The term Streptococcus milleri, traditionally classified under the viridans group, is a heterogeneous group of microorganisms that includes Streptococcus anginosus, intermedius and constellatus. They are oral, nasopharyngeal, gastrointestinal and genitourinary mucosa saprophytes and are pathological in patients with predisposing factors.1 In contrast to the patterns observed some decades ago, bacteria in this group are now the most common pathogens in the etiology of community-acquired pleural infections.1,2 The aim of our study is to analyze the clinical characteristics, associated factors, and clinical course of patients with S. milleri empyema.

This was a retrospective study performed in patients with a diagnosis of pleural empyema in whom S. milleri was isolated in pleural fluid in our hospital between January 2011 and March 2013. The variables studied were age, sex, associated factors, smoking habit, consumption of alcohol, previous endoscopic procedures, symptoms, clinical laboratory and radiological changes, treatment and clinical course.

The SPSS 12.0 statistical software package was used for statistical analysis. A descriptive analysis was performed with qualitative variables expressed as simple frequencies and quantitative variables as mean and standard deviation.

A total of 7 patients were studied, of which 6 (86%) were male. Mean age was 60.2 years (55–67; SD: 3.7). Three were smokers (43%) and 2 were ex-smokers (27%). Most common associated factors were alcoholism in 4 patients (57%), previous pneumonia in 3 (43%) and diabetes mellitus in 3 (43%). None of the patients had neurological disease or swallowing disorders. The main symptoms were fever in 5 patients (71%), pleuritic pain in 4 (57%) and cough with purulent expectoration in 3 (43%). Elevated C-reactive protein (CRP) was found in all patients, with a mean level of 36mg/l (SD: 15). Only 2 patients (29%) had raised procalcitonin (PCT); the mean value was 0.8ng/ml (SD: 0.8). Empyema was primary with no evidence of consolidation in 3 patients (43%). Anaerobic bacteria were isolated in 2 patients (29%): Enterobacter aerogenes, Bacteroides capillosus and Prevotella intermedia. In 3 cases, S. milleri was resistant to erythromycin, clindamycin and amoxicillin. The mean duration of antibiotic treatment was 28 days (SD: 11). Pleural drainage by chest tube was required in 6 patients (86%) and 1 patient underwent ultrasound-guided thoracocentesis for evacuation of fluid. Three patients required fibrinolytics with urokinase for a mean of 6 days (SD: 2). Clinical course was satisfactory in 5 patients (71%). One patient required surgery due to poor clinical progress, undergoing pleural decortication and pulmonary debridement without complications. One patient was admitted to the intensive care unit due to septic shock associated with empyema and sub-diaphragmatic abscess secondary to perforated gangrenous appendicitis. Length of hospitalization was 30 days (8–72; SD: 23.7). None of the patients died. They all had minimal pleural thickening and 2 individuals (29%) had restrictive ventilatory changes.

The main patient characteristics are described in Table 1. Fig. 1 shows the radiological characteristics of one of the patients.

Fig. 1.

Radiological features. Right submassive pleural effusion in the posteroinferior region of the right pleural cavity, 17cm×7cm in diameter.

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S. milleri is the most common causative agent in pyogenic infections, and most of which are located in the abdomen, the central nervous system and the chest.3 These infections usually occur in adults over 50 years of age and more often in patients with predisposing pathological factors. In our series, 86% of the patients had predisposing factors which could cause a certain degree of immunosuppression and their mean hospital stay was prolonged due to the need for pleural procedures and prolonged antimicrobial regimens.

S. milleri bacteremia is uncommon and generally associated with a septic intraabdominal focus,5 as was the case in one of our patients. The chest is involved in one fifth of S. milleri infections, the most common manifestations being empyema and less frequently mediastinitis and lung abscess.1,3–5 In our series, empyema was associated with lung abscess in 2 cases (29%), and 3 cases (43%) were primary empyema.

The pathogenic role of the co-existence of anaerobic bacteria and Streptococcus has been established. This concomitancy leads to synergy that increases virulence, lung tissue damage and the dissemination of the infection4: this was the case in 2 of our patients (29%).

Earlier studies in patients with S. milleri pleuropulmonary infection found that between 67% and 87% of cases required invasive procedures and antimicrobial treatment.5 Pleural fluid drainage may suffice in early stages, but advanced stages require video-assisted thoracic surgery (VATS) that according to most studies should be carried out between 3 and 7 days after failed drainage.5

This group of bacteria are very sensitive to ureidopenicillins, carbapenem and cephalosporins,3–5 although resistance has been reported in up to 33% of cases of S. intermedius.3 In our series, the mean antibiotic treatment time was 28 days, but this varies in other published series (10–65 days).5 Our conclusion is that S. milleri must be considered in community-acquired empyemas, and should be appropriately treated with antibiotics and early pleural drainage in order to avoid major surgical procedures and long periods of hospitalization and to improve prognosis.