We report a rare case of postoperative mediastinitis caused by M. hominis after cardiac surgery in an immunocompetent patient. Postoperative mediastinitis caused by M. hominis has rarely been reported, and Le Guern et al. reviewed 17 cases [6]. The median age was 55 years, and all reported patients were male. The median onset of clinical symptoms was 14 days after surgery. These clinical features were consistent with this patient. M. hominis colonizes the human urogenital tract in sexually active adults [1] and the respiratory tract, but this is less frequent [14]. Although a clear origin of the infection is difficult to recognize, it was hypothesized that invasive medical procedures such as urinary catheterization lead to bloodstream invasion of M. hominis and seeding of the surgical site [15]. In addition, recent reports have shown donor-derived M. hominis infection in lung transplant recipients [4]. In this study, the patient was not immunocompromised and urine culture was positive for M. hominis, but the septum was negative. Therefore, M. hominis colonized the urinary tract, and urinary catheterization during surgery might lead to bacteremia and seeding of the surgical site. As M. hominis is frequently found in the human urogenital tract [1] and catheterization is a common procedure during surgery, the possibility of postoperative M. hominis infections could be underestimated.
The identification of M. hominis infection is often challenging due to the slow growth of the colonies and the absence of a cell wall, which gives a negative Gram stain result. Therefore, it is difficult to detect M. hominis using standard microbiological methods without first suspecting them as a cause of postoperative infections. In addition, cardiovascular surgeons are generally not aware of the fact that M. hominis can cause postoperative infection. In this study, the infectious disease expert suspected M. hominis or Ureaplasma spp. because Gram staining and culture of the purulent discharges obtained from the surgical site were negative and sulbactam/ampicillin was not effective. In addition, Gram staining of the abscess and tissue sampled from the surgical site of re-operation was negative. To identify M. hominis, we used urea-arginine LYO2 broth and anaerobically cultured samples on Brucella HK agar plates for 4 days. As it is not popular to use urea-arginine LYO2 broth in clinical microbiological laboratories of general hospitals in Japan, it may be better to extend the culture period under 5% CO2 on blood agar plates or under anaerobic condition on Brucella HK agar plates to identify M. hominis because of its slow-growing nature. The identification of M. hominis is usually performed by 16S ribosomal DNA sequencing, PCR using M. hominis-specific primers, and MALDI-TOF MS [6, 9, 10]. In this study, we used PCR with M. hominis-specific primers and MALDI-TOF MS. Although MALDI-TOF MS has been reported to be a useful tool for the identification of human Mycoplasma species including M. hominis [16], M. hominis has not been included in the clinical use MALDI Biotyper database. In this case, we found that MALDI-TOF MS analysis performed by a MALDI Biotyper system with the reference database MBT Compass library: Ver.9.0.0.0 (8468MSPs) was very useful for the rapid identification of M. hominis. Because of its slow growth, 16S ribosomal DNA sequencing or PCR with M. hominis-specific primers from positive urea-arginine LYO2 broth might be faster than MALDI-TOF MS using bacterial colonies. Previous reports showed that 16S rDNA was sequenced directly from clinical samples and is a good method to identify M. hominis [6]. Further study is needed to evaluate if MALDI-TOF MS can directly identified M. hominis from positive urea-arginine LYO2 broth and positive blood culture bottles, as well as a pure cultured colony.
Postoperative mediastinitis is a major complication of cardiac surgery, with a low incidence but serious consequences in terms of morbidity and mortality [17]. Early diagnosis can lead to the early use of appropriate antimicrobial agents for M. hominis and avoid repeated surgical interventions. However, the identification of M. hominis is challenging under conventional microbiological identification techniques without suspecting them. Therefore, if a patient develops unexplained postoperative fever in cases of otherwise culture-negative infections, particularly if treated with beta-lactam antibiotics, and has a poor response, it is important to consider M. hominis infection as a differential diagnosis. When clinicians suspect M. hominis, they should inform the clinical microbiological laboratory and request to extend the culture period.
Beta-lactam antibiotics are generally used for antimicrobial prophylaxis in cardiac surgery [18]. In addition, broad-spectrum beta-lactam antibiotics and glycopeptides are usually used for empiric treatment of postoperative infection after cardiac surgery targeting gram-positive cocci, including methicillin-resistant Staphylococcus aureus and gram-negative bacilli [17]. However, beta-lactam antibiotics and glycopeptides are not effective against M. hominis because of the absence of a cell wall. M. hominis is generally susceptible to tetracyclines, clindamycin, and fluoroquinolones, but intrinsically resistant to clarithromycin and erythromycin [13]. M. hominis isolates resistant to these antimicrobial agents have also been reported; however, the resistance rate to fluoroquinolone, tetracyclines, and clindamycin of M. hominis varies by report [19,20,21]. In this case, we started levofloxacin after surgical debridement; however, the culture of the intra-vertical hematoma after 7 days of levofloxacin treatment was positive for M. hominis. Although the MIC of levofloxacin determined using the E-test was not high, we suspected that M. hominis was resistant to levofloxacin, and the antimicrobial agent was changed to minocycline. This condition might be associated with the short treatment duration of levofloxacin or residual intra-vertical hematoma. CLSI M43-A indicates that agar disk diffusion is not useful for testing mycoplasmas because there has been no correlation between inhibitory zones and MICs [13]. In addition, the method using the E-test was not mentioned in CLSI M43-A. However, broth microdilution and agar dilution, which are recommended methods to determine MICs by the CLSI M43-A, are not practical in clinical microbiological laboratories in Japanese hospitals. Therefore, the careful observation of clinical course after the administration of antimicrobial agents for M. hominis is needed.