Laboratory diagnosis of bone and joint infections

Laboratory diagnosis of bone and joint infections

Infection of Bone (Osteomyelitis)

The patient might develop osteomyelitis from:

·       Hematogenous spread of an infectious agent

·       Invasion of bone tissue from an adjacent site eg. joint infection, dental infection

·       Breakdown of tissue caused by trauma or surgery

·       Lack of adequate circulation followed by colonization of skin ulceration with microorganisms.

Infection of bone can progress towards chronicity, particularly if blood supply is insufficient in the affected area.

Causative organisms:

a.     Staphylococcus aureus

b.    Salmonella spp.

c.     Haemophilus spp.

d.    Enterobacteriaceae

e.     Pseudomonas spp.

f.      Fusobacterium necrophorum

g.     Yeasts

Note:

-        Parasites or viruses rarely, if ever, cause osteomyelitis.

-        Human bite may lead to infection with Eikenellacorrodens osteomyelitis whereas animal bite may lead to Pasteurellamultocida osteomyelitis.

-        Poor oral hygiene may lead to osteomyelitis of the jaw with Actinomyces spp., Capnocytophaga spp., and other oral flora, particularly anaerobes.

-        Pelvic infection in the female may result in a mixed aerobic and anaerobic osteomyelitis of the pubic bone.

-        Patients with neuropathy in the extremities, notably patients with diabetes, who may have poor circulation, may experience an unrecognised or notable trauma. They develop ulcers on the feet that do not heal, become infected, and may eventually progress to involve underlying bone. These infections are usually polymicrobial, involving anaerobic and aerobic bacteria.

 

Laboratory diagnosis:

-        To identify the etiologic agent of osteomyelitis, a small piece of bone / pus collection / scrapping material may be sent to the microbiology laboratory.

-        Microscopy – Gram stain

-        Culture – Organism can be isolated by culturing in routine aerobic media (i.e. Blood agar & Mac Conkey agar) & anaerobic media (i.e. Robertson’s cokked meat broth)

 

Infection of Joint ( SepticArthritis / synovitis)

·       Arthritis is an inflammation in a joint space. Infectious arthritis may involve any joint in the body.

·       Infection of the joint usually occurs secondary to hematogenous spread of bacteria, or less often fungi, as a direct extension of the infection of the bone.

·       It may also occur after injection of material, especially corticosteroids, into joints or after insertion of prosthetic material (eg total hip replacement).

·       Although infectious arthritis usually occurs at a single site (monoarticular), a pre-existing bacteremia or fungemia may seed more than one joint to establish polyarticular infection, particularly when multiple joints are diseased, such as in rheumatoid arthritis.

·       In bacterial arthritis, the knees and hips are the most commonly affected joints in all age groups.

·       Sterile, self limited arthritis caused by antigen antibody interactions may follow an episode of infection, such as meningococcal meningitis.

·       Occasionally the causative agent might not be detected due to either absence of viable agents at the site or faults in laboratory procedure.

·       Non-specific test results such as increased WBC, decreased glucose or elevated protein may indicate that the infective agent is present.

Causative organisms:

Causes of septic arthritis:

a.     Staphylococcus aureus is the most common etiological agent

b.    Neisseria gonorrhoea (In adults younger than 30 years)

c.     Hemophilus influenzae (In children less than 2 years)

d.    Streptococcus pyogenes

e.     Streptococcus agalactiae

f.      Pneumococci

g.     Viridans streptococci

h.    Bacteroides fragilis

i.       Fusobacterium necrophorum

Causes of chronic monoarticular arthritis:

a.     Mycobacteria

b.    Nocardia asteroides

c.     Fungi

Causes of arthritis in prosthetic joints:

It is most commonly caused by skin flora including the following

a.     Staphylococcus aureus

b.    Staphylococcus epidermidis

c.     Other coagulase negative staphylococci

d.    Corynebacterium spp.

Laboratory diagnosis:

·       Diagnosis of joint infections requires an aspiration of joint fluid for culture and microscopic examination.

·       Inoculating the fluid directly into blood culture bottles may prevent the fluid from clotting.

·       Some of the fluid may be gram stained and inoculated onto blood as well as chocolate and anaerobic media. The use of AFB and fungal media must also be considered. 

 

Extra note:

Bone marrow aspiration or biopsy may be indicated sometimes for detection of organism causing systemic infections

·       Detection of organisms in the bone marrow can be done for diagnosis of some diseases like brucellosis, histoplasmosis, blastomycosis, tuberculosis and leishmaniasis.

·       Many etiological agents associated with disseminated infections in AIDS patients, can be visualised or isolated from bone marrow, eg Cytomegalovirus, Cryptococcus neoformans and Mycobacterium avium complex.

 

Exercise:

Group discussion on the case of Bone / Joint infection