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Descending necrotizing mediastinitis associated with Lactobacillus plantarum
© Nei et al.; licensee BioMed Central Ltd. 2013
Received: 28 January 2013
Accepted: 28 August 2013
Published: 29 August 2013
Descending necrotizing mediastinitis (DNM), a severe infection with a high fatality rate, develops in mediastinal spaces due mainly to deep cervical abscesses. The majority of causative microbes of DNM are Streptococci and oral anaerobes. DNM associated with Lactobacillus-infection is rather rare.
A 69-year-old male with an unremarkable past medical history was referred to our hospital for surgical resection of advanced laryngeal cancer. Full examination revealed a neck abscess and DNM with a background of untreated diabetes mellitus. Initially, he was treated with meropenem. However, Lactobacillus plantarum was isolated from surgical drainage of a mediastinal abscess. Despite using antibiotics capable of eradicating all isolates with susceptibilities not differing significantly from those of the neck and mediastinal abscesses, we attributed DNM to the L. plantarum detected only in the mediastinal abscess. After DNM treatment, he underwent total pharyngolaryngectomy with bilateral neck dissection followed by reconstruction using free jejunum. He was discharged fully recovered.
We concluded that L. plantarum as the sole cause of the mediastinal abscess in the present case cannot be ruled out. As the number of immunocompromised patients increases, we should be cautious regarding this “familiar” microbe.
Infectious mediastinitis is a life-threatening though rare intrathoracic infection. One of the severe forms of this infection is descending necrotizing mediastinitis (DNM), which is characterized by diffuse necrosis that occurs as a complication of deep cervical infection or an esophageal disorder spreading along the deep fascial planes into the mediastinum [1, 2]. The main etiological factors of DNM are oropharyngeal abscesses. However, fluid collections, abscesses, cellulitis and necrosis are among the local changes and lesions observed. DNM often has a fulminant course, rapidly progressing to sepsis and frequently death. Even in this era of appropriate surgical interventions and highly effective antibiotics, DNM still carries high mortality rates of 10~40% . The majority of causative microbes of DNM are the same as those of oropharyngeal infections; i.e., Streptococci and oral anaerobes . The majority of reported DNM cases had polymicrobial infections including both aerobes and anaerobes, reflecting the indigenous microbiological flora of oral or pharyngeal sites. DNM associated with Lactobacillus-infection was previously described , and L. catendforme and L. jensenii were reportedly isolated with Streptococci and oral anaerobes from mediastinal pus obtained surgically.
Lactobacilli are Gram-positive bacilli that are usually innocuous and exist in daily living environments as probiotics worldwide. They also constitute normal flora of the human vagina, oropharynx, and gastrointestinal tract . However, Lactobacilli can cause infectious diseases, such as bacteremia  and endocarditis [6–8], as well as splenic [9, 10] and hepatic abscesses , in humans. Moreover, Lactobacilli have emerged as pathogenic microbes in both immunocompetent and immunocompromised hosts [12, 13].
L. plantarum has also been regarded as a probiotic, as have L. caseii and L. rhamnosus. They are both commonly isolated as harmless environmental microorganisms, and are used for food fermentation . There are an especially large number of traditional fermented foods in Japan, aside from cheese and yogurt, i.e., Natto and Nuka-zuke (a type of pickle). However, Lactobacilli have recently been identified as potential emerging infectious microorganisms in immunocompromised patients, especially those with cancers and receiving chemotherapy or those with impaired glucose metabolism, including diabetes mellitus, and corticosteroid-treated patients.
A 69-year-old male with no past history of major illness was referred to our hospital for surgical resection of advanced laryngeal cancer. He had a fever (over 38.5°C) and neck swelling at the first visit to our hospital. Emergency computed tomography (CT) of the neck showed a tumor-like lesion and an abscess in the anterior neck region. This lesion was near the thyroid gland. He underwent surgical incision and drainage, and levofloxacin (500 mg/day) was administered for 5 days after the first hospital visit. After incision and drainage, microbiological culture including anaerobic studies of the abscess showed Fusobacterium necrophorum, Prevotella melanigenica, and Streptococcus anginosus/milleri groups. We planned radical surgery for the laryngeal cancer, but prioritized treatment of the abscess.
Though the isolates were identified as L. casei employing conventional biological techniques, we accurately identified the strain with 16 s ribosomal RNA genotyping, as previously described , and a similarity search was conducted using the BLAST program (DDBJ, Shizuoka, Japan). The results (1,501 bp; GenBank accession no. AB755630) showed 100% similarity to the reference strain of Lactobacillus pentosus (GenBank accession no. AJ292254) and Lactobacillus plantarum (GenBank accession no. AL935263). We finally confirmed the identification of L. plantarum using the recA sequence . The results (313 bp; GenBank accession no. AB755631) showed 100% identity with L. plantarum (GenBank accession no. AL935263) [similarity to L. pentosus (GenBank accession no. AJ292254)], ultimately confirming L. plantarum infection.
The origin of the abscess in our patient was difficult to identify, but we suspect the most likely source to have been his oropharyngeal flora. Mediastinal abscess formation is known to be closely related to cervical abscess development and in this case appeared to be comprised of polymicrobial infectious organisms resembling isolates from the neck abscess. It was reasonable to consider the infectious source in this case to most likely have been the oropharyngeal lumen in the view of both the neck abscess and DNM. If so, in this case, the mediastinal abscess was due not only to L. plantarum but also to F. necrophorum, P. melanogenica, and Str. anginosus/milleri groups, cultured from the neck abscess before the diagnosis of DNM. However, only L. plantarum was obtained from the mediastinal abscess. No L. plantarum was isolated from the neck abscess. It was unclear why only L. plantarum was isolated from the mediastinal abscess by direct puncture via thoracic surgery.
We suspected that the phenomenon observed in this patient was attributable to the pharmacodynamics of antibiotics. We initially treated the neck abscess with meropenem, which effectively eradicated all of these isolates. Though we did not measure the MICs of neck isolates, according to previous records, the susceptibilities of organisms from the neck cultures to meropenem were (MIC90, μg/ml): F. necrophorum 0.25 , P. melanogenica 0.25 , and Str. anginosus /milleri groups 0.12 . The MIC90 and MIC of L.plantarum obtained from thoracic surgery (0.5) did not differ significantly. If there was no difference in antibiotic shift into tissue between the neck and mediastinal abscesses, L. plantarum would also be eradicated, along with the other organisms isolated, by meropenem. However, only L. plantarum was isolated from the surgical specimen, despite using the same culture method as that employed for the neck abscess. Though the majority of DNM cases are attributed to polymicrobial infection and we obtained other isolates which may have been involved in DNM from the neck site, we concluded that L. plantarum alone may have been responsible for the present mediastinal abscess.
On the other hand, lactate production and the resulting pH decreasing are powerful antimicrobial activities of Lactobacillus spp. against other microorganisms. Among these secies, L. plantarum can produce not only lactate but also hydrogen peroxide [19, 20], providing protection from manganese catalase but not hem catalase . We thus speculated that a high concentration of hydrogen peroxide produced by L. plantarum inhibited the growth of other microorganisms in mediastinal lymph nodes.
We experienced a case of DNM case associated with an extraordinarily rare microbe, L. plantarum. The present case underscores the importance of being aware of the possibility of Lactobacilli as pathogenic microbes. Furthermore, as the number of immunocompromised patients increases, we should be cautious regarding this “familiar” microbe. (1642 words).
Written informed consent was obtained from the patient for publication of this Case Report and any accompanying images. A copy of the written consent form is available for review by the Editor of this journal.
We thank Bierta Barfod for editing the manuscript, Toshie Sekine for secretarial assistance and Yohei Washio and Ayaka Tashiro for the microbiological cultures. We have no financial relationships with any commercial entity with an interest in the subject of this manuscript.
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