Staphylococcus aureus, Staphylococcus epidermidis, Enterobacteriaceae, Enterobacter cloacae, Enter-ococcus spp., Pseudomonas spp., Proteus spp., Haemophilus spp., Corynobacterium xerosis, Mycoplasma spp., nontuberculous mycobacterium, and Nocardia, Aspergillus, and Candida spp. are the predominant pathogens recovered after cardiovascular surgery (12,13). These organisms can also be recovered mixed with anaerobic bacteria whenever polymicrobial infection is present. Histoplasmosis and tuberculosis are the most common identifiable causes of chronic mediastinitis (Table 1).

The major bacteria recovered from infections originating from the oral flora are Group A streptococci and oral anaerobic bacteria. These include pigmented Prevotella and Porphyromonas, Fusobacterium, and Peptostreptococcus spp. (14). There are also a few cases that reported involvement of Bacteroides fragilis group (15).

The role of anaerobic bacteria in mediastinitis was not established by prospective studies and the data in the literature is based mostly on several case reports.

Ferzil et al. (16) reported a 17-year-old female who developed anaerobic mediastinitis that complicated infectious mononucleosis. They recovered S. aureus, Streptocococcus constellatus, Streptocococcus milleri, and Prevotella melaninogenica. Several reports described the concomitant recovery of Clostridium spp. including Clostridium perfringens in mediastinitis secondary to esophageal perforation (17). Guardia et al. (7) reported a case of fatal necrotizing mediastinitis secondary to acute suppurative parotitis. The infection was the result of synergistic necrotizing cellulitis caused by mixed aerobic and anaerobic bacteria. The causative isolates were E. coli, Enterococcus Escherichia spp., B. fragilis, C. perfringes, P. melaninogenica, and Candida albicans. Isaacs et al. (18) reported a case of a 34-year-old woman with an upper respiratory infection who developed a para, retropharyngeal, and mediastinal abscesses. Peptostreptococcus and Bacteroides spp. were isolated from the infected sites.

Murray and Finegold (14) reported two cases of anaerobic mediastinitis and summarized the literature that included additional 18 cases reported between 1930 and 1981. The predominant origin of the infection in these patients were odontogenic (in 7 instances),

TABLE 1 Predominant Organisms Recovered from Mediastinitis

Aerobic bacteria Staphylococcus aureus Staphylococcus epidermidis Streptococcus pyogenes Microaerophillic streptococcus Enterococcus spp. Haemophilus spp. Enterobacteriaceae Enterobacter cloacae Klebsiella pneumoniae Pseudomonas spp. Proteus spp.

Corynobacterium xerosis Anaerobic bacteria Peptostreptococcus spp. Clostridium spp. Bacteroides spp.

Pigmented Prevotella and Porphyromonas spp. Fusobacterium spp. Other organisms Mycoplasma spp. Nontuberculous mycobacterium Fungi Nocardia spp. Aspergillus spp. Candida spp. Histoplasma spp.

oral abscess (in 3), pleural fluid (in 2), and trauma (in 2). Polymicrobial flora was found in all but one case and the predominant anaerobic bacteria isolated from these patients were Bacteroides, Peptostreptococcus, pigmented Prevotella, and Fusobacteeium spp. Moncada et al. (19) reported five cases of mediastinitis caused by anaerobes, originating from odontogenic and deep cervical infections; two of these were in children.

Wheatley et al. (20) reported two cases of descending necrotizing mediastinitis due to anaerobic bacteria, in which infection arising from the oropharynx spreads to the mediastinum. They also reviewed the English language literature on this disease from 1960 to 1990 summarizing 43 additional cases. Polymicrobial aerobic-anaerobic flora was present in 30 of the 36 (83%) cases where the microbiological result was given; anaerobes only in one (3%) and aerobes alone in five (14%).

Brook and Frazier studied the microbiologic and clinical characteristics of 17 adults with mediastinitis (21). Aerobic or facultative bacteria were present only in three patients (18%), anaerobic bacteria only in seven (41%), and mixed aerobic-anaerobic flora in seven (41%). There were a total of 42 isolates, 13 aerobic or facultative, and 29 anaerobic bacteria, an average of 2.5/specimen. Anaerobic bacteria predominated in infections that originated from esopha-geal perforation and orofacial, odontogenic, and gunshot sources. The predominant aerobes were alpha-hemolytic Streptococcus (3 isolates), S. aureus (2), and Klebsiella pneumoniae (2). The predominant anaerobes were Prevotella and Porphyromonas spp. (8), Peptostreptococcus spp. (7), and B. fragilis group (3). This study highlights the polymicrobial aerobic-anaerobic nature of mediastinitis.

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