Antimicrobial Susceptibility Profile of Streptococcus pseudopneumoniae Isolated from Sputum

Streptococcus pseudopneumoniae is a recently described member of the Streptococcus mitis/oralis group of viridans streptococci that shares some characteristics with Streptococcus pneumoniae (1). Key characteristics of S. pseudopneumoniae are the absence of pneumococcal capsule, insolubility in bile, resistance or indeterminate susceptibility to optochin when incubated in 5% CO2 but susceptibility to optochin when incubated in ambient air, and positive reactions in DNA probe hybridization and antigen detection tests (1, 3). The clinical relevance of S. pseudopneumoniae has not yet been established, although it may be associated with chronic obstructive pulmonary disease (3). As yet, the antibiotic susceptibility profile of S. pseudopneumoniae has not been reported in detail. The aim of this study was to determine the antibiotic susceptibility profile of a large number of S. pseudopneumoniae isolates recovered from clinical specimens. Ninety-five isolates of S. pseudopneumoniae collected between 2000 and 2007 were studied. All isolates had been recovered as the predominant organism from good-quality sputum specimens containing >25 leukocytes and ≤10 squamous epithelial cells per 100× field. The isolates were identified on the basis of phenotypic characteristics as previously described (1, 3). MICs were determined by broth microdilution with the MicroScan Micro STREP plus 1 system (Dade Behring, West Sacramento, CA). The MICs of penicillin, ampicillin, ceftriaxone, cefotaxime, cefepime, meropenem, chloramphenicol, clindamycin, erythromycin, azithromycin, tetracycline, vancomycin, gatifloxacin, levofloxacin, and trimethoprim-sulfamethoxazole were determined. Strains for which the MICs were greater than the highest dilution included on the MicroScan panel were retested with the Etest (AB Biodisk, Solna, Sweden) to determine the precise MIC. Susceptibility breakpoints were based on CLSI guidelines (2). Strains showing resistance to erythromycin but susceptibility to clindamycin were further tested to determine the presence of macrolide-lincosamide-streptogramin B resistance (MLS phenotype). Discs containing erythromycin (15 μg) and clindamycin (2 μg) were placed 20 mm apart on the plate. After incubation, the plates were examined for flattening of the clindamycin zone, which is indicative of the phenotype. The MICs of the 15 antibiotics are shown in Table ​Table1.1. No isolate was resistant to any β-lactam agent, although about one-third showed reduced susceptibility to penicillin and ampicillin. About one-third of the isolates were resistant to macrolides, almost half were resistant to tetracycline, and 71% were resistant to tetracycline and/or macrolide antibiotics. Reduced susceptibility to the macrolide antibiotics only (M phenotype) occurred in seven isolates, with an additional isolate being resistant to clindamycin (intrinsic MLS resistance). Inducible MLS resistance was noted in 3 of 28 erythromycin-resistant, clindamycin-susceptible isolates. In each case, the MIC was within a 1-dilution difference from susceptible. All azithromycin-resistant isolates were resistant to erythromycin. TABLE 1. In vitro activities of antimicrobial agents against 95 isolates of S. pseudopneumoniae One limitation of our study is that all of the isolates were collected from respiratory samples from patients attending a tertiary care center in one geographical area of New Zealand. As such, they may not be representative of the isolates found in the wider community or other regions of the world. Further studies are needed in other geographic regions, and possibly with other sample types, in order to better clarify the antibiotic susceptibility profile of S. pseudopneumoniae.