Identification of sacrococcygeal and pelvic abscesses infected with invasive Mycoplasma hominis by MALDI‐TOF MS

Abstract Background Mycoplasma hominis is the smallest prokaryotic microorganism with no cell wall, high pleomorphism, and slower reproduction than bacteria. It is difficult for clinical technicians to find M. hominis through the negative Gram staining of specimens. Therefore, it is likely to miss detection in routine clinical smear etiological examination. M. hominis is generally considered to be a common colonizing bacterium in urogenital tract with low pathogenicity, and it is usually difficult to invade submucosal tissue and enter the bloodstream. Methods The abscesses of the patient were examined histopathologically, and the pus in the abscesses was extracted for etiological examination. MALDI‐TOF MS was used to identify and confirmed the pathogens in the specimens. The commercial Mycoplasma isolation, culture, and drug sensitivity kit was used to determine antibiotic susceptibility. Results No pathogens were found after pathological and smear microscopic examination of the puncture fluid from the sacrococcygeal and pelvic abscesses. Until 48 h later, small, translucent, and gray‐white colonies were observed in the blood plate culture results. The laboratory physician ultimately determined that the pathogen was M. hominis by MALDI‐TOF MS. Conclusion We report a case of extra‐urogenital cystic abscesses infected by M. hominis, in order to improve clinicians’ comprehensive understanding of the pathogenicity of Mycoplasma. In addition, the clinical laboratory technician should pay attention to the role of Wright–Giemsa staining of puncture fluid smear in the preliminary detection and the application of MALDI‐TOF MS in identification of uncommon pathogenic microorganisms.


| INTRODUC TI ON
Mycoplasma hominis (M. hominis) is a common colonization bacterium in the urogenital tract, it can be universally isolated from sexually mature women, approximately 21% to 53% of asymptomatic women are colonized with M. hominis, and the colonization rate of male urethra can be as high as 20%. 1,2 Under certain conditions, M. hominis can cause urogenital tract infections such as pelvic inflammation and cervicitis, but its pathogenicity is weak. Generally, it only causes genital tract mucosal surface infection and does not invade tissues and blood. 3,4 In this case, the special clinical manifestations of extraurogenital cystic abscesses caused by M. hominis infection were identified by flight mass spectrometry rather than routine Gram staining and colony morphology.

| Patient and basic clinical information
A 70-year-old man with more than half a year's history of an oval cystic abscess in sacrococcygeal region presented to the outpatient department with increasingly unbearable pain and fidgety inconvenience. High-resolution computed tomography (HRCT) of the sacrococcygeal vertebrae showed two connected oval cystic lesions involving in subcutaneous tissue and pelvic cavity ( Figure 1). The surgeon performed resection of sacrococcygeal and pelvic abscesses at the request of the patient. The incision was about 7 cm long along the long axis of the cyst, the skin and subcutaneous tissue were cut in turn, the surrounding adhesive tissue was separated along the surface of the cyst, the cyst was completely stripped, the root was ligated with No. 4 silk thread, continued to separate downward, the pelvic abscess of about 40 × 30 mm was stripped, the subcutaneous fascia was free, sutured intermittently, and the incision was closed layer by layer.

| Routine smear etiological examination of histopathological and Gram staining
Both histopathological examination and Gram staining results of the specimens revealed numerous polymorphonuclear leukocytes with no visible pathogens ( Figure 2). Pinpoint-sized translucent and gray-white colonies were observed on blood agar following 48 h of incubation under 5% CO 2 at 37 °C ( Figure 3A). Microorganisms dyed lavender of different shapes were observed by Wright-Giemsa staining under a 100× oil microscope ( Figure 3B). VITEK 2-compact automatic bacterial detection and analysis system (BioMérieux), one of the most commonly used identification methods of bacteria and Candida in clinic, was failed in this detection.

| MALDI-TOF MS identification and Wright-Giemsa staining results
MALDI-TOF MS was used to identify and confirmed that the pathogen was M. hominis with the confidence of 99. 9% (database VITEK MS IVD 3.0 and SARAMIS V4.14) ( Figure 3D). Wright-Giemsa staining results of the puncture fluid of the abscesses showed that the existence of the microorganisms of different shapes was confirmed again ( Figure 3C).

| Antimicrobial susceptibility of the isolated strain
The commercial Mycoplasma (Ureaplasma urealyticum / M. hominis) isolation, culture, and drug sensitivity kit (Zhuhai DL) was used to determine antibiotic susceptibility by using the Clinical and Laboratory Standards Institute (CLSI) breakpoints. 5 The isolate was susceptible to doxycycline, erythromycin, minocycline, and josamycin, while resistant to ciprofloxacin, clindamycin, tetracycline, levofloxacin, F I G U R E 1 High-resolution computed tomography (HRCT) of the sacrococcygeal vertebrae showed oval cystic lesions involving in subcutaneous tissue and pelvic cavity. Figure A and Figure B represented the location and size of the abscess under the lateral position and frontal position, respectively. There was an oval cystic lesion near the fifth cone of sacrum and subcutaneous tissue, respectively, and the two lesions were connected. The size of the larger abscess was about 50 × 30 mm, and the size of the smaller abscess was about 40 × 30 mm. White arrow indicated the abscesses ofloxacin, roxithromycin, sparfloxacin, and azithromycin (Table 1).
After 3 days of anti-infection treatment, the patient's condition improved significantly and was discharged after expert evaluation.  Figure A and Figure B represented histopathological findings of pelvic and sacrococcygeal abscesses, respectively. The appearance of pelvic abscess was grayish red, the size was 40 × 30 × 14 mm, the section was cystic, the capsule wall thickness was 1 mm, the fibrous capsule wall tissue was accompanied by multifocal lymphocytes infiltration, part of the capsule wall tissue was covered with squamous epithelial cells, the inner wall of the capsule was smooth, and no obvious vegetations were found. The appearance of the sacrococcygeal abscess was grayish red, with the size of 52 × 32 × 25 mm and a cystic appearance in the section, and a brick-red body in the capsule. The capsule wall thickness was 5 mm, locally calcified. Some cystic wall tissues were covered with squamous epithelial cells, and a small amount of keratinocytes and foam cells could be seen. Figure

ACK N OWLED G M ENTS
We thank all members of the microbiology laboratory of Zhejiang Provincial People's Hospital for their help in the collection of clinical data and thank the patient for his cooperation.

CO N FLI C T O F I NTE R E S T
The authors declare that they have no competing interests.

AUTH O R S' CO NTR I B UTI O N S
FS collects the patient clinical information. JWZ, YZZ, and HYL analyzed the data. YMG drawn the manuscript. All authors read and approved the final manuscript.

CO N S E NT FO R PU B LI C ATI O N
Written and informed consent was obtained from the patient for publication of this Case Report and any accompanying images.