The patient is an 82-year-old female with a past medical history significant for type 2 diabetes mellitus (hemoglobin A1c 7.2%), hypertension, chronic venous insufficiency, mild cognitive impairment and post traumatic arthritis of the left foot who presented to the emergency department with progressive atraumatic left foot swelling, erythema, and pain noted over a three-week period. The patient was treated in the outpatient setting with two sequential courses of oral antibiotics (cephalexin 500 mg four times daily for 10 days, followed by amoxicillin-clavulanate 875 mg twice daily for 7 days) without clinical improvement. She denied recent trauma, insect bites, or new footwear, but noted that her current shoe gear felt tight. The patient reported difficulty with nail care due to limited mobility and poor vision.

On presentation, the patient was afebrile (temperature 36.8°C) with stable vital signs. Physical examination of the left lower extremity revealed circumferential pitting edema extending from the mid-calf to the toes, and induration and warmth over the dorsum of the foot. Erythema extended across the dorsal foot into the medial longitudinal arch without distinct borders. There was no ascending lymphangitis. The skin demonstrated xerotic changes with scattered fissuring, and annular scaling bilaterally. Superficial epidermal abrasions were noted in the second and third interdigital web spaces as well as along the dorsal and plantar medial aspects of the foot, with minimal serous drainage but no purulence. No fluctuance, crepitus, or malodor was appreciated. There were no open ulcerations, deep probing wounds, or signs concerning for necrotizing infection.

Vascular examination demonstrated intact dorsalis pedis and posterior tibial pulses bilaterally with brisk capillary refill. Protective sensation was grossly intact on monofilament testing at standard plantar sites; however, the assessment was limited by suboptimal patient participation.

Musculoskeletal examination revealed structural deformities of the forefoot, including prominent metatarsal heads, severe hallux valgus, and rigid hammertoe contractures. Ankle range of motion was limited to approximately 50% of normal with knee extended and flexed. There was evidence of post traumatic arthritis of the left midfoot along with radiographic evidence of metatarsus adductus, contributing to abnormal gait mechanics, forefoot pressure distribution, and resultant toe crowding.

All toenails on both feet were hypertrophic, elongated, and thickened, with accumulation of subungual debris. On closer inspection, the toenails of the affected left foot demonstrated distinct green to blue-green discoloration of the nail plate, particularly prominent in the great toe, second toe, and third toe. This finding was asymmetric, as the toenails on the right foot showed yellow-brown discoloration more typical of onychomycosis or chronic nail dystrophy, and lacked the characteristic green hue. The green discoloration extended from the proximal nail fold to the nails’ free edge and involved both the superficial and deeper layers of the nail plates. Mild periungual erythema was present without purulence or deep probing wounds.

The left lower extremity showed chronic venous stasis changes with hemosiderin deposition and mild edema but no acute inflammatory signs

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  Week 1: Development of left dorsal foot erythema, swelling, and mild pain; initiated cephalexin 500 mg four times daily by primary care physician

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  Week 2: No clinical improvement noted; antibiotic switched to amoxicillin-clavulanate 875 mg twice daily

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  Week 3: Worsening of erythema and edema despite completion of second antibiotic course; presented to emergency department

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  Hospital Day 1: Managed by emergency department with vancomycin for suspected MRSA cellulitis

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  Hospital Day 2: Podiatry consulted, nail debridement performed, cultures obtained, clotrimazole and Zosyn recommended.

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  Hospital Day 3: Preliminary culture results showed gram-negative bacilli; fungal studies negative

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  Hospital Day 5: Final culture identified Pseudomonas aeruginosa and anaerobic polymicrobial flora; antibiotic regimen eventually streamlined to ciprofloxacin based on sensitivities

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  Hospital Day 7: Marked improvement in erythema and edema; patient discharged on oral ciprofloxacin and metronidazole to complete 14-day course

Laboratory Studies:

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  White blood cell count: 8300/μL (normal range 400011,000/μL) without left shift

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  Hemoglobin A1c: 7.2%

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  Serum creatinine: 1.1 mg/dL (baseline)

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  C-reactive protein: 42 mg/L (mildly elevated; reference < 10 mg/L)

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  Erythrocyte sedimentation rate: 35 mm/hr (mildly elevated; reference < 20 mm/hr)

Microbiologic Studies:

Fungal Testing (Nail Plate):

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  Potassium hydroxide (KOH) preparation: No fungal elements visualized

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  Fungal culture: No growth after 4 weeks of incubation

Bacterial Culture (Nail clippings, subungual debris):

• ● Primary Isolate: Pseudomonas aeruginosa – heavy growth

• ○ Antibiotic Sensitivities: Susceptible to ciprofloxacin, levofloxacin, ceftazidime, cefepime, piperacillin-tazobactam, aztreonam, gentamicin, tobramycin, amikacin ○ Resistant to ampicillin, amoxicillin-clavulanate, cephalexin, cefazolin

• ● Additional Isolates (Polymicrobial Anaerobic Growth):

• ○ Brevundimonas diminuta (aerobic gram-negative bacillus)

• ○ Peptoniphilus harei (anaerobic gram-positive coccus)

• ○ Actinotignum schaalii (anaerobic gram-positive bacillus)

• ○ Anaerococcus species (anaerobic gram-positive coccus)

Wound Culture (Interdigital Space Drainage):

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  Mixed skin flora; rare Pseudomonas aeruginosa (consistent with nail source)

Imaging Studies:

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  Plain radiographs of the left foot revealed severe hallux valgus and metatarsus adductus with hammertoe deformities of the lesser rays. Polyarticular arthrosis of the visualized joints is severe in the midfoot. There is a large os peroneum. Chronic osseous remodeling along the medial navicular. There is also a type II accessory navicular. Small dorsal and large plantar calcaneal enthesophytes. No acute fracture, dislocation, or soft tissue gas; no radiographic evidence of osteomyelitis.

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  Non-invasive vascular studies were obtained on hospital day 2. Venous duplex ultrasonography of the bilateral lower extremities demonstrated evidence of chronic deep vein thrombosis in one of the paired gastrocnemius veins of the right lower extremity without evidence of acute DVT bilaterally. Arterial duplex ultrasonography revealed no flow in the mid to distal anterior tibial artery of the right lower extremity, suggestive of occlusion; the posterior tibial, peroneal, and dorsalis pedis arteries were patent bilaterally with diffuse arterial calcifications noted. Ankle-brachial indices were within normal limits bilaterally (right DP 1.22, PT 1.25; left DP 1.07, PT 1.21), and toe-brachial indices were normal (right TBI 0.94, left TBI 1.21), confirming adequate perfusion to the affected extremity.

The pathognomonic green discoloration of the toenails, treatment-refractory cellulitis, and the absence of differential diagnoses including abscess or osteomyelitis strongly supported the diagnosis of GNS, complicated by secondary soft tissue infection.

The additional polymicrobial anaerobic organisms were likely opportunistic colonizers in the setting of chronic moisture, nail dystrophy, and compromised skin barrier function rather than primary pathogens. The negative fungal studies ruled out onychomycosis as the primary etiology of nail discoloration, which in turn successfully altered antimicrobial management. The absence of leukocytosis, combined with imaging findings showing no osteomyelitis, indicated that the infection remained confined to superficial soft tissues without deep extension and thus advanced imaging was not indicated.

The patient was started on empiric broad-spectrum intravenous antibiotics (vancomycin 15 mg/kg every 12 hours) for MRSA cellulitis coverage by the emergency department. After podiatry was consulted, IV piperacillin-tazobactam 4:500 mg every 8 hours was recommended for concern of GNS, for coverage of suspected Pseudomonal infection. After identification of Pseudomonas aeruginosa and various other anaerobic organisms with full antibiotic susceptibilities, the infectious disease team recommended continuing piperacillin-tazobactam and initiated metronidazole 500 mg PO TID for anaerobic coverage. The patient demonstrated excellent clinical response, and therapy was transitioned to oral levofloxacin 750 mg PO daily and metronidazole 500 mg PO TID to complete a 7-day course upon discharge.

Adjunctive local measures included the following:

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  Nail debridement: Mechanical reduction of nail thickness and removal of superficial green-stained nail layers, along with evacuation of subungual debris

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  Wound care: Gentle cleansing of interdigital spaces with normal saline, application of non-adherent dressing with silver sulfadiazine ointment (antipseudomonal topical agent). Clotrimazole 1% cream was added for hydration and management of suspected tinea pedis.

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  Edema management: Leg elevation and compression therapy with

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  multi-layer bandaging once acute infection improved

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  Glycemic optimization: Diabetes management team consultation with adjustment of home insulin regimen

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  Patient education: Instruction on proper foot hygiene, importance of keeping feet dry, use of moisture-wicking socks, and avoidance of occlusive footwear

Referrals were placed for outpatient podiatry follow-up for ongoing nail care and diabetic foot monitoring, wound care nursing for continued compression therapy, and vascular surgery for stable right anterior tibial artery occlusion and chronic DVT.

The patient demonstrated clinical improvement prior to discharge, with reduction in erythema, edema, and pain. No formal outpatient follow-up data were available at the time of manuscript preparation.

Pseudomonas aeruginosa is an opportunistic gram-negative pathogen that thrives in moist environments and demonstrates impressive adaptability to diverse ecological niches. In the nail unit, the bacterium produces characteristic water-soluble pigments, pyocyanin (blue-green) and pyoverdine (yellow-green fluorescent), which diffuse through the nail plate, creating the pathognomonic green discoloration. ^, Unlike dermatophyte infections that invade and destroy keratin, Pseudomonas colonization typically occurs on the nail surface or in the subungual space without true tissue invasion, though biofilm formation can establish persistent colonization which is resistant to antimicrobial therapy.

In immunocompetent individuals, GNS is generally self-limited and responds well to local treatment. However, in patients with diabetes, the combination of peripheral neuropathy, vascular insufficiency, immune dysfunction, and structural foot abnormalities creates an environment that is conducive to the progression from simple colonization to invasive infection. Hyperglycemia impairs neutrophil function, reduces antimicrobial peptide production, and promotes bacterial adherence to epithelial surfaces, while neuropathy induced gait abnormalities lead to increased mechanical stress and microtrauma , creating sites for potential infection.

In our patient, several factors converged to facilitate this: Foot deformities causing abnormal pressure distribution and toe crowding, limited mobility preventing adequate nail hygiene, interdigital skin breakdown providing a portal of entry for bacteria, and chronic moisture accumulation due to impaired foot mechanics and possibly hyperhidrosis.