Bacteriology of Spinal InfectionsCHAPTER 1

Spinal infections, though rare, are particularly challenging as they have a wide spectrum of clinical presentations and may mimic other noninfectious conditions.²

Prompt and accurate diagnosis of spinal infections, the cornerstone of treatment, requires a high index of suspicion in at-risk patients and appropriate evaluation to identify the organism and determine the extent of infection.

Therefore, knowledge about the types of infections and their causative organisms is important for early diagnosis and appropriate treatment.

Hematogenous infection spreads usually from either the skin, respiratory tract, genitourinary tract, gastrointestinal tract or the oral cavity, giving rise to bacteremia.¹ Batson demonstrated retrograde flow from the pelvic venous plexus to the perivertebral plexus.

The extensive prevertebral venous plexes in the vertebral column is thought to be the conduit for the spread of bacterial infection.

Wiley and Trueta suggested that bacteria can become lodged in the arteriolar network at the vertebral endplate. The pathophysiology of infection differs in adults and children. In children, infection of the disc occurs secondary to increased vasculature in the disc space following bacteremia, whereas in the adults, the disc is relatively avascular and the infection of the disc occurs secondary to the spread of infection from the arterial arcades in the adjacent metaphyseal region.

The infection then spreads by direct extension with the rupture of the infective focus through the endplate into the disc. The infection may extend from the vertebral body and paravertebral area to the epidural space and adjoining vertebral bodies.

Spinal instability after destruction of the disc, bone and posterior elements may cause compression of the spinal cord, cauda equina or nerve roots.

In general, Staph. aureus is the most common agent in hematogenous osteomyelitis (Fig. 2). It is the most common agent in osteomyelitis in patients of all ages.

Gram-negative bacilli and anaerobes predominate in patients with decubitus ulcers and in immunocompromised patients. Among them, the most important is E. coli, which mainly affects elderly men with urinary tract infections; on the other hand, Pseudomonas species is associated with epidural infections (Fig. 4). Other members of the Enterobacteriaceae family, like Klebsiella pneumoniae, Enterobacter cloacae and Edwardsiella tarda, have been rarely implicated as the causative agents of spinal infection. Granulomatous spinal disease is seen with Brucella species and is common in endemic areas (Fig. 5). Salmonella infections are frequently seen in endemic countries like India and are associated with sickle cell anemia (Fig. 6). The most common resistant bacteria that is isolated is methicillin-resistant Staphylococcus aureus (MRSA).

Studies have shown that infections caused by resistant organisms may be associated with increased morbidity, mortality and costs. Some risk factors associated with MRSA infections include previous hospitalization, intensive care unit (ICU) stay, indwelling catheters, prolonged antibiotic therapy, advanced age, and exposure to patients colonized or infected with MRSA.⁵

Mycobacterium tuberculosis is the agent responsible for Pott's disease and skeletal tuberculosis accounts for 10 to 20% of all extrapulmonary cases (Fig. 7). Mycobacterium avium-intracellulare infections may mimic Pott's disease even in immunocompetent patients; however, the known association of these agents with human immunodeficiency virus (HIV) is observed for spinal infections as well. Nontuberculosis mycobacteria, like M. xenopi, M. fortuitum and M. kansasii, also have been rarely associated with spinal infections.

Bacterial adherence and biofilm formation in implant-associated infection depend largely on the characteristics of implant surfaces and infecting species of microorganisms. Staphylococcus species, including Staph. aureus and Staph. epidermidis, have the ability to produce biofilms on bones and are one of the most important virulence mechanisms by which they cause infections. The biofilm functions as a protective barrier between the bacterial cells and their environment. It facilitates survival under harsh environmental conditions.

Staphylococcus aureus expresses many potential virulence factors, which help cause infection and spread rapidly to the surrounding tissue.

There are many types of classifications of the infections of spine and the most comprehensive of them are discussed below.

The disc is the largest avascular structure in the body and the infections spread from the end-arterial arcades in the metaphyseal region on to the disc. They cause acute purulent and pyogenic lesions.

The infection typically starts in highly vascular metaphyseal region. It can spread to disc and adjacent vertebral body, frequently destroying the intervertebral disc unlike in tuberculosis. In untreated cases, the vertebral disease progresses to abscess and spread to the adjacent paravertebral structures or spinal canal.⁸ Spinal infections have been found to be secondary to infections in other parts of the body, particularly urinary tract infections and pneumonia (Fig. 8). It has also been found that intravenous drug users injecting substances in the veins of the upper extremities may develop infections of the upper parts of the spine.

Risk factors for vertebral osteomyelitis include diabetes, renal failure, rheumatoid arthritis, acquired immunodeficiency syndrome (AIDS), malignancy and old age. Commonly caused infections by bacterial agents are Stap. aureus (>50%); Streptococcus species; P. aeruginosa gram-negative bacteria, such as E. coli; and Proteus species from the genitourinary tract.

Methicillin-resistant Staphylococcus aureus has emerged as a problematic pathogen with estimates that infection by MRSA in orthopedic wards can double the mortality rate with infection correlating with the increasing length of hospital stay.

Blood parameters, like erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), are elevated (92%). Total leukocytosis and polymorphonuclear leukocytosis may often be absent and are therefore less diagnostic (approximately 40%). X-rays are often normal in the early disease but will pick up disc space narrowing and end plate destruction, localized osteopenia after 3 to 4 weeks or so and later new bone formation (Fig. 9). Magnetic resonance imaging scans are diagnostic. They are sensitive as early as the first week of infection. The earliest response to vertebral osteomyelitis is the accumulation of water in extracellular bone marrow, responsible for bone marrow edema. This edema is easily detected by signal changes with low signal intensity on T1-weighted images and high signal intensity on proton-density or T2-weighted images, but especially on fat saturated spectroscopic inversion recovery (SPIR) techniques or short tau inversion recovery (STIR) sequences. These pulse sequences are more sensitive and reliable in the detection of free water protons than conventional T1- and T2-weighted images. The disc within disc sign is considered pathognomonic of early disc space (Fig. 10) affection. Unlike tuberculous infection, isotope bone scans are sensitive in pyogenic disease (Tech 99 and Gallium 67). Biopsy is critical to the diagnosis of pyogenic infections of the spine and to distinguish it from tuberculosis and tumors of the spine.

In about one-third of the patients with epidural abscess the bacteria gain access from infection in the contiguous areas, in another half of the patients through hematogenous spread and in the remaining cases, the source is not identified. In these infections also Staph. aureus is the causative bacteria in about two-third of the cases.

This is a result of past hematogenous foci, contiguous disease or lymphatic spread from pleural disease. The primary focus is at the anterosuperior or inferior angle of the vertebral body. The area of infection gradually enlarges and spreads to involve two or more adjacent vertebrae by extension beneath the anterior longitudinal ligament or directly across the intervertebral disc.¹¹ This produces the classical X-ray picture of the anterior wedging of two adjacent vertebral bodies with the destruction of the intervening disc.

Five distinct types of spinal column involvement have been described radiologically:

In endemic regions, it commonly affects the children and young adults. Evidence of other foci and systemic symptoms are often absent. Early symptoms may be back pain or stiffness with initially normal radiological features and diagnosis may be delayed until the signs of advanced disease, such as paralysis, deformity or sinus formation, develop.

Bacilli are sparse and smear and culture of pus or tissue are positive only in one-half of the cases. Histologic studies reveal granulomas with or without caseation in three-fourth of the cases.

The common fungal agents causing fungal infections are Candida species, Cryptococcus neoformans and Aspergillus species.¹² Fungal vertebral osteomyelitis caused by the endemic fungi, such as Coccidioides immitis and Blastomyces dermatitidis, has also been described.

Candida species are typically considered commensal organisms and are part of the normal human flora. They become invasive pathogens when the host has impaired immunity or has had repeated intravascular manipulations. Hematogenous seeding of the Candida in the vertebral bodies leads to Candidal vertebral ostomyelitis, the most common form of Candidal infection of the spine. The risk factors for acquiring Candidal vertebral osteomyelitis were the presence of indwelling central venous catheter, use of broad spectrum antibiotics and immunosuppression.

High index of suspicion is essential for the diagnosis of fungal vertebral osteomyelitis followed by appropriate radiographic studies and laboratory confirmation by microbiological tests.¹²11

Although computed tomography (CT) scanning can identify earlier bony changes and can also evaluate the presence of paravertebral or spinal canal extension. Candidal vertebral osteomyelitis has no characteristic CT findings. Magnetic resonance imaging (MRI) has been found to be helpful in a few cases to differentiate from bacterial osteomyelitis.

Isolated bony involvement with Cryptococcus neoformans is rare. The commonest clinical presentation of this organism is meningitis, but involvement of bone has been reported in 10% of cases as a part of the systemic infection.

Osseous involvement is usually a manifestation of disseminated cryptococcosis, appearing in 5 to 10% of cases. The radiological features of skeletal involvement are nonspecific and there is a spectrum of appearances. A lytic lesion within a vertebral body can resemble the cystic form of tuberculosis, with discrete margins and surrounding sclerosis. The infection may appear as a permeative lesion involving a single vertebral body with collapse. In the later stages of the infection, two contiguous bodies may be involved with paravertebral soft-tissue swelling mimicking spinal tuberculosis and, less commonly, as in tuberculosis, the posterior elements may be affected. Since there have been similarities found with tuberculosis, the authors advocate that tissue diagnosis should be carried out as the chemotherapeutic agents are not the same for tuberculosis and cryptococcosis.

Aspergillosis is an infrequent opportunistic fungal infection of the spine that primarily occurs in immunocompromised patients. It involves the spine with the typical radiological features of nonsuppurative osteomyelitis. It is usually secondary to a pre-existing infectious focus most commonly in the lungs. Primary abscess, including spinal epidural abscess, without the involvement of surrounding musculoskeletal structures, occurs less commonly and invariably needs surgical intervention. Prognosis is chiefly related to the patient's general immunological condition, and because of irreversible demyelination, vacuolization and localized necrosis, early diagnosis is important for preventing permanent neurologic deficits and guaranteeing patient's survival.

Primary hydatid disease is common in the liver, spleen and lungs. Musculoskeletal involvement is secondary and uncommon, with an incidence of less than 2.5%. Pelvis, sacrum, metaphysis of the long bones are the more commonly involved sites in the stem.12

Thalgott in 1991¹³ categorized the patients according to two parameters, the first being the severity or type of infection, and the second being the host response or physiologic classification of the patient. This classification scheme is based on the clinical staging system for adult osteomyelitis developed by Cierny. The severity of infection is classified into the following three groups:

The host response, likewise, is classified into the following classes:

Many factors have been identified, which can be classified as the host factor, environmental factor and procedural factor. Certain host factors are known to increase the likelihood of postoperative wound infections such as advanced age, malnutrition, obesity, diabetes mellitus, immunosuppression and preoperative infection. Advances in anesthetic technique and postoperative care allow surgical treatment in older patients. Conversely, poor nutrition and general condition were still common in the elderly. The patients with deep wound infection are on average older than the main group significantly, presenting that increased age as a related risk factor.

The primary pathogens in acute postoperative wound infections are the gram-positive cocci, specifically Staph. aureus, Staph. epidermidis and beta-hemolytic streptococci.

Among the gram-negative bacteria, K. pneumoniae, E. coli, P. aeruginosa and Proteus species are the common gram-negative pathogens isolated. Delayed or chronic infections are usually caused by skin flora of low virulence such as Propionibacterium acnes and diphtheroids. Intravenous drugs users have a higher incidence of infections with gram-negative rods. Infections with nosocomial organisms are more common in patients with a protracted hospital course or ICU stay.

Early recognition of the deep wound infection remains the cornerstone for its appropriate treatment. Suboptimal treatment can lead to poor clinical outcomes, including chronic pain, neurological deficit, pseudarthrosis, unstable implants with loosening and osteomyelitis. Treatment of deep wound infection is based on several important principles including adequate wound drainage, appropriate antibiotic usage (dose, duration and route) and improving the patient's general condition.

Postoperative infection of the spine following posterior instrumentation is not uncommon and is a potentially dangerous complication. Postoperative deep wound infection in such situations can present a number of therapeutic challenges. In early postoperative infection, when the stability of the implant is not in doubt, the implant is left in situ after thorough irrigation. Any loose bone graft may be removed, washed in 3.5% povidone iodine and replaced in position.

Removal of instrumentation for eradicating infection may be required and is indicated when the implants are unstable or loose. However, removal is not always possible without disastrous mechanical consequences. An alternative is to treat the patient with long-term intravenous antibiotics in combination with multiple surgical wound dé bridements and suction irrigation wound dressing systems.

Late onset infection, such as postoperative discitis, occurs typically after several weeks to months after the initial surgery. Clinical findings though delayed are similar to early onset infections. Infection is commonly caused by low virulence organism, such as coagulase-negative Staphylococcus, diphtheroids or Eikenella spp. Diagnosis is done by closed needle biopsy or open biopsy and culture 13of the affected area. The condition is treated on the same surgical and therapeutic principles as for early postoperative wound infections.

Meticulous surgical technique is crucial for reducing postoperative wound infections, since the most likely time of inoculation of the wound is during the surgery. Strict aseptic techniques along with the gentle handling of tissue to minimize hematoma and necrosis are very essential. The method of prepoperative skin preparation is also very crucial factor as shaving of skin prior to surgery has been shown to increase the infection rate. Recommendations are to use hair removing creams and less traumatic hair clipping to shaving.

List of common organisms causing spinal infection is shown in Table 1 and risk factors for developing spinal infection are shown in Table 2.

Identification of the causative organism is essential for a definitive diagnosis. This can be achieved by CT-guided biopsy of the vertebrae or disc space.⁹ Identification of the organism is essential, and this can be accomplished through CT-guided biopsy sampling of the vertebra or disc space. Blood cultures, should always be done and they are positive in about one-third of the patients with pyogenic vertebral osteomyelitis.

Delay in transport to the laboratory may render the microorganisms nonviable. Transport time should be within 2 hours. The sample should be appropriately labeled and sent with clinical details.

Use of microscopy in the diagnosis of tuberculosis is of paramount importance. It does not take up Gram stain because of lipid nature of its cell wall. Rather it will be stained by special differential staining techniques such as Ziehl-Neelsen stain and Auramine O stain. Similar to gram stain, there needs to be 10⁴ bacilli per ml of the specimen for the smear to be positive.⁷ A negative smear, however, does not rule out tuberculosis as some patients harbor fewer number of bacilli, which cannot be detected by direct microscopy.

Moreover, the number of bacilli seen on smear also depends on the host immune response. It is hypothesized that the host immune cells especially the T-cell and macrophages try to localize the tubercle bacilli and retard their multiplication and limit their contiguous spread and further dissemination. It may, thus, be assumed that better the immunity of the patient, the more difficult it would be to get a positive smear with many acid-fast bacilli.

Swabs are not as effective as sampling tissue especially for fungal organisms and tuberculosis. A mini swab has a 1,000 times less specimen than a container with 15 ml of fluid or tissue, i.e. there is a 1,000 times less chance of recovering an organism through swabs. Swabs, in general, are notoriously inadequate for identifying mycobacteria and fungi. Whereas colonization by bacteria is not uncommon on surfaces and membranes, clinically significant bacterial infection usually occurs in tissue and confined to anatomical spaces. Enriched media, such as blood and chocolate agar, are used for culturing the pyogenic organism.

Laboratory diagnosis of tuberculosis is done with a fine needle aspiration (FNA) of the tissue or pus from the affected spine or paraspinal region. This can be used for cytological, histological or bacteriological diagnosis. It is an accurate, safe and cost-effective method of diagnosis. However, samples from other nonspinal sites, such as lung or lymph nodes, can be used as alternate samples in the absence of spinal samples from vertebral tissue.

Mycobacterial culture of Mycobacterium tuberculosis from bone tissue is the gold standard for diagnosis of osseous tuberculosis. Positive Ziehl-Neelsen staining for acid-fast bacilli requires at least 10⁴ acid-fast bacilli per milliliter of specimen and does not differentiate between tuberculous and nontuberculous mycobacteria.⁷ The advent of deoxyribonucleic acid (DNA) detection via polymerase chain reaction (PCR) may increase sensitivity and allow for the exclusion of nontuberculous mycobacteria (atypical mycobacteria) that may also cause spinal and surrounding soft tissue infection.

Tuberculous bacilli grow slowly in culture, and confirmation may not be available for 6 to 8 weeks. Automated culture methods, like BACTEC, MB/BacT and MGIT, use synthetic liquid media, such as Middlebrook 7H9 or 7H11 media, which reduce the positivity time to a mean of 14 days.

There is a small proportion of patients in whom acid-fast bacilli are seen in smear but do not grow in culture. The reasons are not clearly known but there are few hypothesis. The bacteria may be nonviable due to prior treatment with antituberculous chemotherapy or that the organisms become nonviable due to improper sample collection and transport to the laboratory.

Histopathological features in pyogenic infections of the spine are characterized by the infiltration of the tissue and blood vessels with polymorphonuclear leukocytes with or without the areas of necrosis or thrombosis.⁸ Osteoclastic bone resorption is seen in severe cases. Pathologically 15tuberculosis is more specific and can be identified grossly by yellow, cheese-like (caseating) areas of necrosis and histologically by the characteristic caseating granulomas.⁸ Granulomas are organized inflammatory reactions that may be seen in a wide array of infections and noninfectious conditions. Tuberculous granulomas show the central area of necrosis (caseation) with a surrounding inflammatory reaction consisting mostly of mononuclear cells, epithelioid histiocytes (large macrophages mimicking epithelial cells) and the classic Langhans giant cells.