Viridans group streptococci septicaemia and endocarditis : molecular diagnostics, antibiotic susceptibility and cinical aspects

Abstract

Viridans group streptococci (VGS) are inhabitants in the oral cavity and in the gastrointestinal tract. They cause severe infections, they are responsible for up to 39 % of the cases of septicaemia in neutropenic patients with haematological diseases and cause infective endocarditis (IE), mainly in patients with native valves and previous heart disease. Different species cause different clinical picture, therefore the identification of the species is important. The conventional methods for identification of VGS strains, Strep API and API ZYM have not been optimal. A reduced antibiotic susceptibility to penicillin in VGS has developed during the last years, primarily for patients with haematological diseases.

In the present studies we have investigated the rate of infective endocarditis and risk factors, in immunocompetent and immunocompromised patients with septicaemia. We have identified species of VGS septicaemia with old and new diagnostic methods and analysed the antibiotic susceptibility for penicillin and other antimicrobial agents in the oral cavity and blood cultures.

In these studies we found that infective endocarditis was rare in patients with haematological diseases, in this group of patients VGS species as Streptococcus mitis and Streptococcus oralis dominated. When we used rnpB sequencing and PCR, it was possible to identify species of VGS that earlier has been difficult to classify.

In patients with infective endocarditis, strains for the Streptococcus sanguinis group dominated, when using rnpB sequencing we also found Streptococcus gordonii and Streptococcus oralis strains in these patients. We found a reduced susceptibility to pencillin in 18 % (MIC ≥ 0.25 μg/ml) of the VGS isolates in 1998-2003, that is lower compared to studies from Canada where 37 % of the strains had a reduced susceptibility to pencillin. The antibiotic resistance to VGS was increased compared to 1992-1997, however different methods had been used. The highest rate of pencillin resistance in this study was found in oral swabs from haematological patients where 25% of the VGS isolates were resistant to penicillin (MIC ≥ 4.0 μg/ml), which was higher that we had expected. This is an important observation because the oral cavity has been described as a genetic reservoir for transferring resistance genes from VGS to Streptococcus pneumoniae.

We also found that 19% of the isolates had a reduced susceptibility to erythromycin (MIC ≥ 0.5 μg/ml) and 80% of these strains harbored mefA and 40 % ermB. The VGS strains in 1998-2003 had a reduced susceptibility to ciprofloxacin; which has previously been used as antibiotic prophylaxis in neutropenic patients but is not generally recommended because of emergence of resistance. Vancomycin had a high susceptibility to VGS but it should only be used as empiric therapy for severe cases and for resistant strains because of the emergence of resistance. New antimicrobial agent as linezolid seems susceptible but should be saved for cases of antibiotic resistance.