Is Echocardiography Mandatory for All Streptococcus gallolyticus Subsp. pasteurianus Bacteremia?

Background

Formerly, Streptococcus bovis was known as a common pathogen for infective endocarditis, usually associated with colorectal neoplasia.^1,2 However, taxonomy and nomenclature of S. bovis were distinguished using a molecular method, such as 16s rRNA gene and sodA gene, in six different DNA groups. Common human pathogenic subspecies include S. gallolyticus subsp. gallolyticus (formerly S. bovis biotype I), S. gallolyticus subsp. pasteurianus (formerly S. bovis biotype II/2), S. infantarius subsp. infantarius and S. infantarius subsp. coli.^2–6

Since the introduction of the new taxonomic classification, the disparity of clinical syndromes and patient’s underlying diseases between the two subspecies were observed. S. gallolyticus subsp. gallolyticus bacteremia was strongly associated with endocarditis and colorectal neoplasia.^7,8 Nevertheless, S. gallolyticus subsp. pasteurianus was found to be associated with neonatal sepsis, liver diseases and malignancies of the digestive tract, ie, gastric, pancreatic, hepatobiliary and colorectal cancer and is less commonly associated with endocarditis (14–23%) compared with S. gallolyticus subsp. gallolyticus (50–53%).^9–15 Nevertheless, many guidelines still recommend diagnostic work-up for endocarditis in all S. gallolyticus bacteremia cases, which may result in unnecessary procedures and higher costs of hospitalization.^16,17

Because of the lack of large-scale studies investigating the association between S. gallolyticus subsp. pasteurianus and endocarditis, we aimed to find the risk factors for S. gallolyticus subsp. pasteurianus endocarditis to help select appropriate patients for echocardiography, expected to minimize the unnecessary investigation.

Objectives

Our primary objective was to determine the risk factors for developing endocarditis among patients with S. gallolyticus subsp. pasteurianus bacteremia and secondary objective was to determine the 30-day mortality rate of S. gallolyticus subsp. pasteurianus bacteremia.

Materials and Methods

Study Setting and Design

A retrospective cohort study was conducted. We collected all blood isolates of S. gallolyticus subsp. pasteurianus obtained from patients from April 2009 to May 2015 at Phramongkutklao Hospital, a 1200 bed teaching hospital of Phramongkutklao College of Medicine, Royal Thai Army, Bangkok, Thailand. The inclusion criteria were participants aged over 18 years with monomicrobial S. gallolyticus subsp. pasteurianus bacteremia. Participants referred to other hospitals within the first seven days were excluded. Demographic data, patient characteristics, comorbidities, immunosuppressive status, microbiological data and patient outcomes were recorded. The site of infection was defined according to definitions of the Centers for Disease Control and Prevention.¹⁸ Patients whose foci of infection were unidentified were classified as primary bacteremia. Results from echocardiography, ultrasonography, computed tomography and endoscopic examinations searching for endocarditis, hepatobiliary pathology and colonic lesions were collected. All-cause-30-day crude mortality after the onset of bacteremia was recorded. Participants’ medical records were followed up to two years focusing on the finding of new malignancy or endocarditis. The study protocol followed the guidelines of the Declaration of Helsinki and ethics approval was obtained from the Institutional Review Board, Royal Thai army Department.

Microbiological Analysis

Bacterial isolates were collected by the clinical microbiological laboratory at the study hospital. Species were identified using the BACTEC system (Becton Dickson, Sparks, MD). The VITEK 2 automated system (bio-Merieux, Hazelwood, MO) was used to identify bacterial species and subspecies. Antimicrobial susceptibility was determined using the disk diffusion test of various antimicrobial agents including penicillin, gentamicin, clindamycin, ceftriaxone, erythromycin, azithromycin, moxifloxacin, levofloxacin, tigecycline, linezolid, daptomycin and vancomycin. Minimum inhibitory concentrations (MICs) of the isolates to penicillin and ceftriaxone were performed using the E-test. Disk diffusion and MICs were interpreted using the Clinical and Laboratory Standards Institute (CLSI) guidelines.¹⁹

Statistical Analysis

For categorical variables, Fisher’s exact test and the Chi-square test were used. The Mann–Whitney test was used to compare continuous variables. For all analyses, a two-sided p-value of 0.05 was considered significant. Potentially significant predictors in the univariate analyses (p- value <0.10) were included in a forward, stepwise multiple logistic regression model to identify the most important factors related to developing endocarditis. All statistical analyses were performed using Stata 12.0 Software (StataCorp, USA).

Results

During the study period, S. gallolyticus subsp. pasteurianus was isolated in 106 individuals, in whom 191 specimens (88%) blood culture specimens were positive among the overall of 217 specimens taken. The mean age was 66.9±15.6 years. Most patients were male (61.3%), with cirrhosis as the most common underlying disease, followed by diabetes mellitus, malignancy, and chronic kidney disease. Nearly 20% of participants had experienced a malignancy before bacteremic episodes. The most common malignancies included hepatocellular carcinoma, colorectal cancer and hematologic malignancy. The most common clinical manifestations comprised primary bacteremia, followed by spontaneous bacterial peritonitis and endocarditis. Acute kidney injury, septic shock, and respiratory failure were the three most common complications. Almost all participants had bacteria isolated in two blood culture specimens. The 30-day crude mortality rate was 21.7%, and baseline characteristics of all participants are shown in the Table 1.

Table 1 Baseline Characteristics of All S. gallolyticus Subsp. pasteurianus Bacteremic Participants

Thirty-five patients underwent echocardiography based on clinical presentation and physician decision as Figure 1. Nine patients were diagnosed with infective endocarditis as described in Table 2. All of whom had positive blood culture in more than two specimens (RR 1.35; p-value 0.079). None of the cirrhosis patients had endocarditis (RR 0; p-value 0.003). The common complication associated with endocarditis was acute respiratory failure (RR 4.32; p-value 0.05), whereas acute kidney injury served as a protective factor (RR 0; p-value 0.01) as shown in Table 3. Because of the small number of patients with endocarditis, multivariate analysis was not performed.

Figure 1 Patients underwent echocardiography.

Table 2 Characteristics of Nine Patients with Endocarditis

Table 3 Univariate Analysis of Factors Associated with Infective Endocarditis in S. gallolyticus Subsp. pasteurianus Bacteremia

Among 76 patients with records of two-year follow-up, no new diagnosis of endocarditis or malignancy was observed.

The majority of S. gallolyticus subsp. pasteurianus was susceptible to antimicrobial agents, with penicillin susceptibility at 97%. However, a high rate of erythromycin resistance (24.5%) was observed as shown in the Table 4.

Table 4 Susceptibility Data of 106 Isolates of S. gallolyticus Subsp. pasteurianus

Discussion

In this cohort study, nearly one half of the patients with S. gallolyticus subsp. pasteurianus bacteremia presented cirrhosis. This finding was consistent with those of related studies reporting that 15 to 39% of patients had liver diseases or cirrhosis.^8,14,15

Regarding the clinical presentations, this study was consistent with others reporting primary bacteremia as the most common clinical presentation, followed by spontaneous bacterial peritonitis, and infective endocarditis.^8,14,15 About 20% of participants were diagnosed with malignancy before the onset of bacteremia. As in other studies, a correlation between gastrointestinal and other organ malignancies with the bacteremia could not be demonstrated.¹⁴ Although colonoscopy was performed in only 10% of patients, which might have underestimated the true prevalence of precancerous colorectal lesions, no new malignancy was observed after two years among 76 participants.

With the proportion of 8.5%, endocarditis was still an uncommon finding, similar to what had been reported in related studies.^8,14 None of the endocarditis patients had cirrhosis, which was demonstrated to be a significantly protective factor. The different clinical syndromes between cirrhotic and noncirrhotic patients may be explained by the distinction pathophysiology, host-microbe interaction and bacterial virulence factors. Although, no study to date have compared the adherence potential of S. gallolyticus subsp. pasteurianus and S. gallolyticus subsp. gallolyticus to endothelial cells. The surface adhesins such as pili - and in particular the pilus Pil1- is absent from the S. gallolyticus subsp. pasteurianus sequenced strain, and that this pilus plays a major role in infective endocarditis.²⁰ In addition, the adhesin located at the top of the pilus Pil1 is also able to manipulate the host coagulation system which can further promote endocarditis²¹ accompanied with the expression of blood group antigen sialyl lewis-X (sLex) on human leukocytes increases the adhesion ability of S. gallolyticus to endothelial cells.^22–24 In contrast, the main origin of bacteremia among patients with cirrhosis arise from the fecal microbiome, which might have inferior virulence and less adhesion ability to the endothelium.²⁵

Because S. gallolyticus subsp. pasteurianus, isolated from two or more different blood cultures, is one of the two major Duke criteria raising the suspicion of endocarditis, echocardiography should be performed.^16,17 From our findings, because of the lower incidence of endocarditis other than S. bovis, this diagnostic procedure might not be needed in patients with cirrhosis, which will decrease the excessive investigation by one half of patients with the bacteremia.

The mortality rate of the bacteremia was higher than related reports.^8,14 This observation may be explained by the higher proportion of patients with cirrhosis, which usually had higher complications and mortality rates.²⁵

Although some isolates were intermediate or resistant to beta-lactams using disk diffusion tests, all isolates were susceptible after the MIC measurement when E-test method was performed. This error is due to the poor discrimination ability of disc diffusion tests between penicillin-susceptible and penicillin-intermediate streptococcal populations.^26,27 As a result, MIC measurement should be primarily used to interpret susceptibility results or to confirm resistance in resistant isolates determined by the disk diffusion method.

Although this is the first study to demonstrate a correlation between S. gallolyticus subsp. pasteurianus and infective endocarditis among noncirrhotic patients, it had certain limitations. First, although a large number of participants were enrolled in this study, only 8.5% presented endocarditis, which might have overestimated the true association. This small proportion helps confirm that, unlike S. gallolyticus subsp. gallolyticus, S. gallolyticus subsp. pasteurianus uncommonly causes endocarditis. Second, the ability of species identification using VITEK 2 system to discriminate between the two subspecies of S. gallolyticus is not apparent. Nevertheless, studies comparing between the Vitek 2, conventional biochemistry test and molecular testing methods (sodA sequencing, pyrosequencing of 16S rRNA gene) showed generally acceptable agreement.^28–30 Finally, due to the retrospective design, factors influencing the physician’s reason for the decision to perform colonoscopy and echocardiography was unidentified, which might have underestimated the prevalence of endocarditis and cancer. However, this bias was minimized by the long-term follow-up.

Conclusion

Among patients with S. gallolyticus subsp. pasteurianus bacteremia, echocardiography might not be needed among patients with cirrhosis and without sustained bacteremia.