Comparison of microscan MICroSTREP, PASCO, and sensititre MIC panels for determining antimicrobial susceptibilities of Streptococcus pneumoniae

Comparison of MicroScan MICroSTREP, PASCO, and Sensititre MIC Panels for Determining Antimicrobial Susceptibilities of Streptococcus pneumoniae Linda L. Guthrie, Shawn Banks, Wendy Setiawan, and Ken B. Waites

The MicroScan MICroSTREP MIC panel was compared with PASCO and Sensititre systems against 157 isolates of Streptococcus pneumoniae chosen to include penicillinsusceptible, intermediate, and resistant strains. Arbitration testing was performed by microbroth dilution using National Committee for Clinical Laboratory Standards guidelines. Overall essential agreement of 94–97% and categorical agreement of 91–94% with the reference method was achieved for the three systems. There were 8 very major errors (false susceptibility) for PASCO, 10 for Sensititre, and 9 for MICroSTREP;

4 major errors (false resistance) each for PASCO and MICroSTREP, and 6 for Sensititre. Most of these errors occurred with trimethoprim/sulfamethoxazole. Minor errors (susceptible or resistant versus intermediate) totaled 47 for PASCO, 69 for Sensititre, and 53 for MICroSTREP. Minor interpretive errors were most common with penicillin and ceftriaxone. This study showed that all three MIC panels provided interpretive results comparable to one another and to the reference method. © 1999 Elsevier Science Inc.

INTRODUCTION

also a major cause of bacteremia and meningitis in both adults and children (Block et al. 1995; Jernigan et al. 1996; Schreiber and Jacobs 1995). Resistance of S. pneumoniae to antimicrobial agents is increasing throughout the world (Breiman et al. 1994; Goldstein et al. 1996; Jernigan et al. 1996). Over 40% of isolates of S. pneumoniae at the University of Alabama at Birmingham are not susceptible to penicillin, a similar or even higher percentage than reported in other communities in the United States (Breiman et al. 1994; Jernigan et al. 1996; Schreiber and Jacobs 1995, Waites et al. 1997). The mechanism of penicillin resistance occurs through spontaneous mutation by which the organism requires increased concentrations of penicillin to saturate the penicillin-binding proteins (Cohen and Tartasky 1997). In addition, there has been an emergence of pneumococcal isolates resistant to multiple antimicrobials (Breiman et al. 1994; Jernigan et al. 1996; Schreiber and Jacobs 1995; Waites et al. 1997), making treatment decisions

Streptococcus pneumoniae is an extremely common and important pathogen implicated in several types of diseases. It is a leading cause of communityacquired pneumonia, acute sinusitis, and otitis media, conditions that are among the most common reasons for physician office visits and for which antibiotics are prescribed (Block et al. 1995; Jernigan et al. 1996; Schreiber and Jacobs 1995). This organism is From the Department of Pathology (LLG, SB, KBW), University of Alabama at Birmingham, Birmingham, Alabama; and Department of Pathology and Laboratory Medicine (WS), University of California at Los Angeles, Los Angeles, California. Address reprint requests to Dr. Ken B. Waites, Department of Pathology, WP 230, 618 South 18th Street South, University of Alabama at Birmingham, Birmingham, AL 35233-7331. This work was presented in part at the 98th General Meeting of the American Society for Microbiology, Atlanta, GA, May, 1998 (Abstract C-411). Received 2 September 1998; revised and accepted 23 November 1998.

DIAGN MICROBIOL INFECT DIS 1999;33:267–273 © 1999 Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010

0732-8893/98/$–see front matter PII S0732-8893(98)00151-5

L.L. Guthrie et al.

268 even more complex and underscoring the importance of reliable methods of antimicrobial susceptibility testing to guide chemotherapy. Several microbroth dilution methods for determining MICs for S. pneumoniae, as well as quantitative agar-based diffusion gradient systems, such as the Etest, are now commercially available for determining in vitro susceptibilities (Clark et al. 1993; Kiska et al. 1995; Krisher et al. 1994; Macias et al. 1994; Nolte et al. 1995; Tenover et al. 1996). Dade MicroScan (West Sacramento, California, USA) has developed a new product, the MICroSTREP MIC system, containing lysed horse blood (LHB) and prepared according to National Committee for Clinical Laboratory Standards (NCCLS) methodology (NCCLS 1998) to meet the needs of clinical laboratories for determining susceptibilities for aerobic streptococci, including S. pneumoniae. The MICroSTREP MIC panel was evaluated recently against the NCCLS reference method (Jorgensen et al. 1998), but there have been no published studies directly evaluating this product against other commercial MIC panels. In the present investigation, clinical isolates of S. pneumoniae were tested for susceptibilities with MICroSTREP, PASCO (Becton Dickinson Microbiology Systems, Sparks, Maryland, USA), and Sensititre (AccuMed International, Inc., Westlake, Ohio, USA) panels, using the NCCLS reference method for comparison (NCCLS 1998).

centrations tested were: MICroSTREP—penicillin (0.03– 4 mg/mL), ceftriaxone (0.03–2 mg/mL), chloramphenicol (0.25–16 mg/mL), erythromycin 0.03–1 mg/mL), tetracycline (0.06 – 8 mg/mL), and trimethoprim/sulfamethoxazole (T/S) (0.06/1.19 – 4/76 mg/mL). PASCO—penicillin (0.03–2 mg/mL), ceftriaxone (0.25– 8 mg/mL), chloramphenicol (1– 8 mg/mL), erythromycin (0.25– 4 mg/mL), tetracycline (1– 8 mg/mL), T/S (0.5/9.5–2/38 mg/mL). Sensititre—penicillin (0.015– 8 mg/mL), ceftriaxone (0.015–2 mg/mL), chloramphenicol (0.25–32 mg/mL), erythromycin (0.25–16 mg/mL), tetracycline (0.25–32 mg/ mL), and T/S (0.06/1.19 – 4/76 mg/mL).

Susceptibility Testing

A total of 157 nonduplicate clinical isolates of S. pneumoniae obtained primarily from respiratory tract specimens of adults and children in Birmingham, Alabama were evaluated. Isolates were selected to include penicillin-susceptible, intermediate, and resistant strains. Organisms were stored frozen in skim milk at 270°C until tested. Stock cultures were thawed and inoculated onto trypticase soy agar with 5% sheep blood (REMEL, Inc., Lenexa, Kansas, USA) and incubated overnight at 35°C in an atmosphere supplemented with 5% CO2. An additional overnight passage was performed before conducting susceptibility tests. Organisms were checked for purity, reidentified by Gram stain reaction, colony morphology, alpha hemolysis, and optochin susceptibility, with additional confirmation by bile solubility testing, when necessary.

For dried Sensititre panels, dilutions of each isolate equivalent to a 0.5 McFarland turbidity standard were prepared in Mueller-Hinton broth using a nephelometer according to the manufacturer’s instructions. Organism suspensions were then diluted 1/100 in Mueller-Hinton broth supplemented with 2% LHB. For frozen PASCO panels, a 1 McFarland standard was prepared for each isolate in 0.85% saline according to the manufacturer’s instructions. A 1.5-mL aliquot was diluted further by inoculation into 12.5 mL SP blood supplement so that the final concentration of LHB was approximately 2.5%. For MICroSTREP panels, a 0.5 McFarland dilution of each isolate was prepared in 3 mL of MicroScan inoculum water, then 2 mL of this suspension were transferred to 25 mL of inoculum water with Pluronic F. Organism suspensions were transferred to the frozen MICroSTREP and PASCO MIC panels using inoculation devices provided. For Sensititre, a multichannel pipette was used. The frozen MICroSTREP panel already contains Mueller Hinton broth supplemented with 3% LHB so that further additives are not required. The final inoculum concentration used for the MICroSTREP and Sensititre panels was approximately 5 3 105 CFU/mL. For PASCO, the final inoculum concentration used was approximately 1 3 106 CFU/mL. Although the inoculum of organisms was not determined individually for each isolate tested, the ability of the dilution methods as described to yield the appropriate numbers of organisms was determined by plate counts for each method prior to the initiation of the study. All panels were incubated at 35°C in ambient air for 20 to 24 h and MICs were read using a lighted mirror reader. MICs were interpreted as indicated in the manufacturers’ instructions.

Antimicrobial Agents

Quality Control

Six antimicrobial agents common to all three MIC panels were evaluated. Antimicrobials and con-

Quality control was performed daily with all test systems using S. pneumoniae ATCC 49619. All MIC

MATERIALS AND METHODS Bacterial Isolates

Susceptibility Testing of S. pneumoniae results were within the expected ranges according to NCCLS recommendations (NCCLS 1998).

269 either the test system or reference method, and either susceptible or resistant by the other method.

Reference Testing Isolates were shipped to the University of California at Los Angeles, Medical Center Clinical Microbiology Laboratory on chocolate agar slants (REMEL). MIC panels prepared on site were used to determine MICs in accordance with current NCCLS guidelines (NCCLS 1998). The medium used was a cationadjusted Mueller-Hinton broth supplemented with 2.5% LHB. Colonies were suspended in 0.85% saline to obtain a suspension equivalent to a 3.0 McFarland turbidity standard. The suspension was further diluted 1:26 in 25-mL water-Tween 80 and organisms were transferred to the MIC panels with a MIC 2000 inoculator (Dynatech Laboratories, Inc., Chantilly, Virginia, USA) to produce a final inoculum concentration of approximately 3 to 5 3 105 CFU/mL in the wells of the inoculated panels. Panels were incubated in ambient air for 20 to 24 h before visual determination of MICs. All reference testing was performed by an individual without knowledge of the results obtained by the three tests under evaluation. The antimicrobials tested for comparative purposes were: penicillin (0.015 to 8 mg/mL), ceftriaxone (0.015 to 8 mg/mL), chloramphenicol (2 to 16 mg/mL), erythromycin (0.06 to 8 mg/mL), tetracycline (0.12 to 16 mg/mL), and T/S (0.25/5 to 4/80 mg/mL).

Data Analysis Comparisons were limited to the six antimicrobial agents common to all three systems. Testing was repeated once if MIC values obtained for any drug common to all three systems differed by two or more doubling dilutions between any two systems. Repeated results were used for analytical purposes and comparison with the reference method. The percentage of isolates for which there was essential agreement for MICs, i.e., within a single doubling dilution, with respect to the reference method, was calculated for each test system and for each antimicrobial agent. The percentage of isolates for which there was complete categorical agreement for MICs, i.e., susceptible, intermediate or resistant (NCCLS 1998), with respect to the reference method, was calculated for each test system and for each antimicrobial agent. A very major error (false susceptibility) was recorded when an isolate was susceptible by the test system, but resistant by the reference method. A major error (false resistance) was recorded when an isolate was resistant by the test system, but susceptible by the reference method. A minor error was recorded for the test system when an isolate was intermediate by

RESULTS This study compared the MICroSTREP panel with PASCO and Sensititre MIC systems against 157 nonduplicate isolates of S. pneumoniae. There was adequate distribution of antimicrobial susceptibility patterns as determined by the reference method to sufficiently challenge each of the test systems. Table 1 shows a summary of susceptibilities to each antimicrobial agent as determined by the three MIC panels versus the reference method, along with essential and categorical agreements, and a breakdown of the interpretive category discrepancies. Each test system had similar overall essential agreements (6 1 doubling dilution) with the reference method for the six common drugs, ranging from 94 –97%. Each test system also had similar overall categorical interpretive agreements with the reference method, ranging from 91 to 94%. There were no very major or major errors for penicillin detected, but all three panels had a high number of minor errors, totalling (52), distributed relatively evenly among them. PASCO and MICroSTREP had the greatest essential agreements with the reference method (96%), versus 89% for Sensititre. All three test systems had similar categorical agreement ($89%). The Sensititre panel showed one major error for ceftriaxone. There were more minor errors (83) with ceftriaxone than any other drug for all test systems in aggregate, with Sensititre having the most (39). All test systems had similar overall essential agreements ($95%). PASCO had the highest overall categorical agreement for ceftriaxone (87.9%), versus 84.1% for MICroSTREP and 74.5% for Sensititre. Only one very major error was detected for chloramphenicol and erythromycin. This error involved a single isolate and all three panels were affected. There were no very major or major errors for tetracycline and no major errors for chloramphenicol. Erythromycin had a single major error. A total of five minor errors were detected for erythromycin, three for tetracycline, and zero for chloramphenicol (the latter drug has no intermediate category). All test systems had similar overall essential ($94%) and categorical agreements ($97%) for these three drugs. T/S showed the greatest number of very major (21), major (10), and minor errors (26) for all systems combined, divided among the three panels in a relatively equal distribution. All systems had similar overall essential agreements ($90%) and overall categorical agreements ($87%).

L.L. Guthrie et al.

270

TABLE 1 Comparison of Three Commercial Systems for Determination of S. pneumoniae MICs versus NCCLS Reference Method

Method/ Drug

No. (%) NCCLS Interpretive Resultsa

Penicillin MICroSTREP Sb Ic Rd Total PASCO S I R Total Sensititre S I R Total Ceftriaxone MICroSTREP S I R Total PASCO S I R Total Sensititre S I R Total Chloramphenicol MICroSTREP S I R Total PASCO S I R Total Sensititre S I R Total Erythromycin MICroSTREP S I R Total PASCO S I R Total Sensititre S I R Total

No. (%) Categorical Agreement versus Reference

45 (28.7) 57 (36.3) 55 (35) 157 (100) 45 (28.7) 57 (36.3) 55 (35) 157 (100) 45 (28.7) 57 (36.3) 55 (35) 157 (100)

42 (97.3) 44 (77.2) 53 (96.4) 139 (88.5) 44 (97.8) 46 (80.7) 51 (92.7) 141 (89.8) 42 (93.3) 42 (73.7) 55 (100) 139 (88.5)

107 (68.2) 32 (20.4) 18 (11.5) 157 (100) 107 (68.2) 32 (20.4) 18 (11.5) 157 (100) 107 (68.2) 32 (20.4) 18 (11.5) 157 (100)

100 (93.4) 26 (81.3) 12 (66.7) 138 (87.9) 93 (86.9) 23 (71.9) 16 (88.9) 132 (84.1) 85 (79.4) 14 (43.8) 18 (100) 117 (74.5)

131 (83.4) N/Ae 26 (16.6) 157 (100) 131 (83.4) N/Ae 26 (16.6) 157 (100) 131 (83.4) N/Ae 26 (16.6) 157 (100)

131 (100)

91 (58) 1 (0.6) 65 (41.4) 157 (100) 91 (58) 1 (0.6) 65 (41.4) 157 (100) 91 (58) 1 (0.6) 65 (41.4) 157 (100)

No. (%) Agreement 61 dilution versus Reference

No. (%) Interpretive Category Discrepancies versus Reference Very Major

Major

Minor

151 (96.2)

0

0

18 (11.5)

150 (95.5)

0

0

16 (10.2)

140 (89.2)

0

0

18 (11.5)

152 (96.8)

0

0

19 (12.1)

153 (97.5)

0

0

25 (15.9)

149 (94.9)

0

1 (0.9)

39 (24.8)

25 (96.2) 156 (99.4) 131 (100)

156 (99.4)

1 (3.9)

0

0

25 (96.2) 156 (99.4) 131 (100)

156 (99.4)

1 (3.8)

0

0

156 (99.4)

1 (3.9)

0

0

155 (98.7)

1 (1.5)

1 (1.1)

1 (0.6)

152 (96.8)

1 (1.5)

1 (1.1)

1 (0.6)

147 (93.6)

1 (1.5)

1 (1.1)

3 (1.9)

25 (96.2) 156 (99.4) 90 (98.9) 0 64 (98.5) 154 (98.1) 90 (98.9) 0 64 (98.5) 154 (98.1) 88 (96.7) 0 64 (98.5) 152 (96.8)

Susceptibility Testing of S. pneumoniae

271

TABLE 1 Continued No. (%) NCCLS Interpretive Resultsa

Method/ Drug

No. (%) Categorical Agreement versus Reference

Tetracycline MICroSTREP S 111 (70.7) I 3 (1.9) R 43 (27.4) Total 157 (100) PASCO S 111 (70.7) I 3 (1.9) R 43 (27.4) Total 157 (100) Sensititre S 111 (70.7) I 3 (1.9) R 43 (27.4) Total 157 (100) Trimethoprim/sulfamethoxazole MICroSTREP S 56 (35.7) I 3 (1) R 98 (62.4) Total 157 (100) PASCO S 56 (35.7) I 3 (1.9) R 98 (62.4) Total 157 (100) Sensititre S 56 (35.7) I 3 (1) R 98 (62.4) Total 157 (100) a

b

c

111 (100) 2 (66.7) 42 (97.7) 155 (98.7) 111 (100) 2 (66.7) 43 (100) 156 (99.4) 111 (100) 3 (100) 43 (100) 157 (100) 51 (91.2) 3 (100) 86 (87.8) 140 (89.2) 47 (83.9) 3 (100) 88 (89.8) 138 (87.9) 48 (85.7) 3 (100) 85 (86.7) 136 (86.6) d

No. (%) Agreement 61 dilution versus Reference

No. (%) Interpretive Category Discrepancies versus Reference Very Major

Major

Minor

155 (98.7)

0

0

2 (1.3)

156 (99.4)

0

0

1 (0.6)

155 (98.7)

0

0

0

143 (91.1)

7 (7.1)

3 (5.4)

7 (4.5)

142 (90.5)

6 (6.1)

3 (5.4)

10 (6.4)

144 (91.7)

8 (8.2)

4 (7.1)

9 (5.7)

e

NCCLS 1998; susceptible; intermediate; resistant; chloramphenicol has no designated intermediate category (NCCLS 1998).

DISCUSSION This is the first study to evaluate MICroSTREP, PASCO, and Sensititre MIC panels simultaneously against the NCCLS reference methodology for antimicrobial susceptibility testing of S. pneumoniae. All three panels worked very well in comparison to the reference method, but there were some differences among the results for individual antimicrobials tested. The lack of very major or major errors for penicillin is consistent with previous studies evaluating commercial microbroth dilution systems (Tenover et al. 1996; Jorgensen et al. 1998; Lovgren et al. 1998). However, there was a high number of minor errors for penicillin with all three panels, probably related to the characteristics of the isolates chosen for testing, many of which had intermediate resistance with MICs clustering at or near the breakpoints. For most minor errors, the reference method classified an isolate as intermediate, whereas all three test systems would typically classify it as resistant (data not shown). Dade MicroScan’s previous products used for antimicrobial susceptibility testing for S. pneumoniae, i.e., MicroScan overnight panel 6 and MicroScan rapid panels, did not accurately detect penicil-

lin resistance (Clark et al. 1993; Kiska et al. 1995; Nolte et al. 1995; Shanholtzer et al. 1986; Tenover et al. 1996). Nolte et al. (1995) reported the PASCO system was unable to support the growth of S. pneumoniae when a commercial LHB supplement was used. However, Tenover et al. (1996) reported no problems when performing tests with LHB supplement supplied by the manufacturer, as this study utilized. The large number of minor errors detected for ceftriaxone was probably attributable to clustering of MICs around the breakpoints, similar to what was observed with penicillin. Previous studies (Jorgensen et al. 1998; Tenover et al. 1996) have detected somewhat similar error rates with this antimicrobial, when tested against S. pneumoniae using various commercial microbroth dilution panels. Most minor errors with ceftriaxone occurred when the reference method classified isolates as intermediate. Sensititre and MICroSTREP usually classified them as resistant, whereas PASCO most often classified them as susceptible (data not shown). All MIC panels performed well with chloramphenicol, erythromycin, and tetracycline, consistent

272 with findings of Tenover et al. (1996) and Jorgensen et al. (1998). T/S proved to be the most problematic drug for all three panels, with a total of 21 very major, 10 major, and 26 minor errors. Two other studies using commercial microbroth dilution MIC panels (Jorgensen et al. 1998; Tenover et al. 1996) identified no very major or major errors for T/S, but found a similar number of minor errors to our investigation. Lovgren et al. (1998) used MICroSTREP panels to test 102 pneumococcal isolates for which MICs had been previously determined by NCCLS microbroth dilution. They found only three major errors and no very major errors for T/S, but detected 20% minor errors and an overall essential agreement of only 71% with the reference method. The high error rates are likely attributable to the well-known trailing effect of this drug in broth MIC systems. Product inserts for each MIC panel mention potential problems with reading endpoints with T/S and make specific recommendations. The color change endpoint recommended by the manufacturer of the MICroSTREP could be a significant factor in the poor agreement for T/S with reference methods, according to Lovgren et al. (1998). If a commercial microbroth dilution MIC system is to be adopted for routine diagnostic use testing S. pneumoniae, the choice of which system to use should include factors such as cost, ease of use, technical time needed for setup and reading results, storage requirements, and availability of specific drugs as needed by individual institutions. The three panels evaluated and their required disposables are comparable in cost. However, prices paid by an institution will likely vary according to volume of testing performed and purchase agreements with the manufacturer for other products. Each of these MIC panels was relatively easy and convenient to use, requiring approximately the same amount of setup time. There was no handling of LHB supplement necessary with the MICroSTREP system because it is already incorporated in the panel. The MICroSTREP panel is faster to read because endpoints can be determined

L.L. Guthrie et al. using a color change, versus determining endpoints through visualizing a button or turbidity throughout the well with an indirectly lighted background systems. However, color discrimination with the MICroSTREP panel is sometimes difficult when visualizing the MIC endpoints as reported by Jorgensen et al. (1998). PASCO and MICroSTREP panels both require storage at minus (2) 20°C or colder in a noncycling (nonfrost free) freezer. In contrast, dried Sensititre panels can be stored at room temperature. The antimicrobials found in each panel vary with the exception of the six common drugs compared in this study. NCCLS documents for 1998 (NCCLS 1998) include suggestions for primary testing of a fluoroquinolone such as levofloxacin, sparfloxacin, or ofloxacin and vancomycin. PASCO is the only one of the three panels evaluated that contains all the NCCLS recommended antimicrobial agents. Currently, there are no fluoroquinolones contained in the MICroSTREP panels and vancomycin is not contained in the Sensititre panel. This study demonstrates the comparability of each of these commercial MIC panels to the NCCLS reference method for determining antimicrobial susceptibilities for S. pneumoniae. Each can provide accurate data for routine diagnostic use in clinical laboratories for all drugs evaluated, with the possible exception of T/S.

The authors thank Eneida Brookings and Marybeth Minyard for technical support. The assistance and advice of Dr. Barbara Zimmer of Dade MicroScan, Janet Hindler of the University of California at Los Angeles, Linda Jeff, and Dr. Pat Greenup of the School of Health Related Professions at the University of Alabama at Birmingham regarding the study design and its execution is also gratefully acknowledged. Becton Dickinson Microbiology Systems, Sparks, Maryland, and AccuMed International, Inc., Westlake, Ohio facilitated this work by donation of their products. Dade MicroScan, West Sacramento, California provided MICroSTREP panels as well as financial assistance for the performance of reference testing.

REFERENCES Block SL, Harrison CJ, Hedrick JA, Tyler RA, Smith RA, Keegan E, Chertrand SA (1995) Penicillin-resistant Streptococcus pneumoniae in acute otitis media: risk factors, susceptibility patterns, and antimicrobial management. Pediatr Infect Dis J 14:751–759. Breiman RF, Butler JC, Tenover FC, Elliott J, Facklam RR (1994) Emergence of drug-resistant pneumococcal infections in the United States. JAMA 271:1831–1835. Clark RB, Giger O, Mortensen JE (1993) Comparison of susceptibility test methods to detect penicillin-resistant Streptococcus pneumoniae. Diagn Microbiol Infect Dis 17: 213–217.

Cohen FL, Tartasky D (1997) Microbial resistance to drug therapy: a review. Am J Infect Control 25:51–64. Goldstein FW, Acar JF, the Alexander Project Collaborative Group (1996) Antimicrobial resistance among lower respiratory tract isolates of Streptococcus pneumoniae: results of a 1992–1993 western Europe and USA collaborative study. J Antimicrob Chemother 38(suppl A):71–84. Jernigan DB, Cetron MS, Breiman RF (1996) Defining the public health impact of drug-resistant Streptococcus pneumoniae: report of a working group. MMWR 45:1–20. Jorgensen JH, McElmeel ML, Crawford AS (1998) Evalua-

Susceptibility Testing of S. pneumoniae

tion of the Dade MicroScan MICroSTREP antimicrobial susceptibility testing panel with selected Streptococcus pneumoniae challenge strains and recent clinical isolates. J Clin Microbiol 36:788–791. Kiska DL, Kerr A, Jones MC, Chazotte NN, Eskridge B, Miller S, Jordan M, Sheaffer C. Gilligan PH (1995) Comparison of antimicrobial susceptibility methods for detection of penicillin-resistant Streptococcus pneumoniae. J Clin Microbiol 33:229–232. Krisher KK, Linscott A (1994) Comparison of three commercial MIC systems, E test, fastidious antimicrobial susceptibility panel, and FOX fastidious panel, for confirmation of penicillin and cephalosporin resistance in Streptococcus pneumoniae. J Clin Microbiol 32:2242–2245. Lovgren M, Forwick B, McKercher L, Ng T, Weeks J, Tyrrell G (1998) Evaluation of MicroScan MICroSTREP antimicrobial susceptibility testing panels for determination of minimum inhibitory concentration (MIC) of Streptococcus pneumoniae. 98th General Meeting of the American Society for Microbiology. Washington, DC: American Society for Microbiology, Abstract C-405. Macias EA, Mason EO Jr, Ocera HY, LaRocco MT (1994) Comparison of E Test with standard broth microdilution for determining antibiotic susceptibilities of penicillin-resistant strains of Streptococcus pneumoniae. J Clin Microbiol 32:430–432.

273

National Committee for Clinical Laboratory Standards (1998) Performance standards for antimicrobial susceptibility testing; eighth informational supplement (M100–S8). Wayne, PA: NCCLS. Nolte FS, Metchock B, Williams T, Diem L, Bressler A, Tenover FC (1995). Detection of penicillin-resistant Streptococcus pneumoniae with commercially available broth microdilution panels. J Clin Microbiol 33:1804– 1806. Schreiber JR, Jacobs MR (1995) Antibiotic-resistant pneumococci. Pediatr Clin North Am 42:519–537. Tenover FC, Baker CN, Swenson JM (1996) Evaluation of commercial methods for determining antimicrobial susceptibility of Streptococcus pneumoniae. J Clin Microbiol 34:10–14. Shanholtzer CJ, Peterson LR (1986) False susceptible penicillin G MICs for Streptococcus pneumoniae with a commercial microdilution system. Am J Clin Pathol 85:626– 629. Waites K, Swiatlo E, Gray B, Brookings E. (1997) In vitro activities of oral antimicrobial agents against penicillinresistant Streptococcus pneumoniae and implications for outpatient treatment. Southern Med J 90:621–626.