Antimicrobic susceptibility testing. A personal perspective.

The development of antimicrobic susceptibility testing is outlined with particular reference to standardization of procedures and organizational developments that have led to substantially improved performance. Special problems continue to be posed by increased proportions of opportunistic pathogens and newly recognized mechanisms of resistance, and these require updating. The role of automated procedures and technical problems in MBC and serum bactericidal testing are considered.

Development of resistance to cephalosporins in clinical strains of Citrobacter spp.

The predominant beta-lactam antibiogram of Citrobacter freundii resembles that of Enterobacter cloacae in demonstrating resistance to cephalothin and cefoxitin with susceptibility to the newer cephalosporins. Four representative strains of C. freundii were reversibly induced to high-level beta-lactamase production by cefoxitin, and mutants with stable, high-level production were selected with cefamandole. The mutants were resistant to several second- and third-generation cephalosporins. Comparisons of isoelectric points and substrate profiles of beta-lactamases from wild-type, induced wild-type, and mutant organisms suggested a close relationship to those from E. cloacae and indicated that C. freundii mutants, like those of E. cloacae, were derepressed for production of beta-lactamase. One primary isolate of C. freundii resembled the mutants in all characteristics. In contrast, most strains of Citrobacter diversus were susceptible to all cephalosporins, and two representative strains showed neither inducible nor mutational resistance. Cefoxitin induction to enhanced beta-lactamase production was demonstrated in a cephalothin-resistant isolate, and a derepressed mutant was selected with cefotaxime. The beta-lactamase from this C. diversus strain differed substantially in substrate profile from that of E. cloacae and C. freundii.

Determination of minimum bactericidal concentrations of oxacillin for Staphylococcus aureus: influence and significance of technical factors.

The minimum bactericidal concentration of oxacillin for Staphylococcus aureus was shown to be considerably influenced by technical and definitional factors, particularly by the survival of some organisms on the walls of test tubes and by the growth phase of the inoculum. Attention to technical detail greatly improved reproducibility, and log-phase cultures of all strains showed greater than 99.9% killing in 24 h, at or close to the minimum inhibitory concentration, including eight strains described as tolerant. Some strains showed the paradoxical phenomenon of having more survivors in higher concentrations above the minimum bactericidal concentration. An accepted reference minimum bactericidal concentration procedure is needed for establishing clinical correlates and for a review of endpoint criteria. Routine minimum bactericidal concentration tests on S. aureus should be interpreted with great caution.

Mutational enzymatic resistance of Enterobacter species to beta-lactam antibiotics.

Mutants with enhanced beta-lactam resistance were selected from strains of Enterobacter cloacae and E. aerogenes by using three antibiotics. High-level beta-lactamase-producing mutants had similar degrees of increased resistance, enzyme substrate profiles, and isoelectric (pI) values irrespective of the selective agent. Reverse mutants from a resistant E. cloacae mutant regained the susceptibility pattern originally exhibited by the wild type, or were of enhanced susceptibility, and no longer expressed increased beta-lactamase production. beta-Lactamases of the mutants were similar in pI values to the wild-type enzyme. The increased resistance of the mutants therefore appeared to be accounted for by increased beta-lactamase production.

Clinical laboratory evaluation of the Abbott MS-2 automated antimicrobial susceptibility testing system: report of a collaborative study.

The MS-2 system (Abbott Diagnostics, Division of Abbott Laboratories, Dallas, Tex.) was evaluated for its efficacy in determining the susceptibilities of both clinical and selected challenge (nonfastidious, facultative, and aerobic) isolates. The MS-2 results were compared with standard Kirby-Bauer disk diffusion and microdilution results by using fresh clinical isolates. For gram-positive isolates other than enterococci, overall agreement between MS-2 and reference results was 93 to 98%. With enterococci, MS-2 agreement with disk diffusion was 68% but with microdilution was 86% (agreement between disk diffusion and microdilution was 73%). The main discrepancies with enterococci were with cephalothin, penicillin, gentamicin, and kanamycin. With clinical gram-negative isolates, the overall agreement was 91 to 93%, with most discrepancies occurring with Enterobacter spp. and beta-lactam antibiotics (MS-2 versus disk diffusion, 84%; MS-2 versus microdilution, 84%; disk diffusion versus microdilution, 87%) and with Serratia spp. and colistin (false-susceptible results). The agreement of MS-2 results with established reference antibiograms of a special collection of challenge strains was 91 to 97% for the gram-positive cocci and 86 to 98% for the gram-negative strains. (With Enterobacter spp., agreement was 86%, but was greater than 90% for all other organism groups.) Of the 98 finite MS-2 minimum inhibitory concentrations (MICs) that could be directly compared with microdilution MICs, 77 (79%) were within +/- 1 well of the geometric mean microdilution MIC. MS-2 analysis time ranged from 2.8 to 6.5 h (mean, 4.2 h). On the basis of these results, we conclude that the MS-2 can be expected to yield rapid and accurate results with most nonfastidious, facultative, and aerobic pathogens.

In-vitro activities of cefamandole and cephalothin against 1,881 clinical isolates. A multi-center study.

By use of an agardilution technic, 1,881 clinical isolates were tested against cefamandole and cephalothin. The isolates represented 18 genera, recovered in five geographically separate centers within the United States. The majority of strains were susceptible (MICs less than or equal to 8 micrograms/ml) to both drugs. Cefamandole showed greater activity against most of the bacterial pathogens. Enterococci, Serratia spp., and Acinetobacter spp. were resistant to both drugs. Cephalothin was more active against Staphylococcus aureus, and both cephalosporins were relatively inactive against methicillin-resistant strains of S. aureus. Enterobacter spp. and indole-positive Proteus spp. were susceptible to cefamandole but resistant to cephalothin.

In vitro response of Enterobacter to ampicillin.

Three strains of Enterobacter were studied for their response to ampicillin. They exhibited a basic level of resistance that depended on the medium used and high-level mutational resistance at a frequency of 10(-5) to 10(-7). Two classes of mutants were selected, one of which showed markedly enhanced antibiotic inactivation as indicated by a biological assay and the other of which resembled the wild type in this regard. Both mutants showed cross-resistance to other beta-lactam antibiotics. The results explained discrepancies between traditional broth dilution minimum inhibitory concentration tests and early read automated procedures.

Interpretive criteria for cefamandole and cephalothin disk diffusion susceptibility tests.

A multi-center study of 1,838 clinical isolates established the accuracy of diffusion susceptibility tests with 30-mug cephalothin disks and 30-mug cefamandole disks. The same interpretive zone standards can be applied to tests with either disk but the two drugs cannot be tested interchangeably.

Emergence in a burn center of populations of bacteria resistant to gentamicin, tobramycin, and amikacin: evidence for the need for changes in zone diameter interpretative standards.

From July 1974 through June 1976, a number of isolates of Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa from the Burn Center exhibited a shift to smaller zone diameters with gentamicin than did isolates from the general hospital population. Although many had zone diameters >/=13 mm and would have been considered susceptible by this breakpoint, they were found to have minimal inhibitory concentrations (MICs) of >/=8 mug of gentamicin per ml by agar dilution testing. Zone diameters and MICs of gentamicin, tobramycin, and amikacin were subsequently compared for 168 isolates from both the Burn Center and general hospital. The results revealed many isolates that fell into presently used gentamicin- and tobramycin-"susceptible" categories by disk diffusion tests but were resistant by MIC. The data indicated that criteria for gentamicin disk diffusion testing should include an intermediate or indeterminate category, and that the limits of the intermediate category for tobramycin and amikacin should be expanded.

Electrical impedance measurements in the reading and monitoring of broth dilution susceptibility tests.

Electrical impedance changes in the medium were studied during traditional broth dilution tests. Tests involved clinical isolates of Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Enterobacter sp., Klebsiella, and enterococcus and ten antibiotics. Minimal inhibitory concentration (MIC) values from overnight visual readings were compared with MIC values determined from electrical impedance end points, using inocula of 10(6) organisms per ml. Ninety-three percent of the results were within one twofold dilution of each other. When the impedance end point was determined at 6 h, this correlation was lowered to 34%. By increasing the initial electrical impedance inoculum 10-fold, the correlation between the 6-h impedance MIC and the overnight visual MIC was improved to 74%. Ampicillin, tetracycline, and polymyxin E results accounted for most discrepancies. Continuous monitoring of impedance changes suggested that better correspondence could be obtained by adjusting the end point criteria for the 6-h impedance MIC with different antibiotics. Electrical impedance methods for reading bacterial end points in automated clinical laboratory instruments appear promising.

Reproducibility of control strains for antibiotic susceptibility testing.

Inter- and intralaboratory reproducibility of susceptibility testing requires stable control strains. The Food and Drug Administration diffusion procedure recommends the Seattle strains of Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922) for this purpose. It was of interest to determine the present reproducibility of control cultures maintained in various laboratories over several years. Fifteen cultures each of S. aureus and E. coli were obtained from laboratories in different parts of the country. Their performance was compared with strains directly derived from ATCC. Diffusion susceptibility tests using a modified overlay technique were made with four replicates. Seven of the eight statistically significant differences in responses of the staphylococci were to penicillin, methicillin, or cephalothin. One culture was a penicillinase producer with a zone 15 mm less than the standard strain. Eleven of the 15 cultures showed no significant deviations or differences greater than 2 mm from the results with the strain derived directly from ATCC. All except the penicillinase producer were of identical phage type. Among 150 organism-antibiotic combinations tested with E. coli, all but one reading were within 2 mm of the standard. Four of the six statistically significant differences were in a culture from one laboratory. The stability of the cultures appears to have been influenced by the method of storage. Cultures that were kept frozen during extended storage were remarkably stable. Significant differences were found in cultures from four of five laboratories that maintained cultures in refrigerators or at ambient temperature.

Susceptibility of Enterobacter to cefamandole: evidence for a high mutation rate to resistance.

Cefamandole minimum inhibitory concentrations (MICs) of 10 strains of Enterobacter were determined by the ICS agar dilution and broth dilution procedures. Agar dilution MICs ranged from 1 to 8 mug/ml, with an inoculum of 10(4) organisms/spot. Broth dilution MICs were consistently higher, with an inoculum of approximately 7 x 10(5) organisms/ml. Seven strains showed MICs of >/=64 mug/ml. There was a marked inoculum effect in broth, and skipped tubes were often observed. Variants resistant to 32 mug/ml or more were isolated by direct selection and were shown to occur at a frequency of approximately 10(-6) to 10(-7). A mutant showing a 16-fold increase in agar dilution MIC was also isolated by indirect selection. These variants and others isolated from broth in the presence of cefamandole were tested for ability to inactivate the antibiotic, using both a biological and a chemical procedure. Two distinct classes of variants were seen. Twelve of 28 were shown by both methods to inactivate the antibiotic, whereas the others, including the indirectly selected mutant, did not. The wild types were also negative by both tests. The higher cefamandole MICs of Enterobacter in broth, thus, appeared to reflect a high frequency of resistant variants that were not detected with the inoculum and end point criteria usually used in agar dilution methods. The ability of some variants to inactivate cefamandole may have resulted from a mutation that extended the activity of Enterobacter cephalosporinase to include this antibiotic.

Antimicrobial susceptibility testing.

Considerable advances have been made in antimicrobial susceptibility testing. This review emphasizes the continued efforts toward standardization of methods for dilution and diffusion testing, particularly in the area of variation in medium performance, methods for detection of ampicillin resistant Haemophilus influenzae, and attempts to develop rapid automated systems for susceptibility testing. Susceptibility testing of anaerobes continues to be controversial from the standpoint of both the selection of methods and the application of susceptibility results to the clinical situation. It is expected that these and other problems in susceptibility testing will be solved by continued application of the investigative approaches that have brought us where we are today.

Relationship of early readings of minimal inhibitory concentrations to the results of overnight tests.

Broth dilution minimal inhibitory concentration (MIC) readings were compared after different incubation periods and with different inoculum concentrations. The purpose was to determine the best conditions for obtaining early results as close as possible to overnight readings. Initially, 76 antibiotic-organism combinations were tested using the International Collaborative Study technique and inoculum and were read after 3, 8, and 18 h of incubation. Approximately 28% of tests showed fourfold or greater increases in MICs after 18 h of incubation compared with the 3-h readings. No overnight MICs were lower than early readings. MICs of single antibiotics against seven organisms were also read with an automatic particle counter to confirm the validity of the visual readings. Experiments were made to determine whether inoculum manipulation could reconcile the differences between 3- and 18-h MIC results. One hundred and eight organism-antibiotic combinations were tested comparing 3-h MIC readings using an inoculum of 10(7) organisms per ml with overnight readings using 10(5) per ml. In 71 cases, readings with both inocula were within the range tested and 57 (86%) were within +/-1 log(2) of each other and followed an approximately normal distribution. Improved comparability between early read and overnight MICs thus may be achieved by inoculum manipulation, and this may be a suitable approach in the future development of automated procedures.

Laboratory evaluation of a rapid, automatic susceptibility testing system: report of a collaborative study.

Seven laboratories participated in a collaborative study to evaluate the Autobac 1 system. Results obtained with this assay system were compared to those obtained by the standardized Bauer-Kirby disk diffusion test, and each of these two methods was compared to the agar dilution technique. Comparison of the Autobac 1 and the disk diffusion results from the seven laboratories showed an overall average of 91.5% interpretive agreement with the 17 antimicrobial agents tested. The distribution in the levels of Autobac 1/disk diffusion agreement was such that with 13 antimicrobial drugs agreement was 90% or higher; with three, between 85 and 90%; and with one, 77% (nitrofurantoin). Comparison of the Autobac 1 and disk diffusion tests with the International Collaborative Study agar dilution test showed that both methods gave levels of agreement with the International Collaborative Study agar dilution technique that were generally high and equivalent. The average overall agreement between the agar dilution test and each of the other two methods was approximately 90%. Disagreements that did occur tended to involve organisms that were drug susceptible by the Autobac 1 system but intermediate or resistant by the other two methods. This was in part due to the narrow intermediate interpretive zone of the Autobac 1 test. In reproducibility studies with the Autobac 1 and disk diffusion methods, no significant differences were observed between the interpretive reproducibility of the two methods.

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-mug discs of oxacillin and nafcillin or the 5-mug discs of methicillin may be used for this purpose but not the 1-mug 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.