Interpretation of Diffusion Susceptibility Data Obtained with 50-μg Carbenicillin Discs Against Gram-Negative Organisms

A total of 284 clinical isolates of various species of Enterobacteriaceae, Pseudomonas aeruginosa, P. maltophilia, and Acinetobacter anitratum were tested for susceptibility to carbenicillin by the standardized Bauer-Kirby disc diffusion technique and a microtiter broth dilution method. The data obtained led to the following proposed criteria for the interpretation of the results of disc susceptibility tests. Enterobacteriaceae that yield zones of inhibition equal to or greater than 20 mm in diameter around 50-mug discs of carbenicillin are designated as sensitive to the drug; isolates that yield zones measuring from 18 to 19 mm in diameter are reported as of equivocal (intermediate) susceptibility to the drug, whereas those enterobacterial isolates that are characterized by zones of inhibition of 17 mm or less in diameter are interpreted as resistant to carbenicillin. Isolates of P. aeruginosa, P. maltophilia, and A. anitratum yielding zones of 14 mm or more in diameter around 50-mug discs of carbenicillin are reported as sensitive, whereas those isolates that are characterized by zones of 13 mm or less in diameter are reported as resistant to this drug.     

Susceptibility of Pseudomonas aeruginosa to Gentamicin Sulfate In Vitro: Lack of Correlation Between Disc Diffusion and Broth Dilution Sensitivity Data

Seventy-eight of 420 clinical isolates of Pseudomonas aeruginosa yielded zones of inhibition of less than 12 mm in diameter around 10-mug discs of gentamicin sulfate when tested by the standardized Bauer-Kirby disc diffusion method. Of 153 strains chosen from these isolates, one strain (0.65%) required 25 mug of gentamicin per ml for inhibition; the remainder (99.35%) were inhibited by 6 mug/ml or less of the antibiotic. It is recommended that those isolates of P. aeruginosa that yield zones of inhibition less than 12 mm in diameter be disc susceptibility-tested once more; those isolates that give zones of inhibition of less than 12 mm upon repeated examination should then be subjected to the broth dilution test before they are designated as sensitive or resistant to gentamicin.     

In Vitro Activity of Doxycycline Against Bacteria from Clinical Material

Doxycycline (alpha-6-deoxy-oxytetracycline) was tested against various bacteria of recent clinical origin with 30-mug discs. The antibiotic susceptibility of these bacteria to commonly used antimicrobial agents was also established. Those bacteria which responded with equivocal zones of inhibition about the tetracycline compounds were tested by the tube dilution technique. Staphylococci and enterococci consistently displayed greater in vitro susceptibility to doxycycline than to tetracycline or demethylchlortetracycline.     

Medium-Dependent Activity of Gentamicin Sulfate Against Enterococci

Routine disc diffusion susceptibility tests (Bauer-Kirby technique), employing 5% sheep blood-Mueller-Hinton agar and 10-mug gentamicin sulfate discs, disclosed that a significant number of clinical enterococcal isolates were sensitive to the antibiotic, as also revealed by the agar dilution technique. With few exceptions, the isolates proved resistant to this antibiotic when tested for susceptibility in Brain Heart Infusion and Trypticase soy broth or agar. The addition of 5% sheep blood to Trypticase soy and Brain Heart Infusion agars resulted in markedly enhanced activity of the antibiotic, indicating medium-dependent activity of gentamicin against enterococci. Human serum and urine failed to support optimal growth of enterococci. Thus, it was not possible to correlate the activity of gentamicin in any of the media examined with that in serum or urine.     

药物敏感试验药片法研究

药敏药片对国际三株标准菌试验,抑菌圈直径范围均符合判定标准。与进口药敏药片符合率为１００％。与药敏纸片片间差比较试验,药片抑菌圈＝１～２ｍｍ,精密度高,符合国际要求,而纸片抑菌圈＝９～１３ｍｍ,均匀度检测明显不合格。药片变异系数ＣＶ＝１．３１～２．９２,纸片ＣＶ＝１４．７８～２１．６７表明药片变量离散度小,片间差很小;而纸片变量离散度明显大,故纸片片间差很大。     

Evaluation of the In Vitro Activity of Tobramycin as Compared with That of Gentamicin Sulfate

The in vitro activity of tobramycin was quantitatively compared with that of gentamicin sulfate against 195 bacterial isolates from clinical material. Tobramycin was found to be twice as active as gentamicin against isolates of Pseudomonas aeruginosa. Conversely, gentamicin proved fourfold more active than tobramycin against isolates of Serratia marcescens. Both drugs were of comparable activity against isolates of Staphylococcus aureus and the majority of the enterobacterial isolates other than S. marcescens. On the basis of the obtained data, the following criteria are proposed for the interpretation of diffusion susceptibility tests with 10-μg discs of gentamicin and tobramycin. Enterobacteriaceae and isolates of S. aureus are designated as susceptible to gentamicin and tobramycin if the zones of inhibition measure 15 mm or more in diameter; zones of 14 mm or less are indicative of resistance. Pseudomonadaceae are interpreted as sensitive to tobramycin and gentamicin if the inhibition zones measure at least 15 and 12 mm in diameter, respectively.     

Standardized Antimicrobial Disc Susceptibility Testing of Anaerobic Bacteria. I. Susceptibility of Bacteroides fragilis to Tetracycline

A modified Bauer-Kirby-Sherris-Turck method for disc susceptibility testing of anaerobic bacteria is presented. When tetracycline was used against 100 strains of Bacteroides fragilis as a model, reasonably reproducible results were obtained after overnight incubation in both the GasPak atmosphere and an atmosphere achieved by adding 10% CO(2) to a mixture of 10% H(2) and 90% N(2). The minimal inhibitory concentration for the strains determined by the agar dilution technique correlated well with the results of disc tests performed in the GasPak atmosphere with 30-mug tetracycline discs. Among 63 strains isolated from 1970 to the present, only 24 (38.1%) were found to be susceptible to tetracycline.     

Rapid Sulfonamide Disc Sensitivity Test for Meningococci

Minimal inhibitory concentrations (MIC) of 90 strains of Neisseria meningitidis were determined by a plate dilution technique that employed twofold changes in concentrations of sulfadiazine. The geometric mean of three MIC determinations on each strain was correlated with inhibition zones produced by a 300-mug sulfathiazole disc. The linear relationship between the logarithm of the geometric mean MIC values and the zone diameters was highly significant. Strains were separated into sensitive and resistant populations by both test procedures. Quantitative criteria for interpreting the sensitivity of a strain by the disc test were established.     

Development of a method for the quantitative assessment of the microorganism sensitivity to antibiotics by using discs. A study of the patterns of doxycycline diffusion from discs into the nutrient agar]

Doxycycline concentrations provided by the antibiotic diffusion from paper discs into sterile agarized medium and the medium plated with staphylococci and Coli bacteria were studied at different distances from the disc center. Infection of the nutrient medium with the test cultures had no effect on the antibiotic diffusion rate. A linear relation between the logarithm of the antibiotic concentration in the agar and distance from the disc centre were found. Probably it is possible to determine the MIC of the antibiotic with respect to various microorganisms by the value of the radius of the growth inhibition zone around the disc using diagrams expressing such a relation.     

Gonococcal Antibiotic Resistance in Los Angeles

One hundred and thirty-five gonococcal isolates collected from Los Angeles in 1972 were studied for antibiotic susceptibility to penicillin, ampicillin, carbenicillin, tetracycline, minocycline, doxycycline and spectinomycin. Only 12 percent of the isolates were sensitive to 0.05 μg per ml of penicillin while 35 percent required at least 0.5 μg per ml for inhibition of growth. The results were slightly better with ampicillin and nearly the same with carbenicillin. Nineteen percent of the isolates required at least 1.0 μg per ml of tetracycline for inhibition of growth and the results were similar with either minocycline or doxycycline. Forty-nine percent were sensitive to 2.0 μg per ml spectinomycin, but 37 percent required at least 8.0 μg per ml for inhibition of growth.In this study nine of eleven isolates resistant to 1.0 μg per ml of tetracycline were also resistant to both penicillin and spectinomycin. Six came from endocervical sites of female patients who contributed only 37 percent of the total number of isolates studied.Correlation between the agar dilution and disc diffusion methods was satisfactory with penicillin but not with ether tetracycline or spectinomycin.     

Mathematical model for the control and standardization of discs]

Analysis of tetracycline and erythromycin diagnostic discs manufactured in the USSR showed their conformity with the requirements of the USA Federal Register. Comparison of the antibiotic amounts extracting and diffusing from the discs showed that as an average 92 and 90 per cent of the extracted amounts of erythromycin and tetracycline respectively diffused into the agar. Subsequently, it is desirable that on control testing of the disc quality both the similarity of the antibiotic content in the discs and the real amount of the antibiotic diffusing into the agar should be considered. A statistically reliable correlation between the values of the growth inhibition zones around the discs with definite and constant amounts of erythromycin, tetracycline or oxacillin and different resistance levels for every staphylococcal strain was found. On the basis of such a control system it is possible to divide the staphyloccal strains into the groups with high, low and intermediate resistance levels to the above antibiotics. However, it is not possible to use such data for accurate calculation of the value of the minimum inhibitory concentration of unknown strains because of a high value of the main error.     

Antibiotic Susceptibility Patterns of Recent Isolates of Corynebacterium diphtheriae

A variety of factors which might affect zone sizes were studied with strains of Corynebacterium diphtheriae; a standard disc method for antimicrobial sensitivity testing was used. Moderate variations in inoculum size, inoculum preparation, and pH of Mueller Hinton agar (MHA) did not appreciably affect zone sizes. The addition of blood to MHA was necessary to insure the growth of all C. diphtheriae strains on all lots of MHA. Zone diameters on MHA with blood were consistently 4 to 9 mm smaller than on plain MHA; however, zone diameters were within the sensitive range for seven antibiotic discs used on both media. Minimal inhibitory concentration (MIC) values for penicillin, erythromycin, and rifampin were determined by a plate dilution method. The geographical source, toxigenicity, and type of the strains showed no significant correlation with MIC values or zone diameters for eight antibiotic discs. When MIC values were compared to obtainable blood levels, all of the strains appeared to be sensitive with MIC values of 相似文献   

Comparison of Different Agar Diffusion Methods for the Detection of Antimicrobial Residues in Slaughter Animals

The different agar diffusion methods were compared using antibiotic and sulphonamide-impregnated filter-paper discs and the kidneys of healthy and emergency-slaughtered pigs and cows after slaughter. No method used seemed to be sensitive to all antimicrobial drugs preimpregnated onto discs. Tetracycline yielded a greater zone of inhibition at pH 6 than at pH 8 and aminoglycosides, erythromycin, polymyxin B and lin cornycin at pH 8 than at pH 6. It therefore seems necessary to use different pHs (6 and 8). The addition of trimethoprim to the medium is necessary for the detection of sulphonamides. Bacillus subtilis BGA used as the test organism was more sensitive to sulphonamides on the “Test agar for the inhibitor test” containing trimethoprim than on the “Iso-Sensitest agar” also containing trimethoprim. The addition of trimethoprim to “Test agar for the inhibitor test” is recommended at pH 8 but not at pH 6 because false-positive cases (with inhibition zones > 2 mm) were observed at pH 6 with trimethoprim on the kidneys of healthy pigs.     

Reliability of the Kirby-Bauer Disc Diffusion Method for Detecting Methicillin-Resistant Strains of Staphylococcus aureus

The resistance of Staphylococcus aureus to methicillin and related drugs can be reliably determined by using the Kirby-Bauer method of susceptibility testing if the incubation temperature is 35 C or below, but resistance may be missed at 37 C. The 1-μg discs of oxacillin and nafcillin or the 5-μg discs of methicillin may be used for this purpose but not the 1-μg discs of cloxacillin. The latter fail to discriminate between sensitive and resistant staphylococci by zone measurement; some resistant strains of staphylococci may show larger zones of inhibition than sensitive strains. Stability of these antibiotic-containing discs was studied under conditions of temperature and humidity variation that might be encountered in a clinical laboratory refrigerator. Oxacillin discs were the most stable and are to be preferred for susceptibility testing. Nafcillin discs were less stable, and methicillin discs lose their potency rapidly unless carefully stored in a refrigerator with a desiccant.     

Assessment of the susceptibility of the gram-negative rods isolated from hospitalized patients to cefoperazone/sulbactam

The susceptibility to cefoperazone/sulbactam of 197 strains of Gram-negative rods demonstrating an ESBL-positive phenotype was determined. The assortment of the investigated strains was as follows (numbers of strains are given in the brackets): E. cloacae (63), S. marcescens (46), K. pneumoniae (21), P. mirabilis (17), E. coli (9), P. vulgaris (8), P. aeruginosa (20) and A. baumanni (13). 83 strains from 197 were susceptible (42.1%). The MIC values were determined and the disc-diffusion method was performed. The susceptibilities among particular species were as follows (the order of data in the brackets is: % of the susceptible strains/MIC50/MIC90): E. cloacae (54.0/16/64), S. marcescens (23.9/64/> or = 128), K. pneumoniae (38.1/32/64), P. mirabilis (41.2/32/64), E. coli (44.4/32/32), P. vulgaris (75.0/8/32), P. aeruginosa (35.0/32/64), A. baumannii (46.2/32/64). Using disc-diffusion method, for 184 strains the difference between diameter of the inhibition zone around the disc with cefoperazone and the disc with cefoperazone/sulbactam was calculated. This difference amounted 5 mm or more in the case of 76.6% of the investigated strains. The results indicate that the comparison of the inhibition zones around cefoperazone and cefoperazone/sulbactam discs may be an additional method useful for phenotypic detection of ESBL producing organisms. These results highly correlated with results obtained by using analogous test with cefpirome and cefpirome/clavulanic acid (85.6% of concordance).     

Influence of the culture medium on antibiotic susceptibility testing of food-associated lactic acid bacteria with the agar overlay disc diffusion method

AIMS: To investigate the influence of the culture medium on antibiotic susceptibility testing of food-associated lactic acid bacteria (LAB) with the agar overlay disc diffusion (DD) method. METHOD: The antibiotic resistance profile of 39 food-associated lactobacilli and enterococci was determined with the agar overlay DD method using a defined medium (i.e. Iso-sensitest agar; ISA) or an undefined medium (i.e. de Man, Rogosa, Sharpe or MRS agar). RESULTS: The study revealed that ampicillin discs and, although to a lesser extent, also tetracycline discs consistently produced larger zones on MRS medium compared to ISA medium. For the antibiotics gentamicin, bacitracin and erythromycin, the radius of the inhibition zones produced on MRS medium was significantly smaller in relation to ISA. For categorizing LAB isolates into resistant, intermediate and susceptible groups, it was demonstrated that major errors can occur in determining bacitracin and gentamicin resistance if MRS medium instead of ISA medium is used. On the other hand, the performance of both media was found to be equivalent for testing tetracycline resistance. CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY: Despite the fact that MRS medium generally supports the growth of lactic acid bacteria much better than the nutrient-poor ISA medium, the present study clearly demonstrates that both media are not compatible in susceptibility testing against various classes of antibiotics. These results may stimulate future discussions on a generally recommended DD method for susceptibility testing of food LAB strains.     

Effect of Plate Size and Location of Disc on Zone Diameter in the Disc Antimicrobial Susceptibility Test

An established standard antimicrobial disc susceptibility test recommends the use of a 150-mm petri dish. Many workers substitute 100-mm plates and use various types of mechanical dispensers. A comparison of zone diameters was made by testing Staphylococcus aureus and Escherichia coli against penicillin and chloramphenicol with the use of these different sized plates and dispensers. Zone diameters were consistently smaller at the edges of plates than in the center (0.4 to 1.2 mm). Significant small differences of up to 1.2 mm or no differences were observed when the mean diameters from these various plates were compared. All of the zone diameter measurements fell within accepted values for analytical variability in this procedure. There is no evidence that any individual combination of plates, dispensers, or disc locations provides greater precision in zone diameter measurement. More difficulty was encountered in measuring zones at the periphery of small plates. This suggests that busy clinical laboratory workers might not produce work of comparable quality with the smaller plates.     

Effect of Antibiotics on Staphylococcal Hemolysin Production

Thirty-seven per cent of 126 strains of Staphylococcus aureus, when tested on sheep blood-agar with sensitivity discs containing cephalothin, carbenicillin, oxacillin, penicillin, and cycloserine, produced rings of beta-hemolysis surrounding zones of inhibition of bacterial growth. Each strain capable of producing a ring of beta-hemolysis did so with at least two of the mentioned antibiotics. None of the other 15 antibiotics tested was associated with a ring of beta-hemolysis surrounding any of the zones of inhibition. It appears, therefore, that the beta-hemolysis observed was produced by certain strains of S. aureus only in association with certain antibiotics.     

The use of batumin-containing disks for the rapid identification of staphylococci

The study of 467 microbial strains obtained from collections and from clinical sources revealed that microorganisms of the genus Staphylococcus were highly sensitive to batumin, a new antibiotic obtained from bacteria of the genus Pseudomonas. 378 strains of 15 Staphylococcus species proved to be highly sensitive to the diagnostic preparation "Diastaph", developed on the basis of batumin (antibiotic-impregnated discs); After 18-hour incubation the diameter of the growth inhibition zones on agar-containing culture media was 18-38 mm. Strains belonging to the genera Micrococcus, Dermacoccus, Kocuria and Kytococcus, as well as the tested representatives of other taxa (Planococcus, Streptococcus, Corynebacterium, Acinetobacter, Pseudomonas, Neisseria, the representatives of all tested genera of the family Enterobacteriaceae, fungi of the genus Candida) were insensitive to the diagnosticum. "Diastaph" permits not only the rapid identification of staphylococci pure cultures, but also the determination of their presence in association with other microbial species directly in pathological material, which makes it possible to recommend this diagnostic preparation for use in medical, veterinary and sanitary microbiology.     

Experimental criteria for interpretation of bacterial sensitivity to dioxidine determined by diffusion from the disks

Antibacterial activity of dioxidine against aerobic and facultative anaerobic organisms under conditions of anaerobiosis i. e. conditions really observed for example in abscess cavities or necrotic tissues is 30 to 100 times as high as that under aerobic conditions. There is a relationship between sensitivity of bacteria to dioxidine under aerobic and anaerobic conditions which is expressed by the regression equation. Therefore, comparison of the MICs determined under anaerobic conditions with the growth inhibition zones formed by disks with the drug under aerobic conditions is possible. The MIC of dioxidine was determined under anaerobic conditions for 179 clinical strains of aerobic and facultative anaerobic bacteria and the growth inhibition zones of the same bacteria under aerobic conditions were evaluated with the use of disks containing 100, 75, 50, 25, 20, and 15 micrograms of the drug. The border line. MIC differentiating between resistant and sensitive strains was chosen to be equal to 4 micrograms/ml. Differentiation of the strains into sensitive and resistant ones by the values of the growth inhibition zones was performed with the method of error minimization described by C. Metzler and R. De Haan in 1974. Disks containing 25 micrograms of the drug allowed one to differentiate the strains under aerobic conditions into sensitive and resistant ones: the growth inhibition zones greater than 11 mm corresponded to the sensitive strains (the MIC smaller than 4 micrograms/ml) and the growth inhibition zones smaller than 11 mm corresponded to the resistant strains (the MIC greater than 4 micrograms/ml).