Screening for extended-spectrum cephalosporin resistance in pneumococci

Predicting susceptibility of Streptococcus pneumoniae to ceftriaxone and cefotaxime by cefuroxime and ceftizoxime disk diffusion testing

In this study, disk diffusion testing with ceftizoxime and cefuroxime was evaluated for use in predicting the susceptibility of Streptococcus pneumoniae to ceftriaxone and cefotaxime. Of the 194 isolates included in this study, 138 were susceptible, 34 were intermediate, and 22 were resistant to cefotaxime by MIC testing; 138 isolates were susceptible, 35 were intermediate, and 21 were resistant to ceftriaxone by MIC testing. A zone of inhibition around the cefuroxime disk of >/=32 mm correctly categorized 101 of 138 isolates as susceptible to cefotaxime and ceftriaxone. A zone of inhibition around the ceftizoxime disk of >/=26 mm correctly categorized 111 of 138 isolates as susceptible to cefotaxime and 114 of 138 as susceptible to ceftriaxone. We conclude that disk diffusion can separate S. pneumoniae isolates susceptible to ceftriaxone and cefotaxime from those that are not susceptible. Isolates not falling into the susceptible category by disk diffusion require additional testing to determine the MIC.

Evaluation of the E-test for routine testing of the susceptibility of Streptococcus pneumoniae to benzylpenicillin, amoxicillin and cefotaxime

OBJECTIVE: To study the routine use of the E-test for susceptibility testing of penicillin-resistant Streptococcus pneumoniae. METHODS: A multicenter study of penicillin-resistant S. pneumoniae (PRSP) was carried out in Brittany, France (10 general hospitals, and two university hospitals including a coordinating center). Each hospital detected PRSP by the oxacillin (5- micro g) disk method and determined the MICs of penicillin G, amoxicillin and cefotaxime by the E-test under routine conditions. All the PRSP strains were collected in a coordinating center and the MICs were checked by the agar dilution method. The classifications obtained from the MICs determined by the E-test and by the reference method were compared. RESULTS: Between 1 July 1993 and 30 June 1994, 128 PRSP strains were collected. Agreement within 1 log2 dilution was obtained for only 62% of strains with benzylpenicillin, 72.5% with amoxicillin and 76% with cefotaxime. These data are well below published values. In addition, 52% of the strains found to be penicillin-resistant by the reference technique were of intermediate resistance according to the E-test. There were major differences in the quality of the results obtained by the participating laboratories. CONCLUSIONS: There are problems of standardization in the routine use of the E-test. Microbiologists should therefore take particular care when performing the test and when reading the results, and ensure that reference strains are included in the assay.

Use of ceftizoxime screening for the detection of cephalosporin-resistant pneumococci

Two hundred and ten pneumococcal isolates underwent ceftizoxime disk (30 micrograms) screening to predict cephalosporin resistance. Forty-six isolates failed screening with 36 (78%) demonstrating intermediate/resistant MICs. Sensitivity and specificity were 100 and 95%, respectively. The ceftizoxime disk screen is an effective method of identifying potentially cephalosporin-resistant isolates.

Evaluation of penicillin susceptibility in clinical isolates of Streptococcus pneumoniae oxacillin resistant

Antibiotic resistant pneumococci have been reported from all continents. Because of the importance of pneumococci in the aetiology of life-threatening diseases, the screening for penicillin resistance with oxacillin disc on all clinically significant isolates is suggested. However, discrepancy between the determination of penicillin resistance by oxacillin disc diffusion and the determination of penicillin minimum inhibitory concentration (MIC) has been reported. On this basis we have examined seven strains of Streptococcus pneumoniae isolated from patients recovered for the exacerbation of chronic bronchitis which were oxacillin-resistant. The assay of penicillin MICs showed that three isolates were moderately resistant to this agent, while four isolates resulted sensitive to penicillin as well as to cefotaxime. These results suggest that a further evaluation of penicillin MIC should be performed on those strains of S. pneumoniae resulting oxacillin-resistant.

Surrogate disks for predicting cefotaxime and ceftriaxone susceptibilities of Streptococcus pneumoniae

Cefotaxime- and ceftriaxone-resistant Streptococcus pneumoniae is now appearing in some medical centers, but 30-micrograms cefotaxime or 30-micrograms ceftriaxone disks are not reliable for detecting such strains. Studies were undertaken to select another cephalosporin disk that might be used as a screening test that could be used in conjunction with a 1-micrograms oxacillin disk. A 30-micrograms cefuroxime disk is proposed: strains with zones > or = 28 mm in diameter are predictably susceptible to cefotaxime and ceftriaxone, and those with smaller zones should be further studied to confirm resistance to either drug. A 30-micrograms ceftizoxime disk may also be used as a screening test with zones > or = 26 mm indicating susceptibility, but cefuroxime disks are preferred.

Penicillin-resistant pneumococci--an emerging threat to successful therapy

Pneumococci highly resistant to penicillin G [minimum inhibitory concentration (MIC) > or = 2 mg L-1] have become prevalent in many parts of the world since their emergence and spread in the late 1970s. In the USA, such organisms are seen primarily in two populations: infants and children, and adults with AIDS. Surveys in both rural and urban areas have revealed presence of these organisms, as well as an increasing frequency of Streptococcus pneumoniae strains relatively resistant to penicillin (MIC 0.1-1.0 mg L-1--now defined by some as 'intermediate' resistance). Predisposing factors are not yet clear. Prior antimicrobial therapy was given to some of the children and most of the adults who are colonized or infected with resistant strains. Prior or concurrent use of cotrimoxazole prophylaxis for Pneumocystis carinii pneumonia has been frequent in our cases in adults, most of whom had a concurrent diagnosis of AIDS. Children with disease often have a history of long-term prophylaxis with a beta-lactam drug (for sickle cell disease, etc). Many strains are also resistant to newer cephalosporins like cefotaxime and ceftriaxone (MIC > or = 2 mg L-1). The organisms are frequently multi-resistant, with high MIC values common as well for chloramphenicol and variable for tetracycline, macrolides, cotrimoxazole, and fluoroquinolones. Only to vancomycin are the organisms consistently susceptible. These findings raise alarms about the future of pneumococcal disease in both community and nosocomial disease. Increasing prevalence in otitis and pneumonia in children and in community-acquired pneumonia in adults may lead to use of vancomycin as empirical therapy for these clinical situations. This would increase the selective pressure for emergence of vancomycin-resistant organisms, whether S. pneumoniae or others. Moreover, the pneumococcus was a common cause of hospital infection prior to the introduction of penicillin. The potential now exists for nosocomial pneumococcal infection again to become a feared and ominous occurrence.

The growing threat of antibiotic-resistant Streptococcus pneumoniae

Resistance to penicillin has spread worldwide during the past 25 years. Strains resistant to alternative antibiotics have also emerged. Strains resistant to multiple antibiotics increasingly are isolated worldwide. Recently, isolates of penicillin-resistant S. pneumoniae resistant to cefotaxime and ceftriaxone have caused meningitis. As a result, recommendations for the empiric therapy of pneumococcal infections, especially meningitis, are changing.

Development of interpretive criteria and quality control limits for broth microdilution and disk diffusion antimicrobial susceptibility testing of Streptococcus pneumoniae

A five-center collaborative study was undertaken to develop quality control and specific interpretive criteria for susceptibility testing of Streptococcus pneumoniae against 12 antimicrobial agents. MICs were determined for 248 pneumococcal clinical isolates (with an emphasis on resistant strains) by use of the National Committee for Clinical Laboratory Standards (NCCLS)-recommended broth microdilution procedure incorporating lysed horse blood-supplemented Mueller-Hinton broth. NCCLS disk diffusion testing was also performed for each isolate by using Mueller-Hinton sheep blood agar incubated in 5% CO2. Repetitive testing of S. pneumoniae ATCC 49619 with different sources and lots of media and disks allowed development of quality control ranges which encompassed approximately 95% of MIC and zone size values observed in the study. Good intra- and interlaboratory reproducibilities were seen with these testing methods and all of the drugs examined. On the basis of the results of this study, MIC interpretive criteria are proposed for 11 agents. Comparisons of MICs and disk diffusion zone sizes allowed disk diffusion zone size interpretive criteria to be proposed for five drugs and confirmed the use of the oxacillin disk test for prediction of penicillin susceptibility among pneumococci. Excessive numbers of minor-category interpretive errors precludes recommendation at this time of the disk diffusion method for testing of pneumococci against five of the drugs. Use of these proposed quality control and interpretive criteria should provide for reproducible test results and allow recognition of recently emerging resistance among pneumococcal clinical isolates.

Antibiotic therapy and acute outcome of meningitis due to Streptococcus pneumoniae considered intermediately susceptible to broad-spectrum cephalosporins

Children with meningitis due to Streptococcus pneumoniae isolates that are relatively or fully resistant to penicillin and have decreased susceptibility to broad-spectrum cephalosporins (MIC, > or = 2.0 micrograms/ml) who have failed treatment with broad-spectrum cephalosporins have been reported. The National Committee for Clinical Laboratory Standards has newly revised guidelines indicating that S. pneumoniae isolates associated with meningitis for which the MICs are > or = 0.5 micrograms/ml should be considered resistant to broad-spectrum cephalosporins. This recommendation is not clearly based on data related to clinical outcome and may be too conservative. We present data on five children who had S. pneumoniae meningitis due to isolates that were relatively or fully resistant to penicillin (MIC range, 0.125 to 4.0 micrograms/ml) and had cefotaxime or ceftriaxone MICs of 0.50 to 2.0 micrograms/ml. Their clinical courses and outcomes were comparable to those of five children with S. pneumoniae meningitis due to strains that were relatively or fully resistant to penicillin and were inhibited by cefotaxime at concentrations of < or = 0.25 micrograms/ml, as well as to those of 25 patients with S. pneumoniae meningitis due to penicillin-susceptible isolates identified during the same period. Children with meningitis due to S. pneumoniae with cefotaxime or ceftriaxone MICs of < or = 1.0 micrograms/ml may be adequately treated with these antibiotics. Further clinical data are required before solid recommendations can be made regarding cephalosporin breakpoints for S. pneumoniae.

Detection of penicillin and extended-spectrum cephalosporin resistance among Streptococcus pneumoniae clinical isolates by use of the E test

Increasing penicillin resistance and the initial recognition of resistance to extended-spectrum cephalosporins among Streptococcus pneumoniae isolates have placed greater emphasis on accurate methods for susceptibility testing of clinical isolates. This study has evaluated the use of the E test (AB Biodisk NA, Piscataway, N.J.) for the detection of penicillin and cefotaxime resistance among 147 pneumococcal clinical isolates in three geographically separate laboratories. These included 42 penicillin-resistant (MIC, > or = 2 micrograms/ml) and 14 cefotaxime-resistant (defined here as an MIC of > or = 2 micrograms/ml) isolates. E test strips were applied to the surface of Mueller-Hinton sheep blood agar plates and incubated at 35 degrees C in 5% CO2 for 20 to 24 h. E test MICs were compared with MICs determined with lysed horse blood-supplemented Mueller-Hinton broth in a microdilution format as recommended by the National Committee for Clinical Laboratory Standards. Penicillin MICs agreed within one log2 dilution for 136 of 147 (92.5%) isolates, and cefotaxime MICs agreed within one log2 dilution for 142 of 147 (96.6%) isolates. No very major or major interpretive errors occurred with either penicillin or cefotaxime E test MIC results. There were 9.5 and 5.4% minor interpretive category errors with penicillin and cefotaxime E test MICs, respectively. These data indicate that the E test represents a convenient and reliable method for the detection of penicillin or cephalosporin resistance in pneumococci.

Screening for cephalosporin-resistant Streptococcus pneumoniae with the Kirby-Bauer disk susceptibility test

Kirby-Bauer disk susceptibility tests with five standard cephalosporin disks were performed on 23 penicillin-resistant Streptococcus pneumoniae isolates for which ceftriaxone MICs were 0.125 to 4 micrograms/ml. Cefuroxime disk inhibition zone diameters distinguished clearly isolates for which ceftriaxone MICs were > or = 2 micrograms/ml from more susceptible strains, whereas cephalothin, ceftizoxime, cefotaxime, and ceftriaxone disks distinguished these isolates less clearly than the cefuroxime disk did.