A Case Report of Wound Infection Caused by Mycobacterium porcinum

Introduction

Mycobacterium porcinum, a rapidly growing non-tuberculous mycobacterium, was classified from the Mycobacterium fortuitum group. These mycobacteria are distinguished by their ability to form visible colonies within 3 to 5 days of incubation, with the majority exhibiting robust growth within the first week.¹ They are omnipresent in various environmental niches, including water sources, soil, and carriage in animals, and can be transmitted through aerosols, dust particles, or via contact with contaminated water and soil, classifying them as opportunistic pathogens.² The conventional bacterial culture methods frequently fail to isolate these organisms, potentially leading to misdiagnoses or oversights, and their management necessitates an extended course of anti-infective therapy. In addition to conventional antibiotic therapy, surgical debridement plays a crucial role in the treatment of rapidly growing mycobacterial wound infections.³ A literature review revealed that naturally derived multifunctional hydrogels can be applied to wound sites to modulate the inflammatory microenvironment and macrophage polarization, thereby achieving anti-inflammatory and antibacterial effects while promoting tissue repair.⁴ Robot-assisted debridement is an emerging therapeutic modality for cavitary and luminal infectionsThis article reports a case of wound infection caused by Mycobacterium porcinum in a diabetic patient, investigating the infection mechanisms and wound healing approaches to emphasize the critical importance of accurate diagnosis and appropriate treatment strategies in clinical practice.

Case Presentation

A 64-year-old female was admitted to the cardiovascular surgery department on July 5, 2022, presenting with symptoms of shortness of breath, palpitations, and fever following activity for over 10 days. Her admission vital signs included a temperature of 36.5°C, a heart rate of 83 beats per minute, a respiratory rate of 22 breaths per minute, and a blood pressure of 106/59 mmHg. The patient was conscious, displayed no signs of distress, had no jugular venous distension, and exhibited moist rales at the lung bases. There was no cardiac apical lift, the heart rhythm was irregular, and a grade III systolic murmur was auscultated at the left apical region; no cardiac murmurs were detected on the right side. The abdominal examination revealed a soft and non-tender abdomen, and there was no edema in the lower limbs. Chest X-ray imaging indicated bilateral pneumonia, and echocardiography confirmed the diagnoses of infective endocarditis, severe non-rheumatic mitral valve insufficiency, and left ventricular enlargement. The admission diagnoses included infective endocarditis, severe non-rheumatic mitral valve insufficiency, left ventricular enlargement, class IV heart function, acute heart failure, type 2 diabetes mellitus, and chronic HBV carrier status.

Laboratory tests: Blood gas pH 7.50↑, oxygen partial pressure 126 mmHg↑, bicarbonate concentration 30.5 mmol/L↑, white blood cell count 9.44×10^9/L, neutrophil percentage 68.9%, international normalized ratio (INR) 1.05, fibrinogen 6.03 g/L↑, D-dimer quantification 1.61 mg/L, high-sensitivity troponin T 0.006 ng/mL, high-sensitivity troponin I 0.004 ng/mL, erythrocyte sedimentation rate 70 mm/h by the Westergren method, procalcitonin 0.041 ng/mL, N-terminal pro-B-type natriuretic peptide (NT-proBNP) 808 pg/mL↑, glycated hemoglobin (HbA1c) 7.64%↑, anti-SS-A antibody/Ro52KDa weakly positive. Bilateral double sets of blood cultures were drawn immediately upon admission, and the results were negative.

On July 15, the patient underwent mitral valve bioprosthesis replacement and temporary cardiac pacemaker implantation. Postoperative examination on the first day revealed a clean surgical wound dressing. Five days post-surgery, the patient exhibited a low-grade fever, with temperatures ranging between 37.5°C and 38.3°C. Laboratory investigations showed a leukocyte count peaking at 14.15 × 10^9/L and a neutrophil percentage at 91.9%. Procalcitonin (PCT) levels peaked at 1.88 ng/mL, indicating a systemic inflammatory response. Despite negative sputum and blood cultures, the patient was treated with ceftazidime (1g, Q12h) from July 6 to August 30, and vancomycin (1g, Q12h) from July 17 to July 25 for anti-infective therapy. On August 11, the patient reported wound pain and a slight decrease in appetite. On August 13, dark red serous discharge was noted from the upper part of the wound (Figure 1), prompting wound opening for drainage. Subsequent laboratory tests revealed an ultra-sensitive C-reactive protein level of 129.19 mg/L, a leukocyte count of 9.91 × 10^9/L, and a neutrophil percentage of 76.7%. Ceftazidime was continued for anti-infective purposes, and albumin was administered due to hypoalbuminemia (28.7 g/L). On August 21, the patient presented with fat liquefaction at the mid-segment wound, which was managed by draining the liquefied fat and performing bedside debridement and suture. On August 23, bone wax fragments were visible in the wound effusion, and the effusion was sent to the microbiology lab for pathogen culture. On August 23, Gram staining was performed on the wound exudate to identify bacteria and fungi, with no predominant organisms observed. The exudate was cultured on Columbia sheep blood agar and MacConkey agar and incubated at incubator, with daily observations. On the fourth day of incubation, small, rough, white, dry colonies were observed on the Columbia sheep blood agar. The semi-quantitative colony count result was 3+. (Figure 2) Gram staining revealed Gram-positive bacilli under optical microscopy at 1000x magnification,3+ semi-quantitative (Figure 3). Acid-fast staining under similar conditions demonstrated red-stained bacilli,3+ semi-quantitative (Figure 4). The strain was sent to a third-party testing company (Guangzhou KingMed Diagnostics Company Limited) for identification. 16sRNA gene sequencing was employed to identify the colonies cultured on Columbia sheep blood agar, and the result was identified as Mycobacterium porcinum. Additionally. The sequencing results were compared using the BLAST algorithm, showing a 100% similarity to the reference sequence of Mycobacterium porcinum strains.

Figure 2 After enrichment, the morphology of colonies on Columbia agar plates can be observed after 4 days of cultivation.

Figure 3 Microscopic appearance of Mycobacterium porcinum in pure culture (Gram staining, ×1000).

Figure 4 Microscopic appearance of Mycobacterium porcinum in pure culture (acid-fast staining, ×1000).

After learning that the wound was infected with Mycobacterium porcinum, the clinicians, in accordance with the Sanford Guide to Antimicrobial Therapy⁵ and referring to the treatment regimen for sporadic mycobacterial infections, initiated anti- Mycobacterium porcinum therapy on August 30th. The regimen included a combination of three drugs: amikacin (0.4g,QD), azithromycin (0.5g,QD), and moxifloxacin (0.4g,QD), with a treatment course of at least 1 month. Concurrently, debridement of the wound was performed. After the initiation of the treatment, the white blood cell count was monitored at 11.25×109/L, with a neutrophil percentage of 68.9% and a procalcitonin (PCT) level of 0.053 ng/mL. The wound improved following the antimicrobial therapy, and the patient was discharged.

As shown in Figure 5, the patient was treated with ceftazidime from August 13 to August 30, during which body temperature remained stable However, the white blood cell (WBC) count and neutrophil percentage showed an increasing trend, while high-sensitivity C-reactive protein (hs-CRP) levels remained abnormally elevated. On August 29, the patient suddenly developed chills and high fever, with body temperature spiking to 39.5°C, indicating that ceftazidime was only partially effective in controlling the infection. When the bacterial load increased and disrupted the immune balance, Mycobacterium porcinum triggered a robust immune response. Subsequent clinical evaluation confirmed that M. porcinum infection requires prolonged combination therapy due to its extended recovery period.

Figure 5 The changes in infection indicator levels throughout the patient’s disease course.

Discussion and Conclusion

Rapidly growing mycobacteria are omnipresent in various environmental niches, exhibiting natural resistance to a spectrum of disinfectants, heavy metals, and antibiotics. This resistance is attributed to their lipid-rich cell walls, which facilitate their survival in aquatic systems. These mycobacteria can endure on both natural and artificial surfaces owing to their capacity to form protective biofilms, shielding them from detrimental external factors.^6–10 The biofilm produced by Mycobacterium spp. encapsulates bacterial cells in a sophisticated extracellular matrix, which acts as a barrier against antibiotic penetration. Studies have revealed that certain mycobacteria exhibit susceptibility to antibiotics such as amikacin and clarithromycin in their planktonic state, but their antibiotic activity is significantly diminished when these bacteria reside in a biofilm state.^11,12 The mechanisms underlying this reduction in antibiotic efficacy may involve altered metabolic states, activation of resistance genes (eg, inducible methyltransferases), and the formation of persister cells.^13–15 Bacteria embedded within biofilms can survive antibiotic treatment and resume proliferation upon cessation of therapy, leading to recurrent infections.¹⁶ These mycobacteria can endure on both natural and artificial surfaces owing to their capacity to form protective biofilms, shielding them from detrimental external factors.¹⁷

Mycobacterium porcinum, initially isolated from the lymph nodes of pigs exhibiting tuberculous-like infections in 1973¹⁸ was formerly classified within the Mycobacterium fortuitum group. Schinsky et al¹⁹ re-evaluated certain strains previously identified as the third biovariant of Mycobacterium fortuitum using molecular techniques, distinguishing Mycobacterium porcinum from other rhamnose-negative strains within the complex and identifying it as a human pathogen for the first time in 2004. This Gram-positive bacillus exhibits red staining with both acid-fast and weakly acid-fast methods. It is capable of growth on blood culture medium at temperatures of 28°C, 37°C, and 42°C after three days of incubation.²⁰ Detection of Mycobacterium porcinum in tap water samples has been reported.²¹ Unlike other mycobacterial species, it has not been linked to outbreaks or pseudo-outbreaks.¹⁹ Mycobacterium porcinum is known to cause infections involving the skin, soft tissue, respiratory tract, bone marrow, and catheters.^20–23

The pathogenesis of Mycobacterium porcinum infection is primarily attributed to cell-mediated immunity and delayed-type hypersensitivity reactions mediated by CD4+ T cells. Various cytokines, including interferon-γ, interleukin-12, and tumor necrosis factor-α, are implicated in the inflammatory response, leading to granuloma formation and potentially resulting in tissue necrosis and cavity formation.^24–27 There is evidence in the literature suggesting that the presence of anti-IFN-γ autoantibodies is a significant risk factor for extrapulmonary non-tuberculous mycobacterial (NTM) infections.²⁸

In studies investigating host-microbiome interactions at the transcriptomic level, emerging research has demonstrated that the host may regulate microbial metabolism and immune responses through immunoglobulin-like domain-containing proteins as well as ITIH2, PAEP, and TDRD9 factors.²⁹ Notably, host gene expression has been shown to reciprocally influence microbiota composition, with significant associations observed between specific bacterial abundances and human genomic variations.³⁰ Chronic hyperglycemia in diabetic patients impairs immune function through multiple mechanisms, significantly compromising host defenses against rapidly growing mycobacteria (RGM). The hyperglycemic microenvironment suppresses neutrophil chemotaxis, phagocytic activity, and bactericidal capacity³¹ while impairing macrophage antigen presentation, thereby reducing pathogen clearance efficiency.³² Furthermore, diabetes disrupts the critical balance between pro-inflammatory (eg,TNF-α,IL-6) and anti-inflammatory cytokines (eg, IL-10), resulting in immune dysregulation that hinders effective RGM elimination.^33,34 Notably, T-cell dysfunction – particularly in CD4+ and CD8+ T cell populations – further diminishes immune surveillance and compromises the host’s ability to control RGM infections.^35,36

A review of recent case reports on Mycobacterium infections revealed only three documented cases of Mycobacterium porcinum infection, with the remainder involving other mycobacterial species. The literature indicates that the majority of affected patients are middle-aged or elderly individuals, with infections predominantly occurring in immunocompromised hosts with underlying conditions such as diabetes mellitus and end-stage renal disease (ESRD). However, cases have also been reported in immunocompetent hosts following invasive procedures (eg, cosmetic injections). All cases necessitated multidrug regimens, typically including macrolides (eg, clarithromycin), aminoglycosides (eg, amikacin), and fluoroquinolones, with dynamic adjustments based on antimicrobial susceptibility testing. Although clinical improvement was achieved in most patients, treatment failures were observed, such as in cases of catheter-related Mycobacterium abscessus bacteremia. When cultures were negative, molecular diagnostic techniques like 16S rRNA sequencing played a pivotal role in diagnosis and management. Patients with multiple comorbidities who developed disseminated nontuberculous mycobacterial (NTM) infections had poorer prognoses. The patient in this study had diabetes mellitus and underwent cardiac surgery, rendering him immunocompromised and at risk for M. porcinum infection. Prolonged conventional culture incubation increased the detection yield, and 16S rRNA sequencing confirmed the presence of M. porcinum. Combination therapy with amikacin, azithromycin, moxifloxacin, and surgical debridement successfully controlled the infection (Table 1).

Table 1 Review of the Literature on Reported Cases of M. Porcinum Infection and Rapidly Growing Mycobacteria

Extensive literature review revealed that, to date, approximately 200 species of rapidly growing mycobacteria have been identified.⁴⁴ Of these, 95% are ubiquitously present in the environment, being found in soil, air⁴⁵ and water.⁴⁶ The majority of these mycobacteria are saprophytic or non-pathogenic to humans and animals, with only a minority of species capable of causing infections in animals or humans. The increasing global population age, underlying metabolic diseases, pulmonary conditions,^47,48 immunosuppression,^49,50 and broad-spectrum antibiotic therapy^47,51 are all host factors that predispose individuals to infections caused by nontuberculous mycobacteria (NTM). Given the high abundance of NTM in environmental niches such as soil, natural water sources, and drinking water supplies, there is often a high rate of human-pathogen contact, which in turn leads to an increased incidence of NTM infections.⁵²

In this case report, the patient presented with elevated infection indicators, characterized by red, swollen, and painful wounds, along with tissue necrosis and granuloma formation. Serial cultures of wound secretions consistently detected Mycobacterium porcinum. Following the confirmation of Mycobacterium porcinum infection, the patient was promptly initiated on anti-infective treatment. The clinical management of rapidly growing mycobacterial infections varies slightly depending on the body site affected. This patient was treated with a regimen consisting of amikacin (0.4g, QD), azithromycin (0.5g, QD), and moxifloxacin (0.4g, QD) for anti-infective purposes, concurrent with active surgical debridement to remove foreign bodies. Post-treatment, infection indicators, including leukocyte count, procalcitonin (PCT), and interleukin-6 (IL-6), exhibited a stable downward trend, coincident with symptomatic improvement and overall condition amelioration, thereby validating the treatment efficacy.

Owing to the extended culture time and the inherent difficulty in cultivation, Mycobacterium porcinum is frequently overlooked in clinical settings. To enhance detection, the culture duration for abscesses, wound secretions, and necrotic tissues can be extended to 5 or 7 days, with interim reporting at 3, 5, and 7 days to capture the rapid growth of mycobacteria. In this instance, the specimen only demonstrated 4cfu small colonies on the Columbia sheep blood agar plate on the fourth day of culture. For non-open abscesses, deep wound secretions, and necrotic tissues, pre-enrichment followed by subculturing on Columbia sheep blood agar plates is recommended to augment the positive detection rate. In this case, the laboratory pre-enriched the second specimen on brain heart infusion agar and concurrently inoculated it onto Columbia sheep blood agar plates. After one day of enrichment, subculturing onto Columbia sheep blood agar plates resulted in dense colonies of Mycobacterium porcinum by the third day (Figure 2). This enrichment method is preferable for non-open specimens or deep tissue specimens to prevent the interference of fast-growing colonies with the growth of slow-growing ones, or for simultaneous inoculation on Columbia sheep blood agar plates for comparative growth observation. In cases of recurrent infections where Gram staining smears do not reveal specific bacteria, various staining methods, such as weakly acid-fast and acid-fast staining, can be employed to identify pathogens. It is imperative for the laboratory to establish a communication channel with the clinic, closely liaise to understand the condition, and conduct relevant tests to expedite the identification of the pathogen, facilitating the formulation of an effective anti-infective plan and treatment strategy.

The patient in this case had diabetes mellitus. Diabetic patients frequently exhibit impaired wound healing due to persistent hyperglycemia-induced pathophysiological alterations. Recent advances in biomaterial engineering have developed multifunctional hydrogels capable of modulating the diabetic wound microenvironment through three key mechanisms: immunomodulation, antimicrobial activity, and tissue regeneration. Naturally derived dual dynamic crosslinked hydrogels4 demonstrate significant potential in reprogramming macrophage phenotypes from pro-inflammatory M1 to anti-inflammatory M2, thereby attenuating chronic inflammation and promoting physiological healing. For instance, obakunone-incorporated hydrogels facilitate the critical transition from the inflammatory to proliferative phase, while protocatechualdehyde (PA) and ferric ion (Fe³⁺)-loaded systems⁵³ provide synergistic antibacterial effects via sustained drug release. Additionally, dynamically crosslinked self-healing hydrogels enable sequential delivery of antimicrobial agents and growth factors, optimizing the temporal requirements of wound repair. Furthermore, collagen/hyaluronic acid-based hydrogels releasing sulfated hyaluronan⁵⁴ have been shown to improve diabetic wound healing by modulating inflammatory macrophage activity and providing extracellular matrix-mimetic support. These advanced hydrogel systems represent a promising therapeutic strategy for refractory diabetic wounds, offering targeted immunomodulation, enhanced antimicrobial efficacy, and controlled regenerative factor delivery while minimizing systemic side effects. Further clinical studies are warranted to establish standardized treatment protocols for their translation into routine practice.

For established cavitary and lumenal infections, emerging research has demonstrated the potential of nature-inspired magnetically actuated microrobots to penetrate infected sites. These microdevices utilize low-frequency magnetic fields to perform localized biofilm disruption through mechanical scraping action while enabling targeted antimicrobial delivery.⁵⁵ Furthermore, advanced systems incorporating high-frequency alternating magnetic fields can achieve precise thermal ablation of bacterial biofilms via controlled heating of low-friction soft robotic platforms. Complementary approaches include self-propelled microrocket systems that utilize chemically generated bubble propulsion to actively penetrate gastrointestinal biofilms and enhance drug delivery efficacy.⁵⁶ Notably, recent developments in magnetic living hydrogel composites demonstrate multifunctional capabilities for intestinal infection management. These biohybrid systems combine magnetic navigation for precise localization with controlled therapeutic release and diagnostic functions, offering a comprehensive theranostic platform.⁵⁷ These innovative technologies represent a paradigm shift in the treatment of complex cavitary infections by combining mechanical biofilm disruption with spatially controlled therapeutic delivery.

The present study highlights the critical importance of optimizing culture strategies to improve the detection of Mycobacterium porcinum, a slow-growing yet easily overlooked pathogen in chronic wound infections, particularly in diabetic patients. By extending incubation periods to 7 days with interim reporting at 3, 5, and 7 days, alongside selective enrichment in Brain Heart Infusion Broth for deep or non-open wounds, we significantly enhanced recovery rates, reducing diagnostic delays. Furthermore, integrating alternative staining methods (weak acid-fast, Ziehl-Neelsen) proved valuable when conventional Gram staining failed to identify pathogens. Beyond diagnostics, emerging therapies such as immunomodulatory hydrogels—engineered to shift macrophage polarization from pro-inflammatory M1 to reparative M2 phenotypes—demonstrate promise in disrupting the chronic inflammatory milieu of diabetic wounds. Innovations like dynamic crosslinked hydrogels enabling sequential drug/growth factor release, or magnetically guided microrobots for biofilm penetration and targeted antimicrobial delivery, represent transformative approaches for resistant infections. Future efforts should prioritize rapid molecular diagnostics, clinically adaptable biomaterials, and interdisciplinary strategies bridging microbiology, nanotechnology, and immunology to address the multifaceted challenges of chronic wound management.