SMALL COLONY VARIANT STAPHYLOCOCCUS AUREUS MULTIORGAN... : The Pediatric Infectious Disease Journal (original) (raw)
Small colony variant (SCVs) Staphylococcus aureus are a subpopulation of S. aureus that grow slowly on routine media and yield small, nonpigmented, nonhemolytic colonies as a consequence of disruption of the electron transport chain in SCVs.1,2 Clinical microbiology laboratories isolate SCVs in 1–2% of S. aureus infections.3 Case studies have commonly identified SCVs in persistent or relapsing S. aureus infections after prolonged antibiotic therapy.4–6 We report an unusual case of SCVs infection in a pediatric patient manifesting as multifocal pyomyositis and osteomyelitis, spinal epidural abscess and acute acalculous cholecystitis in the absence of prolonged antibiotic therapy.
CASE REPORT
A 14-year-old adolescent was referred to our hospital for management of jaundice. He initially presented to the pediatrician’s office with back pain following football practice. He was treated with naproxen medication without improvement and 3 days later he developed jaundice and fever. He was admitted to a local hospital for further management. Laboratory tests revealed a white blood cell count of 4500/cmm with 90% neutrophils, total bilirubin 13.4 mg/dL, conjugated bilirubin 10.1 mg/dL, AST 145 U/L, ALT 144 U/L and alkaline phosphatase 345 U/L. He was treated with intravenous ampicillin without clinical improvement and transferred to our hospital for further management.
His past medical history revealed an isolated subcutaneous abscess in the left lower leg 3 months earlier. The abscess was surgically drained and treated with oral amoxicillin/clavulanate for 7 days. Bacterial culture of the abscess fluid revealed S. aureus colonies susceptible to all antibiotics tested except penicillin. There was no other previous history of hospital admissions, recurrent infections, drug abuse or recent travel history.
On arrival at our hospital, he was febrile (101°F), and icteric with right upper quadrant abdominal tenderness and lumbar paraspinal tenderness. Liver function tests revealed total bilirubin 16.7 mg/dL, conjugated bilirubin 13.1 mg/dL, AST 89 U/L, ALT 77 U/L and alkaline phosphatase 361 U/L. Ultrasound examination of the abdomen revealed a dilated gall bladder measuring 12.2 cm × 4.4 cm filled with biliary sludge. Technetium-labeled hepatic iminodiacetic acid scan did not visualize the gall bladder both after the dye injection and after an additional injection of intravenous morphine sulfate consistent with the diagnosis of acute acalculous cholecystitis. A magnetic resonance scan of the spine revealed a spinal epidural abscess extending from T4 to T9 and T12 to L3 vertebral bodies, bilateral small psoas muscle abscesses and irregularity of endplates of T11–L1 vertebral bodies. Blood cultures were obtained. He was treated with intravenous ampicillin, gentamicin, cefotaxime and doxycycline and anti-inflammatory drugs. Surgical intervention was withheld since he had long-segment epidural abscess with no spinal cord involvement or neurologic deficit.
The preliminary microbiology reports of the blood culture revealed Gram-positive cocci in clusters. Antibiotic therapy was changed to vancomycin, cefotaxime and doxycycline. The next day, he developed swelling and tenderness of his right forearm followed by similar complaints in his left forearm. A CT scan and ultrasonography of his forearms revealed multiple abscesses in flexor and extensor muscle groups. The large abscesses in the flexor muscle groups were drained surgically. The final microbiology report of the blood culture revealed S. aureus susceptible to all antibiotics except penicillin and antibiotic therapy was changed to oxacillin and rifampin. During the next 3 days, jaundice and forearm tenderness subsided, but he continued to have low back pain that was now associated with pain in his legs. A CT scan of his pelvis and legs revealed persistent abscesses in bilateral iliopsoas muscles and new abscesses in flexor, extensor and hamstring muscle groups in both thigh regions and tibialis group of muscles in both legs. An ultrasound guided percutaneous technique was used to drain the bigger abscesses in his back and thigh muscles.
The bacterial culture from his forearm muscle abscesses grew SCVs (identified as slow growing, pinpoint colonies with no coagulase and beta hemolytic activity and resistance to gentamicin) and large colony S. aureus (Fig. 1). Both normal S. aureus and SCVs were positive for catalase and SCVs subsequently tested positive for coagulase by tube agglutination method. On reexamination, the previous blood cultures also revealed SCVs. Both SCVs and large colony S. aureus colonies were negative for mecA gene and had indistinguishable pulsed field gel electrophoresis patterns.7 Treatment was continued with oxacillin and rifampin and trimethoprim-sulfamethoxazole and vitamin K were added to the regimen. He demonstrated gradual clinical improvement and was discharged home a week later on intravenous oxacillin and oral rifampin, trimethoprim-sulfamethoxazole and vitamin K therapy. He received a total of 12 weeks of antibiotic therapy based on sustained clinical, radiology and laboratory (normal erythrocyte sedimentation rate) improvement. The patient is currently undergoing rehabilitation and physical therapy for the lumbar spine involvement.
Microcolonies of SCVs (→) and large colonies of S. aureus (≥) on chocolate agar plate.
DISCUSSION
Multiorgan infection by SCVs in our patient manifested as multifocal pyomyositis, multifocal osteomyelitis, spinal epidural abscess and acute acalculous cholecystitis. SCVs have commonly been isolated in single organ infections of lungs, bones, soft tissues and skin.4–6 Increased uptake by endothelial and epithelial cell lines due to enhanced expression of fibronectin binding proteins in SCVs8 may have contributed to the multiorgan involvement.
Isolation and susceptibility testing of SCVs in the microbiology laboratory is not easy, as seen in our patient. SCVs are easily overgrown and missed in the presence of normal S. aureus because SCVs divide at about one ninth the rate of normal S. aureus.9 This slow growth rate of SCVs also alters diffusion and antibiotic susceptibility tests.9 An increased inoculum of bacteria and use of mecA gene identification or antipenicillin-binding-protein-2a latex agglutination test may be more reliable for diagnosis and validation of SCVs.10
Conversion of normal strains of S. aureus to SCVs is not completely understood. They usually emerge in S. aureus infections that persist despite prolonged antibiotic treatment or that recur after long disease-free intervals.4 Lack of efficient clearance of S. aureus by antibiotics has commonly been proposed as a mechanism to produce a reservoir of viable intracellular SCVs.5,6 In vitro studies have demonstrated that endothelial cell intracellular milieu as well as intracellular cationic proteins may also cause SCV emergence in the absence of antibiotic exposure.11,12 It is possible that SCVs in our patient evolved from the S. aureus isolated from the subcutaneous abscess 3 months ago. Multiple mechanisms, including decreased alpha toxin (a pore-forming toxin that causes early cell death) production by SCVs, decreased concentration of beta lactam antibiotics and aminoglycosides intracellularly and endothelial intracellular protection from cell-mediated immunity are responsible for SCVs to persist within host cells.5,13 Decreased production of alpha toxin is due to reduced expression of hla gene at the transcription level.14
Controlled therapeutic trials for management of SCV infections are lacking. SCVs are difficult to eradicate as they have unique biochemical characteristics, including intracellular location, slow growth rate and defective oxidative metabolism.4 Our patient was successfully treated with a 12-week course of oxacillin, rifampin and trimethoprim-sulfamethoxazole with 8 weeks of vitamin K supplementation. Rifampin was included in the treatment as SCVs, like S. aureus, are ingested by phagocytes such as fibroblasts and endothelial cells.14 Addition of vitamin K to the antibiotic regimen may be helpful as intracellular concentration of hemin or menadione (vitamin K) is typically low. Auxotrophy testing of SCVs, which is an additional laboratory test for SCVs, has demonstrated that hemin or menadione supplementation of the media causes rapid bacterial growth of SCVs with decreased intracellular persistence in endothelial cells.15
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Keywords:
Staphylococcus aureus; small colony variant Staphylococcus aureus; pyomyositis; acute acalculous cholecystitis; spinal epidural abscess
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