Streptococcus mitis Strains Causing Severe Clinical Disease in Cancer Patients - Volume 20, Number 5—May 2014 - Emerging Infectious Disease journal - CDC
Volume 20, Number 5—May 2014
Streptococcus mitis Strains Causing Severe Clinical Disease in Cancer Patients
Samuel A. Shelburne , Pranoti Sahasrabhojane, Miguel Saldana, Hui Yao, Xiaoping Su, Nicola Horstmann, Erika Thompson, and Anthony R. Flores
Author affiliations: MD Anderson Cancer Center, Houston, Texas, USA (S.A. Shelburne, P. Sahasrabhojane, M. Saldana, H. Yao, X. Su, N. Horstmann, E. Thompson); Baylor College of Medicine, Houston, Texas, USA (A.R. Flores)
Viridans group streptococci (VGS), a genetically heterogeneous group of bacteria, are the predominant bacteria in the human oropharynx (1). VGS cause a wide range of infections in humans, including bacteremia in patients with neutropenia, infective endocarditis, and orbital cellulitis (2–5). However, despite the substantial clinical effect of VGS, the epidemiology and pathogenesis of these bacteria are minimally understood (6).
A major impediment to the study of VGS has been the inability to consistently and accurately assign VGS strains to specific species, which has resulted in numerous changes in species designation and classification schemes over time (7). From a clinical microbiology laboratory standpoint, automated systems have considerable limitations in VGS species identification (8,9). The problematic nature of VGS species assignment also extends to16S rRNA sequencing, the most widely used genetic tool for species identification in clinical and research settings (9,10).
Outcomes for patients with VGS bacteremia are highly variable: some patients have minimal symptoms, and others have a severe infection characterized by hypotension and acute respiratory distress syndrome (11). The severe infections have been termed VGS shock syndrome (12). Numerous studies have examined the species distribution of VGS that cause bacteremia (9,13–16). However, these studies have found inconsistent results between a particular VGS species and disease occurrence or clinical severity of infection (9,13,14,16,17). Moreover, the recently recognized limitations of previously used techniques of VGS species identification and the low number of clinical cases analyzed preclude definitive conclusions regarding the relationship between VGS species type and clinical disease (8,9,18). Thus, we sought to combine the species identification of a large number of VGS bloodstream isolates, which we typed by using a recently developed multilocus sequence analysis (MLSA) technique (19), with patient-specific clinical data to determine relationships between VGS species and clinical endpoints.
Materials and Methods
Study Cohort and Data Abstraction
The study cohort comprised patients at MD Anderson Cancer (MDACC) who had VGS isolated from their blood between July 1, 2011, and December 1, 2012. MDAAC is a 600-bed referral cancer hospital in Houston, Texas, USA. We used a standardized data collection form to abstract clinical data from the comprehensive electronic medical records of patients with blood culture results positive for VGS. Antimicrobial drug resistance was determined in accordance with guidelines of the Clinical and Laboratory Standards Institute (www.clsi.org/standards/). VGS are known to contaminate blood cultures and to cause clinically minor, transient bacteremia, and differentiating between contamination and infection is problematic (20). Thus, for the purpose of this study, we considered that patients without signs or symptoms of infection had clinically minor bacteremia, even though they may represent cases of blood culture contamination.
Severity of infection, as measured by the Pitt bacteremia score, was determined as described (21). Pitt bacteremia scores were not determined for patients with polymicrobial bacteremia. VGS shock syndrome was defined by using the accepted definition for septic shock (i.e., hypotension refractory to fluid replacement in the setting of an infection) (22). A focus of the bloodstream infection was defined as isolation of a VGS species from a nonsterile site (e.g., liver abscess) at the same time that VGS were isolated from the blood, with the exception of infective endocarditis, which was defined according the modified Duke criteria (23,24). Neutropenia was defined as an absolute neutrophil count of <500 cells/μL.
Some patients had signs and symptoms of a lower respiratory infection and x-ray findings compatible with a pneumonic process that could not be explained (i.e., no known respiratory pathogens were isolated and no other alternative explanation, e.g., congestive heart failure, was found). Such patients were defined as having unexplained pulmonary infiltrates. Because VGS are considered normal flora, isolation of these organisms from a respiratory specimen would not have been considered clinically meaningful by the clinical microbiology laboratory and thus would not have been reported. The study protocol was approved by the MDCC institutional review board.