Primary septic arthritis of the knee caused by Neisseria meningitidis serogroup B in an elderly patient. Case report and review of the literature

Primary meningococcal arthritis (PMA) represents an uncommon clinical presentation of meningococcal infection, mainly reported among young people. Herein, a case of PMA of the knee in an elderly patient is described. On January 2022, an 87-year-old patient arrived at hospital with continuous fever persisting for three days and a picture of pain, swelling, redness, and warmth of her left knee. An arthrocentesis was promptly performed and the inoculated synovial fluid turned positive with numerous Gram-negative diplococci at the microscopic examination. The identification of bacteria was done in 48 h using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) MS systems (VITEK®MS—bioMérieux) and standard microbiological procedures (VITEK®2 NH ID card—bioMérieux). Both methods identified the strain as N. meningitidis. The meningococcal isolate belonged to the serogroup B (MenB), Sequence type (ST)-162/clonal complex (cc)162. Two grams of ceftriaxone twice a day were administered for 21 days; than cefditoren pivoxil 400 mg twice a day for further 6 weeks after discharge. In Italy, from 2018 to January 2022, among 135 MenB, 31 MenB/cc162 were identified, of which only the case here reported was associated with an atypical clinical presentation. A total of 41 cases of PMA caused by N. meningitidis was reported in the literature, but only four occurred in elderly. To our knowledgements, no cases of PMA caused by MenB were previously reported among patients of more than 65 years of age.


Introduction
Neisseria meningitidis is a Gram-negative diplococcus causing invasive meningococcal disease. Based on differences of the polysaccharide capsule, N. meningitidis strains are classified into 12 serogroups, of which six are most commonly reported from invasive infections: A, B, C, W, X and Y [1]. Multilocus sequence typing (MLST) is used to categorize N. meningitidis into different sequence types (ST s ) and clonal complexes (cc s ) [2,3].
Neisseria meningitidis is often recovered in the nasopharynx without causing disease, a situation described as asymptomatic carriage, with a proportion of 4.5% in infants to a peak of 23.7% in 19-year-olds and subsequently decreasing in adulthood to 7.8% in over-50s [4]. Invasive 1 3 meningococcal disease is rare but has devastating consequences if not promptly treated. Meningitis and bloodstream infections are the most classic manifestations of invasive meningococcal disease; the highest incidence is in young children, followed by adolescents and young adults [3].
Herein, we report a primary septic arthritis of the knee in an elderly patient caused by N. meningitidis of serogroup B. Moreover, we have conducted a review of the literature about previous confirmed cases of PMA, with a special focus on patient age and serogroups of N. meningitidis isolated from synovial fluids.

Case description
We describe the case of an 87-year-old patient, suffering from hypertension and type 2 diabetes and with a history of stroke 1 year earlier, affected by chronic low back pain and painful bilateral gonarthrosis impairing her walking ability. On January 2022, the patient arrived to the Emergency Department of the Hospital of Pontedera (Tuscany, Italy) for continuous fever persisting for three days and a picture of pain, swelling, redness, and warmth of the left knee. The patient did not complain other symptoms, and on physical examination was found to be in good general condition. The neurological examination was normal, no skin lesions were found. The left knee presented with antalgic posture in flexion, painful for minimal movements; no other joints were involved. Blood tests showed leukocytosis (15,030/mm 3 , neutrophils 85.8%, lymphocytes 4.9%), normal platelets count (191,000/mm 3 ), increased C-reactive protein and procalcitonin levels (19.9 mg/dL and 2.45 ng/ mL, respectively). The diabetes resulted to be effectively controlled (HbA1c 52 mmol/mol). The patient had never had anti-meningococcal vaccination before. The pharyngeal swab culture, screening for syphilis and two couples of blood cultures collected before antibiotic starting, were negative.
An arthrocentesis of purulent synovial fluid was promptly performed before starting antibiotic treatment, and a diagnosis of septic gonarthritis was done. In-line with local epidemiology, pending microbiological results, empirical antibiotic intravenous treatment with ceftriaxone 2 g once daily and vancomycin 1 g twice in daily was started, targeting both Gram-negative and Gram-positive bacteria.
Microbiological analyses (first level investigations) First level microbiological analyses of the synovial fluid were performed at the Microbiology laboratory of the Hospital of Pontedera: the specimen was immediately inoculated into a BacT/ALERT ® PF Plus culture bottle (bioMérieux) and loaded on Virtuo systems, according to the manufacturer's instructions. Unfortunately, the organization of the laboratory did not allow the execution of Gram staining on the synovial fluid upon arrival of the sample.
The synovial fluid turned positive after 11 h 52 min: subcultures were carried out on blood agar plate (BAP) incubated aerobically and in 5% CO 2 , on a chocolate agar plate (CAP) incubated in 5% CO 2 and on a blood agar plate and Schaedler-KV agar plate incubated anaerobically. Microscopic examination of the Gram-stained smear revealed numerous Gram-negative diplococci.
Subcultures were maintained at 35-37 °C: 48 h later, gray, unpigmented, round and smooth colonies were observed on blood agar plate and chocolate agar plate. The identification of bacteria was done using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS systems (VITEK ® MS-bioMérieux) and standard microbiological procedures (VITEK ® 2 NH ID card-bioMérieux). Both methods identified the strain as N. meningitidis. The BioFire ® FilmArray ® Meningitis/ Encephalitis (ME) Panel (bioMérieux) was performed to quickly confirm this identification. The original protocol was changed: synovial fluid was not used for the inoculation of the cartridge, but the sample was aspirated from the enrichment bottle up to the second line of the transfer pipette. The BioFire ® ME Panel (bioMérieux) confirmed the presence of N. meningitidis.
A definitive antibiotic therapy with ceftriaxone 2 g twice per day was administered for a total of 21 days, and rifampicin 600 mg/day was initially added for the first 5 days. A low grade fever persisted for 2 weeks, albeit in progressive reduction, with decreasing levels of C-reactive protein. After 3 weeks from the hospital admission, a magnetic resonance (MR) of the knee showed the persistence of articular collection with inflammatory reaction of the synovia and erosion, geodes and multiple osteochondral lesions at the level of the femoral condyle, patella and tibia. No further arthrocentesis was necessary due to sensible reduction of the joint effusion and pain of the knee. The patient was discharged after 21 days of hospitalization with the recommendation to continue cefditoren pivoxil 400 mg twice daily for 6 weeks. At the follow-up visit 2 months after the discharge, the patient has declared to be in fair condition and to do physiotherapy. At 5 months after the discharge the patient has ended the physiotherapy: she had a complete recovery from the septic arthritis.
Microbiological analyses (second level investigations) The meningococcal strain was sent to the National Reference Laboratory (NRL) of the Istituto Superiore di Sanità (ISS) in Rome (Italy), in the frame of the Invasive Meningococcal Disease (IMD) National Surveillance System (NSS), to perform complete microbiological analyses with a pseudonymous code.
The serogroup of the N. meningitidis isolate was identified by slide agglutination with commercial antisera (Thermo Scientific, Waltham, Massachusetts, US) and confirmed by multiplex PCR [7]. The susceptibility to cefotaxime, ceftriaxone, ciprofloxacin, meropenem, penicillin G and rifampicin was defined by the Minimum Inhibitory Concentration (MIC) test strip method (Liofilchem, Roseto degli Abruzzi, Italy) [8]. The QiAampamini kit (Qiagen, Hilden, Germany) was used for the extraction of chromosomal DNA from an overnight culture.

Genome analyses
The method used for the Whole Genome Sequencing (WGS) on the DNA extracted from the cultivated strain was already described [9]. The genome was uploaded to the PubMLST.org database (http:// pubml st. org/ neiss eria/) [10]. Through the PubMLST platform, the Sequence type (ST), the clonal complex (cc) and the finetype (including PorA and FetA types) were defined [11]. Moreover, the genome was compared with other genomes using the BIGSdb Genome Comparator [12]. Based on a phylogenetic analysis on the core genome MLST (cgMLST), a neighbor-net network was created by SplitsTree4 (version 4.13.1) [13,14].

Review of the literature and discussion
A literature search was performed up to May 2022 through PubMed. The following terms were searched in different combinations: meningococcal disease, Neisseria meningitidis, primary septic arthritis, and primary meningococcal arthritis. Variations of these terms were also searched. The selection was limited to articles written in English and published in peer-reviewed journals only. After deduplication, all authors independently screened titles and abstracts, and finally full texts, to identify all potentially relevant papers, resolving discrepancies through discussion and consultation among them. References of retrieved articles were manually searched to ensure identification of relevant studies not found in the initial literature search. Table 1 shows demographic and clinical characteristics and treatment of patients with primary meningococcal arthritis retrieved from literature.
Forty-one cases of primary meningococcal arthritis , 9 occurring in children, 17 in adolescents (as defined by the World Health Organization as those people between 10 and 19 years of age), 11 in under-65, and 4 in elderly (over-65) were reported. PMA is most frequently identified in children and young adults, in fact 90% of patients are less than 65 years of age, and about 63% were 19 years old or less.
Sixteen cases were due to serogroup C, 8 cases to serogroup B, 6 to W and 5 to Y. No cases of PMA due to MenB were previously reported in elderly, therefore, the case here described seems to be atypical. In fact, among the isolates of MenB/cc162 belonging to the genotypic formula B:P1.22,14:F3-6:ST-162(cc162) isolated in Italy from 2018 to January 2022, only the isolate subject of this study was responsible for an atypical clinical presentation (isolate mono-arthritis); all the other isolates B:P1.22,14:F3-6:ST-162(cc162) were responsible for meningitis and/or septicemia.
The most frequently involved joint (alone or together with others) in the literature is the knee (63.4%) [17, 18, 20, 23, 26, 27, 29, 31, 33, 34, 38-41, 43-49, 51, 52]. Although the direct bacterial invasion of the synovium via the bloodstream is the proposed pathogenetic mechanism for PMA, only 17% of the patients had a positive blood culture [26,33,37,40,46,49]. This could be in part explained by the early start of antibiotic treatment, before obtaining a sample of synovial fluid; however, in our case, all blood cultures turned negative despite being collected before antibiotic treatment and during fever. Bacteremic phase could be very transitory and, as expected, before the localization of bacteria to the joint. Therefore, the atypicality of this case was both the peculiar presentation of a meningococcal invasive disease as an isolated  (3), OFX (2) Yes mono-arthritis, and the age of the patient. Moreover, the B-serotype was very atypical as demonstrated by the totality of cases of PMA described in the literature in subjects under 65 years of age. Actually, in this study, the phylogenetic analysis detected a genetic similarity among the genome of the meningococcus Nm3369, isolated from synovial fluid, and the genomes of meningococci with the same genotypic formula but causing meningitis or septicemia.
The gold standard for identification of the causative microorganism is the conventional culture of joint fluid, but this procedure has low sensitivity and is time-consuming. The advent of MALDI-TOF MS for the routine diagnosis of bacterial infections in clinical laboratories has increased the speed and ease of identification of some fastidious bacteria. This technique has been utilized especially in bloodstream infection; however, its use in the setting of joint infection could be very useful to accelerate   [53,54]. Among the 41 cases of PMA that we retrieved in the literature, in only another case the MALDI-TOF MS has been utilized to confirm the identification of N. meningitidis [30]. We found only some examples of its application in the field of septic arthritis of the native joints [55,56] and periprosthetic infection [57][58][59]. Molecular methods bring significant improvements in diagnostic sensitivity, especially in the case of unusual presentations, such as PMA [60]. With a rapid MALDI-TOF MS identification of colonies grown in subculture and an immediate molecular confirmation obtained running the filmarray, we were able to diagnose, in a secondary-referral hospital, a rare case of PMA in less than 48 h. Consequently, we started an appropriate effective antibiotic treatment. However, a crucial role was played by the reference center, equipped with adequate technologies and know-how, in order to perform the "second level" of microbiological characterization for all the meningococcal isolates including those from atypical anatomical sites. Efficient relationships between first and second level centers should therefore always be guaranteed.
Recently, an increase in atypical presentations due to emerging meningococcal genotypes has been reported [61]. This case report underlines the importance to correctly identify the causative agent of any septic arthritis, to know exactly the microorganism involved and its antibiotic susceptibility, to start a prompt and adequate antibiotic therapy. In this case in particular, we decided to continue with ceftriaxone 4 g/day because, due to the high protein binding, the issue of the right dose of ceftriaxone in case of arthritis has been often addressed. Doubling the dose to 2 g twice a day might be more adequate to overcome the protein binding effect. Moreover, in our case, the higher dose seemed more adequate to guarantee an effective treatment in case of asymptomatic or unrecognized meningeal location [62].
Delayed or inadequate treatment of septic arthritis can cause irreversible joint destruction, disability, or even death: sensibilization of all clinicians (including orthopedists) about atypical etiologies and presentations, turning in an early diagnosis and prompt effective treatment, is therefore crucial.