Journal List > Korean J Clin Microbiol > v.13(2) > 1038214

Kim, Kim, Park, Song, Ahn, Park, and Seo: Impact of Revised Penicillin Breakpoints for Streptococcus pneumoniae (CLSI M100-S18) on the Penicillin Susceptibility Rate

Abstract

Background

In January 2008, the Clinical and Laboratory Standards Institute (CLSI) published revised penicillin breakpoints for Streptococcus pneumoniae according to clinical presentation and the route of penicillin administration. The aim of this study was to evaluate the impacts of the new penicillin breakpoints on the susceptibility rates of S. pneumoniae isolated from blood.

Methods

A total of 156 non-duplicated S. pneumoniae strains recovered from blood of hospitalized patients were collected between January 2003 and December 2008. Penicillin and cefotaxime susceptibility tests were performed using an E-test (AB Biodisk, Solna, Sweden). Results of the penicillin susceptibility tests were analyzed using the former and new CLSI guidelines.

Results

Of the 156 S. pneumoniae strains isolated from blood, penicillin susceptibility under the former CLSI guidelines resulted in 42.3% susceptible, 42.3% intermediate, and 15.4% resistant states. According to the new CLSI guidelines (nonmeningitis, parenteral), 87.8% of isolates were susceptible, 9.6% were intermediate, and 2.6% were resistant to penicillin.

Conclusion

When the new CLSI guidelines are applied, the penicillin susceptibility rate of S. pneumoniae strains isolated from blood is considerably increased. This suggests that penicillin should still be useful for the treatment of nonmeningeal pneumococcal infections and that the use of broad-spectrum antimicrobials should not replace this treatment.

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Table 1.
Former and new penicillin breakpoints for Streptococcus pneumoniae - Clinical and Laboratory Standards Institute, 2008
Standards Susceptibility category MIC (μg/mL)
Susceptible Intermediate Resistant
Former ≤0.06 0.12∼1 ≥2
New      
 Meningitis, intravenous penicillin ≤0.06 ≥0.12
 Nonmeningitis, intravenous penicillin ≤2 4 ≥8
 Nonmeningitis, oral penicillin ≤0.06 0.12∼1 ≥2

No intermediate category for meningitis under new penicillin breakpoints.

Table 2.
Penicillin susceptibility of blood isolates of Streptococcus pneumoniae recovered from 2003 to 2008 by former and new CLSI breakpoints (N=156)
Penicillin Former (%) New (%)
Susceptible 66 (42.3) 137 (87.8)
Intermediate Resistant 66 (42.3) 24 (15.4) 15 (9.6) 4 (2.6)

For nonmeningitis, intravenous treatment.

Table 3.
Cross-susceptibility and cross-resistance analysis of minimum inhibitory concentration (MIC) breakpoints of penicillin, compared with MIC breakpoints of cefotaxime
Cefotaxime MIC (μg/mL) Number (%) of isolates, by penicillin MIC (μg/mL)
Susceptible Intermediate Resistant
≤0.06 0.12∼2 4 ≥8
(≤1) Susceptible 66 (42.3) 71 (45.5) 4 (2.6) 0
(2) Intermediate 0 0 10 (6.4) 3 (1.9)
(≥4) Resistant 0 0 1 (0.6) 1 (0.6)

Percentage of absolute categorical agreement between penicillin and cefotaxime (e.g., 94.9%) by use of the new penicillin breakpoints.

Table 4.
Minimum inhibitory concentration (MIC) distributions for Streptococcus pneumoniae isolates
Antimicrobial agent Cumulative percentage of isolates inhibited, by MIC (μg/mL)
≤0.03 0.06 0.12 0.25 0.5 1 2 4 8
Penicillin 40 42 44 48 62 81 88 94 100
Cefotaxime 49 51 53 59 73 90 97 99 100
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