1. Introduction

Staphylococcus aureus (S. aureus) is a commensal organism found in the nose or on the skin and environmental surfaces, and often harmless. S. aureus is considered the classic opportunist in taking advantage of hosts with breakage of the skin or mucosal barrier, indwelling of intravascular devices, and a weakened immunity ^[1]. Its ability to modify virulence factors in adapting to host defense response and antibiotics is a key factor in enabling S. aureus to persist in the bloodstream ^[2] and to spread metastatic infection in deep tissues ^[3]. Recent transmission of antibiotic-resistant strains of S. aureus, particularly methicillin-resistant Staphylococcus aureus (MRSA), is a worldwide problem in clinical medicine. Better approaches to identifying, screening, and treating those at risk will reduce the morbidity and mortality associated with S. aureus infection.

2. Case Presentation

A 69-year-old Asian female presented with fever up to 38°C and non-traumatic pain at the right buttock for one week. The pain was sharp and worsened with weight bearing and movement. The earlier treatment with acetaminophan 3000 mg per day was not effective and her fever remained. Note that two weeks before, she had an episode of watery diarrhea more than 5 times per day for approximately two days. Her job was labor intensive predominantly consists of ditch cleaning and litter removal. She denied trauma or infection. She had tenderness over her lower back and right buttock. There were no sign of respiratory symptoms or neurological deficit. Laboratory investigation revealed leukocytosis (WBC 20500/ml; Seg/Lym: 88/4), C-reactive protein was 9.92 mg/L (normal <5 mg/L). Urinalysis was unremarkable and no abnormal finding of chest radiograph.

Initial KUB plain film (Figure 1A) showed a clear and prominent psoas shadow in the right iliac fossa (arrows), without evidence of a calculus. The findings of echocardiography were basically normal. Gallium-67 citrate inflammation scan for infectious screening (Figure 1B) delineated intense uptake in right iliopsoas muscle, right sacroiliac articulation and the ipsilateral gluteal region (black arrow), implying right iliopsoas abscess. Since blood culture result was positive for methicillin-sensitive Staphylococcus aureus (MSSA), intravenous vancomycin (1 gram every 12 hours) and oral minocycline (100 mg twice a day) were started on the basis of susceptibility results.

On subsequent CT examination (Figure 2), collection of intense fluid (asterisk) and air (arrow) densities was observed, extending from right psoas to gluteus minimus. The appearance was in concordance with muscle abscess. CT-guided drainage was performed and the pus culture was positive for MSSA. Following abscess drainage, intravenous vancomycin for 2 weeks and oral minocycline, the patient recovered uneventfully. She spent 4 weeks in the hospital and was discharged on her home medication of oral minocycline for further 2 weeks. Three months on, she is doing well without any complications.

3. Discussion

S. aureus is often found on the skin and in the nose of healthy people, but capable of transforming into a fatal illness in hosts with compromised immunity or lower socioeconomic status. The ability to modify virulence factors to adapt to host defense response or circulating antibiotics is a key factor in enabling of S. aureus to remain in the bloodstream and spread infections in deep tissues ^[2].

Download as

• PPT
  PowerPoint Slide
• PNG
  Larger image(png format)

Veiw figureFigures index

•  NEW
  View larger figure in new window

View next figure

Figure 1. A plain KUB film (A) did not show any evidence of a calculus, but a clear and prominent psoas shadow in the right iliac fossa, indicating intramuscular space occupying lesion (arrows). A gallium-67 scan for infectious screening (B) delineated a fusiform lesion originates from the right paravertebral region of lower L-spine, extending downwards to the right sacroiliac articulation and the ipsilateral gluteal region (black arrow), implying right iliopsoas abscess. Cortical disruption of the inferior end-plate of T8 with erosion opposite vertebral end (open arrow) was consistent with degenerative spondylosis on sagittal tomographic section (C)

Bacteria may enter the bloodstream from an occult infection or by direct inoculation, and are generally cleared from the blood within minutes, so the bacteremia is silent and transient. In immunocompromised hosts, viable bacteria remain in the blood and bacteremic symptoms would arise ^[4]. S. aureus is a major cause of bloodstream infections in most of the industrialized world ^{[2, 5]}, with wide variation in its manifestations. When present, symptoms may range from fever and chills to septic shock. Staphylococcus aureus bacteremia (SAB) is an urgent medical problem due to its poor outcome and the growing prevalence of methicillin-resistant S. aureus (MRSA). The presence of S. aureus in the blood does not establish the source of septic symptoms, because S. aureus is also in criminated in a number of infectious diseases. The crucial step in the assessment of a bacteremic patient is defining the extent of infection, as this will determine the nature of management. Patients with SAB are at risk for the development of metastatic foci ^{[2, 3]}, as was the case of this patient.

Metastatic infection is defined as deep-seated infection detected within 3 months after the initial positive blood culture result is obtained ^[6]. The prevalence of metastatic infection in patients with SAB varies from 13% to 39% ^{[7, 8, 9]}, with endocarditis, psoas abscess and spondylodiscitis were diagnosed most frequently ^{[6, 10]}. Signs and symptoms guiding the attending physician in the diagnostic workup are present in only a minority (41%) of cases ^[11]. Any complaint of local pain indicates the possibility of underlying complication of SAB and should aggressively be pursued if no alternate explanation can be identified. Incomplete eradication of occult infections may lead to relapsing bacteremia or mortality ^[6].

The psoas is surrounded by a rich venous plexus and has lymphatics overlying the muscle from nearby organs. The psoas is also in close proximity to intra-abdominal organs, including the colon, appendix, terminal ileum, jejunum, ureters, kidneys and pancreas. Hence the psoas is susceptible to the infections of these organs ^[12]. The pathogenesis of psoas abscess is related to two mechanisms: primary, with hematogenic spread; and secondary, when related to adjacent structures infection. Moreover, the causative organisms differ among primary (S. aureus) and secondary (Escherichia coli, Klebsiella spp, Bacteroides spp, Streptococcus spp) psoas abecesses ^{[13, 14]}. In a review of 367 cases, Ricci et al noted that over 99% of iliopsoas abscess are primary in Asia and Africa, in which 88% are staphylococcal abscess, and that the mortality rate in untreated patients is 100% ^[15]. The classic triad of fever, back pain and limping is present in only 30% of patients with psoas abscess ^{[12, 16]}. Other common symptoms include malaise, weight loss, nausea, anorexia, and pain that radiates to the flank, groin, or anterior thigh ^[12].

Download as

• PPT
  PowerPoint Slide
• PNG
  Larger image(png format)

Veiw figureFigures index

•  NEW
  View larger figure in new window
• PREV
  View previous figure

Figure 2. CT of lumbar spine. (A, axial) Collection of intense fluid (asterisk) and air (arrow) densities was observed in the right psoas muscle. Note the collection extending from right iliopsoas muscle, (B, axial) to right gluteus minimus muscle (C, axial). The appearance was in concordance with muscle abscess. The extent of the abscess was also demonstrated on the sagittal reconstruction image (D, sagittal)

A major concern in the present case is about the possibility of simultaneous infections of bloodstream and of the psoas muscle caused by S. aureus. In fact, this event could have occurred in the case here described, considering that the etiologic agent was concomitantly found in samples from blood and pus. Notwithstanding, metastatic infections (endocarditis, osteomyelitis, psoas abscess, etc.) are more often described as a consequence of SAB. Likewise, infection that originates in a deep-seated focus can extend through the bloodstream to other sites. According to the literature, S. aureus is a major microorganism responsible for primary psoas abscess ^[15], metastatic infection ^[16], bacteremia ^[2] and food poisoning ^[17]. The conclusive evidence of staphylococcal food poisoning is the linking of an illness with a specific food or detection of the toxin in the food sample. We have no convincing data to establish that diarrhea, as an initial symptom in our case, is a clue to unexpected SAB or psoas abscess.

4. Conclusion

Patients with SAB are at risk for the development of metastatic infections. These complications are associated with increased morbidity and mortality. Radionuclide imaging may delineate and assess the infectious foci in all parts of the body, based on functional changes of tissues. Early clinical recognition and appropriate management of staphylococcal infections are crucial to avoid debilitating complications.

Conflict of Interest

We declare that we have no conflict of interest.

References

[1]	von EC, Becker K, Machka K, Stammer H, Peters G. Nasal carriage as a source of Staphylococcus aureus bacteremia. N Engl J Med. 2001; 344(1):11-16.
	In article		View Article  PubMed
[2]	Naber CK. Staphylococcus aureus bacteremia: epidemiology, pathophysiology, and management strategies. Clin Infect Dis. 2009; 48(Supplement 4): S231-S237.
	In article		View Article  PubMed
[3]	Troidle L, Eisen T, Pacelli L, Finkelstein F. Complications associated with the development of bacteremia with Staphylococcus aureus. Hemodiallnt. 2007; 11(1):72-75.
	In article		View Article
[4]	Cobe HM. Transitory bacteremia. Oral Surg Oral Med Oral Pathol. 1954; 7(6):609-615.
	In article		View Article
[5]	Corey GR. Staphylococcus aureus bloodstream infections: definitions and treatment. Clin Infect Dis. 2009; 48(Supplement 4):S254-S259.
	In article		View Article  PubMed
[6]	Horino T, Sato F, Hosaka Y, Hoshina T, Tamura K, Nakaharai K, et al. Predictive factors for metastatic infection in patients with bacteremia caused by methicillin-sensitive Staphylococcus aureus. Am J Med Sci. 2015; 349(1):24-28.
	In article		View Article  PubMed
[7]	Mylotte JM, McDermott C, Spooner JA. Prospective study of 114 consecutive episodes of Staphylococcus aureus bacteremia. Rev Infect Dis. 1987; 9(5): 891-907.
	In article		View Article  PubMed
[8]	Jensen AG, Wachmann CH, Espersen F, Scheibel J, Skinhøj P, Frimodt-Møller N. Treatment and outcome of Staphylococcus aureus bacteremia: a prospective study of 278 cases. Arch Intern Med. 2002; 162(1): 25-32.
	In article		View Article  PubMed
[9]	Lesens O, Hansmann Y, Brannigan E, Remy V, Hopkins S, Martinot M, et al. Positive surveillance blood culture is a predictive factor for secondary metastatic infection in patients with Staphylococcus aureus bacteraemia. J Infect. 2004; 48(3): 245-252.
	In article		View Article  PubMed
[10]	Fowler VG Jr, Olsen MK, Corey GR, Woods CW, Cabell CH, Reller LB, et al. Clinical identifiers of complicated Staphylococcus aureus bacteremia. Arch Intern Med. 2003; 163(17):2066-2072.
	In article		View Article  PubMed
[11]	Vos FJ, Kullberg BJ, Sturm PD, Krabbe PF, van Dijk AP, Wanten GJ, et al. Metastatic infectious disease and clinical outcome in Staphylococcus aureus and Streptococcus species bacteremia. Medicine (Baltimore). 2012; 91(2):86-94.
	In article		View Article  PubMed
[12]	Mallick IH, Winslet MC. Iliopsoas abscesses. Postgrad Med J. 2004; 80(946):459-462.
	In article		View Article  PubMed
[13]	Santaella RO, Fishman EK, Lipsett PA. Primary vs secondary iliopsoas abscess. Arch Surg. 1995; 130(12):1309-1313.
	In article		View Article  PubMed
[14]	Charalampopoulos A, Macheras A, Charalabopoulos A, Fotiadis C, Charalabopoulos K. Iliopsoas abscesses: diagnostic, aetiologic and therapeutic approach in five patients with a literature review. Scand J Gastroenterol. 2009; 44(5):594-599.
	In article		View Article  PubMed
[15]	Ricci MA, Rose FB, Meyer KK. Pyogenic psoas abscess: worldwide variations in etiology. World J Surg. 1986; 10(5):834-843.
	In article		View Article  PubMed
[16]	Chern CH, Hu SC, Kao WF, Tsai J, Yen D, Lee CH. Psoas abscess: making an early diagnosis in the ED. Am J Emerg Med. 1997; 15(1):83-88.
	In article		View Article
[17]	Argudín MÁ, Mendoza MC, Rodicio MR. Food poisoning and Staphylococcus aureus enterotoxins. Toxins (Basel). 2010; 2(7):1751-1773.
	In article		View Article  PubMed