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Detection of relatively penicillin G-resistant Neisseria meningitidis by disk susceptibility testing
J Campos1, P M Mendelman, M U Sako
1Department of Microbiology, Hospital Infantil San Juan de Dios, Barcelona, Spain.
Abstract:
Beginning in 1985, relatively penicillin G-resistant (Penr) meningococci which did not produce beta-lactamase were isolated from the blood and cerebrospinal fluid of patients in Spain. We identified 16 Penr (mean MIC, 0.3 microgram/ml; range, 0.1 to 0.7 microgram/ml) and 12 penicillin-susceptible (Pens; mean MIC, less than or equal to 0.06 microgram/ml) strains of Neisseria meningitidis by the agar dilution technique using an inoculum of 10(4) CFU and questioned which disk susceptibility test would best differentiate these two populations. We compared the disk susceptibility of these strains using disks containing 2 (P2) and 10 (P10) U of penicillin G, 2 (Am2) and 10 (Am10) micrograms of ampicillin, and 1 microgram of oxacillin (OX1). We also investigated susceptibility with disks containing 30 micrograms of each of cephalothin (CF30), cefoxitin (FOX30), cefuroxime (CXM30), and cefotaxime (CTX30) and 75 micrograms of cefoperazone (CFP75) and determined by cluster analysis any correlation with the zone diameters obtained with P2 disks. Using the P2 and AM2 disks (in contrast to the P10 and AM10 disks), we correctly differentiated all the Penr from Pens isolates. In addition, the zone diameters with the P2 disk gave the best correlation with the penicillin G MIC determinations. All 16 Penr strains and 3 of 12 Pens strains showed zone diameters of 6 mm around OX1 disks, limiting the usefulness of OX1 disks. The zone diameters obtained with CF30, CXM30, and OX1 disks correlated with those obtained with the P2 disk, which suggests that these antibiotics have similar effects on these strains. In contrast, the data obtained with FOX30, CTX30, and CFP75 disks did not cluster with those obtained with the P2 disk, which suggests that there was a difference in the bacterial target or reflects their greater activity. We conclude that the P2 disk tests more readily identify Penr meningococci than do the standard P10 disk tests.
Insights
The 2-unit penicillin G disk test effectively identifies penicillin-resistant Neisseria meningitidis strains, outperforming the standard 10-unit disk. This method aids in distinguishing resistant from susceptible meningococci for better clinical management.
Area of Science:
- Microbiology
- Clinical Infectious Diseases
- Antimicrobial Resistance
Background:
- Penicillin G-resistant (Penr) Neisseria meningitidis strains emerged in Spain starting in 1985.
- These Penr strains did not produce beta-lactamase, complicating susceptibility testing.
- Differentiating Penr from penicillin-susceptible (Pens) meningococci is crucial for effective treatment.
Purpose of the Study:
- To evaluate disk susceptibility tests for differentiating Penr from Pens Neisseria meningitidis strains.
- To determine which disk diffusion test best correlates with penicillin G minimum inhibitory concentration (MIC) values.
Main Methods:
- Agar dilution technique to determine MICs for 16 Penr and 12 Pens Neisseria meningitidis strains.
- Disk susceptibility testing using various concentrations of penicillin G, ampicillin, oxacillin, cephalothin, cefoxitin, cefuroxime, cefotaxime, and cefoperazone.
- Cluster analysis to correlate zone diameters with P2 disk results.
Main Results:
- The 2-unit penicillin G (P2) and 2-microgram ampicillin (Am2) disks accurately differentiated all Penr from Pens isolates.
- P2 disk zone diameters showed the strongest correlation with penicillin G MICs.
- Oxacillin (OX1) disks were limited in usefulness due to similar zone sizes for both resistant and susceptible strains.
Conclusions:
- The P2 disk test is superior to the standard P10 disk test for identifying Penr meningococci.
- Certain cephalosporins (cephalothin, cefuroxime) showed zone diameters correlating with P2 results, suggesting similar mechanisms.
- Other cephalosporins (cefoxitin, cefotaxime, cefoperazone) did not correlate, indicating potential differences in activity or targets.