method does not seem reliable for the determination of in vitro susceptibility to teicoplanin.     

Antimicrobial activity of fusidic acid and disk diffusion susceptibility testing criteria for gram-positive cocci.

The in vitro activity of fusidic acid was assessed and was compared with those of cloxacillin, cefamandole, vancomycin, teicoplanin, ofloxacin, ciprofloxacin, pefloxacin, and fleroxacin against 500 gram-positive cocci: 151 Staphylococcus aureus, 197 coagulase-negative staphylococci, and 152 Enterococcus faecalis strains. All clinical isolates were concomitantly tested by disk diffusion and agar dilution procedures as outlined by the National Committee for Clinical Laboratory Standards. The results with fusidic acid were further analyzed by regression line and error rate-bounded methods. With control American Type Culture Collection organisms, the values were within the limits of the National Committee for Clinical Laboratory Standards or published limits. The incidence of resistance to fusidic acid was 0.7% for S. aureus, 2.5% for coagulase-negative staphylococci, and 99.3% for E. faecalis. The correlation coefficient between the results of disk diffusion and agar dilution tests with fusidic acid was 0.90. Current interpretive criteria for susceptibility to fusidic acid (i.e., MIC of < 2 micrograms/ml and inhibitory zone of 20 mm) gave 1% false susceptibility (all strains being E. faecalis). This error rate is practically eliminated if a zone diameter of 21 mm is considered the breakpoint for susceptibility.     

Susceptibility testing of carumonam: interpretive criteria for 30-microgram disk tests and quality control guidelines for disk diffusion and broth microdilution methods.

Carumonam 30-microgram disk diffusion tests with 342 gram-negative organisms suggested modifying earlier interpretive zone criteria, i.e., a susceptibility zone diameter of greater than or equal to 23 mm (less than or equal to 8.0 micrograms/ml MIC correlate) and a resistance zone diameter of less than or equal to 17 mm (greater than or equal to 32 micrograms/ml MIC correlate). Quality assurance guidelines were determined by multilaboratory investigations. Recommended limits were calculated for the gram-negative quality control organisms only. For Escherichia coli ATCC 25922, the recommended limits are 30 to 36 mm and 0.03 to 0.25 micrograms/ml, and for Pseudomonas aeruginosa ATCC 27853, they are 24 to 32 mm and 1.0 to 4.0 micrograms/ml.     

Standardization of disk diffusion and agar dilution susceptibility tests for Neisseria gonorrhoeae: interpretive criteria and quality control guidelines for ceftriaxone, penicillin, spectinomycin, and tetracycline.

A six-laboratory study developed a standardized method for determining the susceptibilities of Neisseria gonorrhoeae strains to penicillin, tetracycline, spectinomycin, and ceftriaxone. Three quality control organisms were also selected, and quality assurance guidelines were initially generated for the disk diffusion and agar dilution methods. The medium recommended for gonococcal susceptibility testing was GC agar with a defined "XV-like" supplement. The supplement should be free of cysteine, a component implicated in the inactivation of some newer beta-lactam compounds. Penicillin, tetracycline, spectinomycin, and ceftriaxone were stable in agar plates stored at 3 to 5 degrees C for at least 2 weeks. Numerous GC agar and drug disk lots were used during the trials without significant variation in test results. Several other gonococcal strains were recommended for additional medium quality assurance. The disk quality control zone limits were established for N. gonorrhoeae ATCC 49226 (formerly CDC F-18) and Staphylococcus aureus ATCC 25923. MIC quality control ranges were also developed for N. gonorrhoeae ATCC 49226 and S. aureus ATCC 29213. The interpretive criteria for penicillin were as follows: susceptibility, greater than or equal to 47 mm (diameter of inhibition zone) (less than or equal to 0.06 micrograms/ml [MIC]); resistance, less than or equal to 26 mm (greater than or equal to 2 micrograms/ml). For tetracycline they were as follows: susceptibility, greater than or equal to 38 mm (less than or equal to 0.25 microgram/ml); resistance, less than or equal to 30 mm (greater than or equal to 2 micrograms/ml). For spectinomycin they were as follows: susceptibility, >/= 18 mm (/= 128 micrograms/ml). For ceftriaxone susceptibility, the criterion was >/= 35 mm (相似文献   

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%.     

First report of methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin in Thailand

To investigate whether there are methicillin-resistant Staphylococcus aureus (MRSA) strains with reduced susceptibility to vancomycin in Thailand, a total of 155 MRSA strains isolated from patients hospitalized between 1988 and 1999 in university hospitals in Thailand were tested for glycopeptide susceptibility. All the strains were classified as susceptible to vancomycin and teicoplanin when judged by NCCLS criteria for glycopeptide susceptibility using the agar dilution MIC determination. Vancomycin MICs at which 50 and 90% of the isolates tested were inhibited (MIC50 and MIC(90), respectively) were 0.5 and 1 microg/ml, respectively, with a range of 0.25 to 2 microg/ml. For teicoplanin, MIC50 and MIC90 were 2 microg/ml, with a range of 0.5 to 4 microg/ml. However, one-point population analysis identified three MRSA strains, MR135, MR187, and MR209, which contained subpopulations of cells that could grow in 4 microg of vancomycin per ml. The proportions of the subpopulations were 2 x 10(-4), 1.5 x 10(-6), and 4 x 10(-7), respectively. The subsequent performance of a complete population analysis and testing for the emergence of mutants with reduced susceptibility to vancomycin (MIC > or = 8 microg/ml) confirmed that these strains were heterogeneously resistant to vancomycin. Two of these strains caused infection that was refractory to vancomycin therapy. Pulsed-field gel electrophoresis showed that the two strains had identical SmaI macrorestriction patterns and that they were one of the common types of MRSA isolated in the hospital. This is the first report of heterogeneous resistance to vancomycin in Thailand and an early warning for the possible emergence of vancomycin resistance in S. aureus in Southeast Asia.     

Disk susceptibility of ofloxacin, a new carboxyquinolone.

Ofloxacin, a fluorinated carboxyquinolone, was tested against 485 clinical isolates, and the MICs and disk inhibitory zones were correlated. A critical zone of greater than or equal to 19 mm and an MIC of less than or equal to 2 micrograms/ml indicate susceptibility. An MIC of 4 micrograms/ml and a zone size of 16 to 18 mm is intermediate, and an MIC of greater than or equal to 8 micrograms/ml with a zone size of less than 15 mm indicates resistance. Alternatively, organisms inhibited by an MIC of less than or equal to 4 micrograms/ml with a critical zone diameter of greater than or equal to 15 mm could be considered susceptible. By either of these criteria, major errors in judging susceptibility or resistance are less than 1%.     

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.     

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.     

Interpretive criteria for disk diffusion tests using 5-microgram cefdinir disks with rapidly growing clinical isolates.

Preliminary interpretive zone diameter criteria were calculated for the 5-micrograms cefdinir disk diffusion test by using two potential MIC breakpoints (less than or equal to 0.5 and less than or equal to 1 micrograms/ml). The absolute agreement between tests ranged from 85.9 to 92.4%, and the false-susceptibility errors were principally contributed by the Enterobacter spp. (2.2% error). One proposed criterion was greater than or equal to 20-mm zone diameter (less than or equal to 1 micrograms/ml) for susceptibility and less than or equal to 16-mm zone diameter (greater than 2 micrograms/ml) for resistance to cefdinir. Clinical laboratory users of the disk diffusion method should be cautioned about the possibility of very major interpretive errors among enterobacter isolates.     

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).