Laboratory and Epidemiological Characteristics of Streptococcus pseudopneumoniae

Abstract

The species designation Streptococcus pseudopneumoniae has recently been proposed for a streptococcus that is phenotypically and genetically distinct from Streptococcus pneumoniae and other viridans streptococci. The key characteristics of S. pseudopneumoniae are the absence of a pneumococcal capsule, insolubility in bile, resistance or indeterminate susceptibility to optochin when incubated in 5% CO2 but susceptible to optochin when incubated in ambient air and a positive reaction with the AccuProbe DNA hybridization test.

In this thesis, the laboratory characteristics and some preliminary clinical associations are described for a collection of sputum isolates of S. pseudopneumoniae. Streptococcus pseudopneumoniae can be distinguished from S. pneumoniae by some key phenotypic characteristics listed above, and also has a positive result with the Binax NOW pneumococcal antigen test. The pneumolysin gene was present in all the isolates.

No high level penicillin resistance was found amongst the strains of S. pseudopneumoniae tested but intermediate resistance was detected in 32% of strains. Resistance to erythromycin and tetracycline was greater than that reported for New Zealand strains of S. pneumoniae (46% vs 20% and 54% vs 23% respectively). The figures for constitutive clindamycin resistance were the same (17%) but the rate of inducible MLSβ resistance was higher for S. pseudopneumoniae. In a study of blood culture isolates outside New Zealand in 2004, intermediate penicillin resistance for S. mitis was 27% and for erythromycin 50% which compares well with the results obtained in this study.

The clinical significance S. pseudopneumoniae is currently unknown but all the strains were isolated from patients who had symptoms of lower respiratory tract infection. 79% of the patients had chronic obstructive pulmonary disease (COPD) and 33% had chest radiographic infiltrates. In a preliminary study with matched control patients who had S. pneumoniae isolated from sputum, patients with S. pseudopneumoniae infection were more likely to have COPD (OR 5.0, 95% CI) or exacerbation of COPD (OR 6.5, 95% CI). A larger study of 418 adults with and without COPD to investigate the apparent association with COPD was inconclusive but did suggest that there may be an association with smoking. Streptococcus pseudopneumoniae was most commonly isolated from the sputum of participants with stable COPD.

S. pseudopneumoniae was isolated from 5% of the study participants which was higher than that found in the only comparable study, but nasopharyngeal carriage was low (0.7%) and compared well with that of S. pneumoniae in adults.

Although it was found that this new species can be differentiated from S. pneumoniae in the laboratory, more research is needed to better characterise the members of the S.mitis group and the epidemiological role of S. pseudopneumoniae.