[Problems with identification of beta-hemolytic streptococcus resistant to bacitracin isolated from patients with pharyngitis].

Abstract

Introduction: The genus Streptococcus comprises a number of species characterized by a differential pathogenic potential. These bacteria can be considered as members of microbial physiological flora but they can also cause mild infections or severe, life threatening conditions. The majority of infections of streptococcal etiology are caused by beta-hemolysing species. The predominant causative agent of bacterial pharyngitis is Streptococcus pyogenes. This species usually doesn't give rise to any identification difficulties due to the introduction the well determined diagnostic schemes. Problems concerning laboratory identification can be, however, associated with other species of beta-hemolysing streptococci isolated from patients with pharyngitis. These streptococci can demonstrate features similar to those of S. pyogenes and share the group antygen A, such as some strains of Streptococcus anginosus and Streptococcus dysgalactiae subsp. equisimilis. The determination of sensitivity to bacitracin, which is a feature typical of S. pyogenes, is the basic test useful for its preliminary identification. Nevertheless, the identification of some strains by this test can give rise to incompatibility. The aim of the study was characterisation of beta-hemolysing streptococci resistant to bacitracin isolated from patients with pharyngitis. The examined bacterial strains caused identification problems by the use of routine diagnostic methods.

Methods: The material included 14 streptococcal strains resistant to bacitracin which were isolated from adult patients suffering from pharyngitis. The bacteria were cultured on media dedicated for the species. The following routine diagnostic tests were used for the bacterial identification: sensitivity to bacitracin (0.04 U/disc), CAMP test, determination of the group antigens A, B, C, D, F and G (Slidex Strepto-Kit), and determination of biochemical features by the API 20 STREP test (bioMèrieux). The sensitivity of streptococcal isolates to antibiotics (penicillin, clindamycin, erythromycin, tetracycline, vancomycin, ofloxacin) and trimethoprim/sulfamethoxazole, was determined by the disc diffusion method on the Mueller-Hinton agar with 5% sheep blood (the inoculum-0.5 McFarland).

Results: Among the 14 isolates resistant to bacitracin, 6 isolates of S. pyogenes, 6 isolates of S. constellatus, and 2 isolates of S. dysgalactiae subsp. equisimilis were identified. All isolates were sensitive to penicillin and vancomycin. One isolate ofS. pyogenes demonstrated constitutive MLSB resistance mechanism. Seven isolates were resistant to tetracycline: S. dysgalactiae subsp. equisimilis (3 isolates), S. constellatus (3), and S. pyogenes (1). The number of isolates resistant to trimethoprim/sulfamethoxazole was as follows: S. pyogenes (6) and S. dysgalactiae subsp. equisimilis (1), whereas four isolates were resistant to ofloxacin.

Conclusions: The bacitracin test cannot be used as the only test for the laboratory identification of S. pyogenes even if it is combined with the determination of the Lancefield group antigen due to the existence of bacitracin resistant S. pyogenes strains. Therefore, it is necessary to perform biochemical commercial tests in addition to routine phenotypic tests. Isolation of beta-hemolysing streptococci other than S. pyogenes from patients with pharyngitis confirms their role in the etiology of this infection. Taken into account the significance of determination of sensitivity to bacitracin for the preliminary identification of S. pyogenes, it is necessary to standardize this test, which will make obtaining of the comparability of results possible.