Bacteriology of empyema

Parapneumonic effusions and empyema are common clinical problems with varying prognosis and treatment options based upon the organism isolated. A total of 448 clinically suspected cases of empyema were analyzed by retrospective analysis. The male:female ratio was 1.87:1. Most common presenting symptoms were fever (90%), cough (80%), chest pain (60%), expectoration (60%), and dyspnea (60%). The most common underlying predisposing factor was chronic obstructive pulmonary disease (COPD) in 60% of cases. Culture was positive in 11.16% (50/448) of cases. Most common isolate was methicillin-resistant Staphylococcus aureus (MRSA) (24/70) followed by methicillin-sensitive Staphylococcus aureus (MSSA) (8/70), Klebsiella pneumoniae (8/70), Pseudomonas aeruginosa (8/70), and Acinetobacter species (7/70). Five patients from whom multi drug resistant Gram-negative bacilli were isolated expired.

Keywords: Pleural effusion, bacterial isolates

Parapneumonic effusions and empyema are common clinical problems with good variety of treatment options, occasionally having poor prognosis. ^[1] Empyema is usually a complication of pneumonia but may arise from infections at other sites. ^[1],[2] The microbial etiology of pleural space infections has changed since the introduction of antibiotics and is modified by either specific patient factors such as surgical procedures, trauma or underlying conditions like diabetes, chronic obstructive pulmonary disease, and HIV infection. ^{[1],[2],[3],[4]} For this reason, several studies have found different results in the spectrum of pathogens causing pleural effusions. We reviewed our experience with the microbial causes of pleural empyema over a period of 1 year.

A retrospective analysis of 488 clinically suspected cases of pleural effusion admitted in Intensive Respiratory Care Unit and medicine wards in Lokmanya Tilak Municipal Medical College and General Hospital, during a period of 1 year from February 2007 to February 2008 are included in the study. Medical records of the patients were reviewed. Out of the 488 cases, 318 patients were males and 170 were females (male:female ratio of 1.87: 1). A total of 442 patients were adults, while 36 patients were children from 1 month to 12 years. Infected pleural effusion was identified, if they met the following criteria:

1. The thoracocentesis received thick purulent-appearing pleural fluid.
2. Microscopic examination of pleural effusions revealed a WBC count of < 15,000/΅l, with neutrophils predominating, and the microorganisms were identified by microscopic examination and/or isolated in cultures. ^[5]

All the patients underwent diagnostic thoracocentesis under aseptic precautions. The specimens were examined by Gram’s staining and were cultured for aerobic growth on blood agar, bhocolate agar and MacConkey agar. Some portion of the sample was put in the thioglycollate broth. After incubation for 48 h anaerobically in Macintosh Filde’s jar, subcultures were done on blood agar, egg yolk agar, and Wilkins- Charlgreen’s agar, which were incubated anaerobically. Ziehl Neelsen staining was also done for demonstration of acid-fast bacilli and cultures were done on the Lowenstein Jensen medium. Two sets of Sabouraud’s Dextrose agar (SDA) were also inoculated to check for the growth of fungi. The bacteriology of pleural effusion was classified as follows: aerobic or facultative Gram positive; aerobic Gram negative, anaerobic, anaerobes along with aerobes, and mixed and acid-fast bacilli (Mycobacterium tuberculosis). Mixed was defined as the isolation of more than one strain of pathogen from the pleural effusion. The following data were collected for each patient: age, sex, predisposing factors, and clinical symptoms.

From a total 488 clinically suspected cases of pleural effusion, the age varied from 1 year to 60 years. Male patients predominated over female patients with a male to female ratio of 1.87:1. The most common underlying diseases were found to be chronic obstructive pulmonary disease (COPD) in 60% of cases and pneumonia in 10% cases; two patients were HIV seropositive; spontaneous pneumothorax was present in a single case and in one case there was a history of rupture of liver abscess into the pleural cavity in which no trophozoites of E. histolytica were seen, on the other hand methicillin-resistant Staphylocccus aureus (MRSA) was isolated.

Most common manifestation were fever (90%), cough (80%), chest pain (60%). expectoration (60%) and dyspnea (60%).

All the samples whose gram staining revealed organisms showed growth in culture. The results of pleural effusion cultures were positive in 50 cases (11.16 %) [Table 1]. The most predominant isolates were MRSA in 24 cases, followed by Klebsiella pneumoniae 8, methicillin-sensitive Staphylocccus aureus (MSSA) 8, Pseudomonas aeruginosa 8 , Acinetobacter species 7, 3 isolates of E. coli, Streptococcus pneumoniae 3, Streptococcus pyogenes 4, Enterococcus species 1, Enterobacter species 3, Proteus vulgaris 1, and Mycobacterium tuberculosis that were isolated in three cases. No other aerobic or anaerobic organism was isolated. There was also no growth of any fungi on SDA. 10% of culture-positive fluids were parapneumonic in origin. Out of the three isolates of Mycobacterium tuberculosis, two were from HIV infected individuals.

The patients which presented as spontaneous pneumothorax was a 2-year and 6-month-old female child, had a mixed infection with MRSAand E. coli.

The sensitivity patterns of all the isolates are given in [Table 1]. Multidrug- resistant strains were isolated from five of the patients who expired. These multidrug-resistant isolates were Klebsiella pneumoniae (2), Pseudomonas aeruginosa (2) and Acinetobacter species in one case.

In all the patients, the duration of hospitalization varied from minimum 2-4 weeks (median of 3 weeks). Antibiotics were given based on the antibiotic sensitivity pattern. No surgery was performed in any of the cases. The follow-up of the patients showed that most of the patients responded to the treatment except the five patients, who did not respond to treatment and expired, the mortality rate thus being 9% (4/ 442). All these five patients who expired were suffering from infection with multidrug-resistant gram-negative organisms i.e. Klebsiella species (2), Pseudomonas aeruginosa (2) and Acinetobacter species (1), which were resistant to all the antibiotics including the carbapenems. The cause of death in these patients was septicemia, the focus of which was in the pleural cavity (pleural effusion). Three patients who were suffering from tuberculous pleural effusion were discharged after 4 weeks after they showed improvement in the clinical condition following DOTS therapy. The follow-up of these patients showed a complete response after the completion of the DOTS regimen. The most common presenting features were mild-to-moderate fever of more than 3-week duration (90%) and cough since 2-week duration (80%), which was productive in 60% cases. 60% of the cases presented with dyspnea on exertion for 3 weeks and chest pain.

In this study we found that MRSA was the most common isolate (24/70) followed by MSSA, Klebsiella pneumoniae and Pseudomonas aeruginosa each (8/70). In over past 30 years, aerobic Gram-positive organisms have been the most frequent isolates in acute thoracic empyema. ^[5],[7] Study by Brook et al.[6] showed that Staphylococcus aureus and Streptococcus pneumoniae accounted for ~70% of all the aerobic Gram-positive isolates. Similar findings were observed in the present study.MRSA, MSSA and Streptococcus pneumoniae (24, 8, and 4 isolates, respectively) accounted for 91.42% (32/35) of Gram-positive isolates. Gram-negative aerobic bacteria constituted ~30% (15/50) of the isolates. Klebsiella pneumoniae has been reported earlier as the predominant isolate in other studies. ^[8],[9]

Out of the MRSA isolates, the sensitivity pattern shows that 79.16% isolates were sensitive to amikacin, 37.5% of the isolates to ciprofloxacin, 12.5 % to amoxycillin + clavulanic acid, and all the isolates (100%) were senitive to vancomycin, linezolid and netilmycin. This is an important finding in our study, MRSA has emerged as the most common pathogen in case of pleural effusion. Even multidrug resistance has been observed in Gram-negative organisms specially in Pseudomonas aeruginosa and Acinetobacter species in which imepenem or meropenem had to be given. One isolate of Acinetobacter species was even resistant to both the carbapenems and the patient expired. From other four patients who expired, MDR strains of gram-negative bacilli were isolated.

The most common underlying factor observed in this study was COPD, which was observed in other studies also. ^{[1],[2],[3],[4],[5]} HIV seropositive was reported by Joseph et al. ^[4] study. In his study, pleural effusion occurred in 59 patients out of 222 patients with AIDS between January 1986-January 1992, and  Mycobacterium tuberculosis s isolated in 5 (8%) of the cases.

Anaerobic organisms either alone or in combination have been reported as causative agents of pleural effusion. ^{[1],[2],[3],[4],[5],[6],[7]} The paucity of anaerobic organisms in our study is notable and probably depends on the improper methods of collection and transportation of the specimens. Similar facts were noted by a study from Porcel et al. ^[1]

In summary, the incidence ofMRSA, MSSA appears to be increasing in Mumbai. Also, MDR Gram-negative organisms are also increasingly being isolated. Judicious use of antibiotics is important to prevent emergence of MDR strains.

Source of Support: None, Conflict of Interest: None

[Table 1]