Portal of entry
Anaerobic bacteremia is almost invariably secondary to a focal primary infection. As reported for adults , the strain of anaerobic organisms recovered depended to a large extent on the portal of entry and the underlying disease. Bacteroides fragilis is the most frequent anaerobic isolate [13, 23,24,25,26,27,28] and, with other members of the B. fragilis group species, accounts for 36–64% of anaerobic blood isolates. Bacteroides thetaiotaomicron is the second most common member of the group to be isolated from blood. Clostridia, especially C. perfringens, and peptostreptococci are also frequently isolated from blood. The gastrointestinal tract accounted for half of the anaerobic bacteremias and the female genital tract was the source of 20% of these bacteremias [13, 27,28,29,30].
Brook  noted in adults that the gastrointestinal tract was the principal source of B. fragilis and clostridial bacteremias and that the female genital tract was the principal source of peptostreptococcal and fusobacterial bacteremias. Redondo et al.  reported that bacteremias caused by the B. fragilis group of organisms originated from: the gastrointestinal tract (69% of bacteremias); soft-tissue wound infections (16%); the female genitourinary tract (5%); and lung infections (4%). Fainstein et al.  found bacteremia caused by B. fragilis to be common in patients with genitourinary and gynecological tumors, acute leukemia, and gastrointestinal malignancies.
The probable portals of entry for the blood culture isolates in the 28 children studied by Brook and associates  were: the gastrointestinal (GI) tract (13 patients), the respiratory tract (ear, sinus, and oropharynx, seven), the lower respiratory tract (three), cardiovascular shunts and neurologic shunts (three), and skin and soft tissue (three). When the GI tract was the probable portal of entry, Bacteroides sp. (eight isolates, including five B. fragilis) and Clostridium sp. (four isolates) were the organisms most frequently recovered from blood. The predominant anaerobic organisms recovered in association with infections of the ear, sinus and oropharynx were Peptostreptococcus sp. (from four patients) and F. nucleatum (from two patients). Propionibacterium acnes was grown in cultures taken from four patients, three of whom had artificial cardiac valves or ventriculoatrial shunts. Two of these patients also were initially observed to have meningitis caused by a similar organism. All lower respiratory tract infections that served as a probable source of bacteremia were caused by isolates belonging to the B. fragilis group.
No obvious focus of infection was noted in six patients; interestingly, however, all of these patients had some GI problem that might have served as a source of the bacteremia. Furthermore, four of these patients had bacteremia caused by Clostridium species.
These findings therefore support studies of adults [13,32,33] and children [6,14] that report that Bacteroides species, including the B. fragilis group, were the predominant isolates from patients in whom the GI tract was the probable portal of entry. As summarized by Sanders and Stevenson , however, other anaerobic Gram-negative bacilli caused bacteremia in children with otitis media and abscesses.
The ear, sinus, and oropharynx were found to be possible portals of entry that predisposed patients to bacteremia with Peptostreptococcus sp. and Fusobacterium sp. This is not surprising because these organisms are part of the normal flora of these anatomic sites and can be involved in local infections .
Three newborns developed bacteremia in conjunction with pneumonia with organisms belonging to the B. fragilis group . This has also been noted before in newborns  and adults . Although Bacteroides accounted for the majority of the episodes of bacteremia in this study, other studies have shown relatively infrequent isolation of these organisms from children , except during the neonatal period .
An association between surgical procedures and anaerobic septicemia was recently reported. Pass and Waldo  observed anaerobic bacteremia in two infants following suprapubic bladder aspiration. Bacteroides fragilis was isolated in one instance and in another instance was mixed with Veillonella alcalescens. An accidental bowel perforation was the assumed etiology of these infections. Kasik et al.  observed sepsis and meningitis caused by E. coli and Bacteroides sp. after anal dilatation. Fusobacterium mortiferum was also recovered in the blood.
Fisher et al.  described bacteremia caused by B. fragilis in four of 75 children after elective appendectomy in renal transplant recipients. The bacteremia was associated with profound lymphopenia. Fusobacterial infection generally is associated with otolaryngological processes. Seidenfeld et al.  reported five patients, four of whom were children, who developed F. necrophorum septicemia following oropha-ryngeal infection. Septicemia caused by Streptococcus mor-billorum was reported by Rushton to have complicated herpetic pharyngitis .
Bacteroides fragilis, anaerobic Gram-positive cocci, and Fusobacterium sp. were the clinically significant anaerobic organisms most commonly isolated from blood cultures in three recent studies [2,3,4]. Most of the patients described in these studies were either newborns or were over 6 weeks of age and suffered from chronic debilitating disorders such as malignant neoplasms, immunodeficiencies, chronic renal insufficiency, or decubitus ulcers and carried a poor prognosis. Bacteroides sp. were also isolated frequently after perforation of viscus and appendicitis [38,39].
Clostridium sp. may complicate leukemias. Caya et al.  reported 11 children with leukemia who presented with sepsis caused by Clostridium septicum (seven children), C. perfringens (two children), and Clostridium sp. (two children). None of these children survived the sepsis, which was characterized by thrombocytopenia, gastrointestinal lesions, and neutropenia.
Infectious mononucleosis can also predispose to anaerobic bacteremia. Dagan and Powell  observed three patients who developed postanginal anaerobic sepsis following Epstein–Barr virus infection. All three had Fusobacterium species isolated (two were F. necrophorum) and in one case a Peptostreptococcus was also recovered.
Predisposing factors to anaerobic bacteremia in adults include malignant neoplasms [42,43], hematologic disorders , transplantation of organs , recent gastrointestinal or obstetric gynecologic surgery [43,44,46], intestinal obstruction , diabetes mellitus , post-splenectomy , use of cytotoxic agents or corticosteroids , and use of pro-phylactic antimicrobial agents for bowel preparation prior to surgery [43,46].
Predisposing conditions were noted also in one study of pediatric patients . Two patients had malignant neoplasms, two suffered from hematologic abnormalities, and one had an immune deficiency. Interestingly, 82% of the bacteremias in this series of patients  occurred in children who had no immunosuppression or malignant neoplasms. This is in contrast to another study  in which anaerobic bacteremia occurred more frequently in children with these predisposing factors. Dental or oral surgery can also predispose to anaerobic bacteremia in adults and children [13,15,16].