WEEK 1: Microbiology of bone and joint infections Flashcards
Define osteomyelitis.
Osteomyelitis – can be defined as an inflammatory process affecting the bone and bone marrow caused by an infectious organism(s) resulting in localised bone destruction, “necrosis” and deposition of new bone.
Osteomyelitis includes a broad array of infections.
State them.
Osteomyelitis includes a broad array of infections i.e.septic arthritis, prosthetic joint infections, osteomyelitis, spinal infections (discitis, vertebral osteomyelitis and epidural abscess) and diabetic foot osteomyelitis.
State the effects of bone and joint infections.
Bone and joint infections cause serious morbidity and pose significant management challenges.
They may cause acute sepsis with bone and joint destruction, chronic pain, discharging wounds and permanent disability.
Untreated, it progresses to necrosis of bone ______________followed by new bone formation ______________.
Untreated, it progresses to necrosis of bone (sequestra) followed by new bone formation (involucrum)
Describe the routes of infection.
- Hematogenous
Via the bloodstream - Contiguous
Transmitted from infection in overlying tissue e.g. Diabetic foot ulcer. - Direct inoculation
Pathogens reach bone directly e.g., penetrating injuries, contamination during surgery e.g. prosthetic joint implants.
NB. Infection in bone may spread to adjacent joint.
Bone and joint infections may also be categorized as ACUTE or CHRONIC.
Define acute and chronic bone infections.
Acute develops over several days or few weeks .Vs.
Chronic continues over months or years, characterized by relapses, persistence of microorganisms, low-grade inflammation, dead bone & fistulous tracts.
Describe the following categories of osteomyelitis.
1. Primary hematogenous spread
2. Contiguous infection
3. Vascular or neurologic insufficiency associated osteomyelitis
Primary hematogenous spread (A+B) of bacteria primarily affects the metaphysis of young growing children. But can also afflict the vertebra across all ages, (NB, osteomyelitis can also affect other locations.
Contiguous infection (C+D) bacterial transmission to the bone from another contaminated site. Commonly occurs as direct contamination by bacteria in open fractures or joint replacement surgery with an orthopedic implant (Periprostheticjoint infection(PJI ).
Vascular or neurologic insufficiency associated osteomyelitis (E)results from poor blood supply e.g. diabetic wounds, loss of protective sensation and altered immune defenses, commonly affecting the lower extremity.
Discuss the risk factors of osteomyelitis.
1.Diabetes
Poorly managed high blood glucose can cause neuropathy, loss of sensation
Diabetic foot ulcers or other feet injuries may go unnoticed
Due to poor circulation, deceased sensation vascular insufficiency, feet infections spread to the bone.
- Immunocompromised patients esp. with assoc. conditions causing poor blood circulation e.g.
peripheral arterial disease, atherosclerosis & type 1 & 2 diabetes - Orthopedic surgery e.g. prosthetic implants
- Use of unsterilised needles e.g. intravenous drug use, increases risk of introducing bacteria into bloodstream
State the Common clinical symptoms of Acute hematogenous osteomyelitis.
Acutely febrile: a sudden onset of a high fever.
Septicemia: is a severe and potentially life-threatening condition that occurs when the body’s response to an infection leads to widespread inflammation and systemic symptoms.
Pain, tenderness at site of infection worsening with movement & wt. bearing.
Diminished function, reduced range of motion
Primarily acute but chronic infections may occur due to failed antimicrobial therapy.
Neonatal osteomyelitis more likely assoc. with septic arthritis of adjacent joint.
Describe the Etiology & the process…. of Acute hematogenous osteomyelitis (AHO)
Acute hematogenous osteomyelitis (AHO) primarily caused by bacteria in bloodstream.
BUT note: Fungi common cause in immunocompromised.
AHO more common in children
Thought that they experience frequent episodes of asymptomatic bacteremia.
In children (prepubertal), metaphyses richly vascularized, as producing new bone at a rapid rate.
Bacteria in bloodstream may seed bones where blood flow is abundant & slow.
Bacterial seeding of richly vascularized metaphyseal region, triggers inflammation
Describe the process of Acute hematogenous osteomyelitis (AHO) in children.
Valveless sinusoidal loops of metaphyseal venules have sluggish flow which facilitates bacterial invasion.
Bacteria seeding within metaphyseal vessels (nutrient artery & vein)
Infection develops inducing an acute inflammatory reaction in metaphysis.
Abscess escapes via Haversian systems & Volkmann canals, into cortex of bone, lifting or rupturing the periosteum (loosely attached in children).
Growing abscess in cortex & extending along periosteum, impairs blood supply, resulting in an area(s) of bone necrosis “sequestrum”.
State the Indicators of “chronic osteomyelitis”
Dead Bone (Sequestrum):
Chronic osteomyelitis often results in the formation of dead bone tissue, known as sequestrum. This necrotic tissue is a consequence of the infection disrupting blood supply to a portion of the bone, leading to bone death. Sequestra can contribute to the chronic nature of the infection and may need to be surgically removed to facilitate healing.
New Bone Formation (Involucrum):
In response to chronic inflammation and infection, the body may attempt to wall off the affected area by forming new bone around the dead or infected region. This new bone is called involucrum and is a part of the body’s defense mechanism to contain the infection. However, it can also contribute to the chronicity of the condition.
Sinus Tracts: Chronic osteomyelitis may give rise to the formation of sinus tracts or channels that connect the infected bone to the skin surface. These tracts can serve as pathways for the drainage of pus or infected material from the affected bone.
Recurrent Infections: Patients with chronic osteomyelitis may experience recurrent episodes of infection, leading to a persistent or relapsing course of the condition. This can occur if the underlying infection is not adequately treated or if there are factors that impede proper healing.
Persistent Symptoms: Chronic osteomyelitis is characterized by persistent symptoms such as pain, swelling, and drainage from the affected area. These symptoms can persist over an extended period, contributing to the chronic nature of the condition.
Prevalence of acute hematogenous osteomyelitis in different bones in children.
AHO mostly occurs often in ___________children & commonly involves the metaphysis of long bones, esp. ____________and _________. Rather than the flat bones or spine
In children, metaphyses grow the fastest, i.e. around the knee, why?
> 50% infections occur in what bones?
AHO mostly occurs often in prepubertal children & commonly involves the metaphysis of long bones, esp. tibia & femur. Rather than the flat bones or spine
In children, metaphyses grow the fastest, i.e. around the knee, as richly vascularized to produce new bone rapidly.
> 50% infections occur in the femur, tibia or fibula
In adults, long bones less well-served by circulatory system rather ____________ receive more blood flow.
Common site of bone infection in adults: ________________
Why does vertebral osteomyelitis often occur in two contiguous vertebrae bodies & the intervertebral disc?
What is osteomyelitis in long bones mostly from in adults?
In adults, long bones less well-served by circulatory system rather vertebrae receive more blood flow.
Common site of bone infection: vertebrae (followed by long bones, pelvis & clavicle)
Primary blood supply of vertebrae is the segmental arteries, which perfuse adjacent vertebrae. So vertebral osteomyelitis often occurs in two contiguous vertebrae bodies & the intervertebral disc
Osteomyelitis in long bones mostly from a contiguous site e.g. secondary to prosthetic implant infection
Various bacteria can cause osteomyelitisresponsible for most bone infections?
Antimicrobial resistance is a challenge for treatment & significant no. of infections caused by which bacterium?
State the less common causes of osteomyelitis. (bacteria)
Various bacteria can cause osteomyelitis BUT S. aureus& coagulase-negative staphylococci i.e. S. epidermidisresponsible for most bone infections.
Antimicrobial resistance is a challenge for treatment & significant no. of infections caused by methicillin-resistantS. aureus(MRSA) strains.
Less common: Streptococcus spp., *Pseudomonas aeruginosa (foot bone infections),Enterobacteriaceae spp.
Define Peri-prosthetic joint infections.
Peri-prosthetic joint infections (PJIs) - infections occurring after joint reconstruction or replacement surgery (arthroplasty).
How many % of patients post-operatively are affected by Peri-prosthetic joint infections BUT assoc. with significant cost & morbidity.
Acute PJIs present within how many weeks of the index surgery?
May result from intraoperative seeding of implants or via hematogenous spread (blood stream infection) during early post-operative period.
Chronic PJIs occur after how long after the index surgery?
May result from a low-virulence organism seeded intraoperatively or failed treatment of earlier acute PJI.
Affect ~1-2% of patients post-operatively BUT assoc. with significant cost & morbidity
Acute PJIs present ≤4wk of the index surgery. May result from intraoperative seeding of implants or via haematogenous spread (blood stream infection) during early post-operative period
Chronic PJIs occur >1mth after the index surgery. May result from a low-virulence organism seeded intraoperatively or failed treatment of earlier acute PJI
Discuss the process and virulence factors involved to successfully cause Peri-prosthetic joint infections.
When a prosthetic device/ implant is placed in the body it’s quickly coated with adhesins from host’s extracellular fluid e.g. fibronectin, fibrin, fibrinogen. Fibronectin is an important adhesin for S. aureus
Once the implant is coated with these proteins it provides a ‘foundation’ for biofilm formation by certain bacteria
Biofilms: “a community of microorganisms in a structural matrix usually adherent to an underlying substratum”
Then produce an extracellular layer largely composed of polysaccharides e.g. polysaccharide intercellular adhesion, Glycerol teichoic acid
Biofilms encapsulate bacteria, preventing antibiotic perfusion thus reducing antimicrobial effectiveness
Describe PJI by process by S. aureus.
Post surgery traumatized bone releases several factors e.g. Mg2+ & prosthesis covered with extracellular matrix proteins
Biofilm comprises bacteria undergoing various metabolisms, embedded in excreted matrix mainly composed of polysaccharides, extracellular DNA & proteins
Antimicrobial action of immune cells hindered by biofilm
Diffusion gradients of nutrients & O2 develop i.e. higher conc. in upper layer vs. accumulation of metabolic wastes at lower level.
S. aureus quorum sensing (communication) helps regulate metabolism activity, production of virulence factors & other bacterial responses.
& (8) bacteria may persist within abscesses in surrounding tissues &/ OR exist within bone osteoblasts &/ OR colonize osteocyte lacuno-canalicular network (OLCN)
(Enabling S. aureus to evade the immune system & cause future reinfections
Outline the Pathogenic mechanisms by S. aureus during bone infection.
- Intracellular infection of osteoblasts, osteoclasts & osteocytes.
-Reservoir of S. aureus for long-term persistence in the bone. - Invasion of osteocyte-lacuno canalicular network (OLCN)
-Allows evasion of host immune cells - Survival in macrophages
-‘Trojan horse’ macrophages enable bacteria dissemination - Biofilm formation on bone &/or implants
-Resistance to antibiotic therapy - Staphylococcal abscess communities:
-S. aureusat the center of abscess surrounded by a fibrous pseudocapsule & layers of dead & live immune cells
Etiology of Peri-prosthetic joint infections.
PJIs are mostly caused by bacteria, notably:
Coagulase-negative staphylococci (30–43%)
S. aureus (12–23%)
Beta hemolytic Streptococcus spp. (9–10%)
Enterococcus spp. (3–7%)
Gram-negative organisms (3–6%) & anaerobes (2–4%)
Unusual organisms that can also cause PJIs incl.:
*Propionibacterium acnes considered non-pathogenic & may be dismissed as a contaminant BUT it’s become a common cause of shoulder PJIs
*Mycobacterium spp., Mycoplasma hominis, Corynebacterium spp., Actinomyces spp.
*Fungi e.g. Aspergillus fumigatus, Histoplasma capsulatum, Sporothrix schenckii, Candida spp
Osteomyelitis: Leading pathogens according to site
Microorganisms ranked from high to low prevalence or epidemiological importance.
State the micro-organisms ranking for the following.
1. Vertebral osteomyelitis
2. Diabetic foot infection
3. post-traumatic infection
4. Prosthetic-joint infection
- Vertebral osteomyelitis
Staphylococcus aureus
Gram-negative aerobic bacilli
Streptococcus species
Mycobacterium tuberculosis - Diabetic foot infection
Staphylococcus aureus
Pseudomonas aeroginosa
Streptococcus species
Enterococcus species
Coagulase-negative Staphylococci
Gram-negative aerobic bacilli
Anaerobes - post-traumatic infection
Staphylococcus aureus
Polymicrobial
Gram-negative aerobic bacilli
Anaerobes - Prosthetic-joint infection
Staphylococcus epidermidis
Staphylococcus aureus
Polymicrobial
Streptococcus species
Gram-negative aerobic bacilli
Propionibacterium acnes (esp. shoulder PJIs)
P. aeruginosa is an important cause of what osteomyelitis?
An important cause of osteomyelitis of tarsal & metatarsal bones of foot, after penetrating trauma e.g. stepping on protruding nail
P. aeruginosa septic arthritis following nail puncture wound to foot is a further complication.
P. aeruginosa also common cause of hematogenous osteomyelitis in intravenous drug users.
Describe the Diagnosis of PJIs
A multifaceted approach which includes:
i) A complete history; ii) physical examination; iii) diagnostic imaging studies; iv) serum laboratory tests; & v) synovial fluid analysis
Commonly patients present with a recent history of fevers, chills, inability to bear weight, pain on activity & at rest. BUT absence of fever & chills does not rule out PJIs
For chronic PJIs:
Symptoms may include prolonged drainage, sinus tract, wound that had difficulty healing, prolonged administration of antibiotics
But also:
Vague longstanding joint pain without any other signs of infection
Co-morbidities increase the risk of PJIs incl. inflammatory arthritis, obesity, diabetes & chronic renal failure
PJIs can be established with the following criteria:
These give a “strong consensus” BUT PJIs can occur without meeting these criteria, esp. those caused by less virulent organisms
Outline them.
A sinus tract communicating with the joint
Aspirated synovial fluid for differentiated cell count & culture:
*Two positive periprosthetic cultures with identical organisms & AST
*Elevated synovial fluid white blood cell (WBC) count i.e. high leukocyte counts& neutrophil percentage.
*Elevated serum C-Reactive protein & erythrocyte sedimentation rate
Elevated synovial polymorphonuclear cells
Presence of leukocyte esterase
Positive histologic analysis of periprosthetic tissue, bone fragments
Culture & testing of removed prosthetic implant
NB. radiographs are helpful for “ruling out” fractures or loosening, as opposed to diagnosing PJI.
Elevated C-Reactive protein (CRP) - an acute phase response protein produced in hepatocytes by cytokine stimulation (especially interleukin-6)
Elevated erythrocyte sedimentation rate (ESR) - measure of increased rbc aggregation. RBCs have a -ve charge whichkeeps them repelled from each other. During inflammation coating with fibrinogen causes them to aggregate.
Elevated CRP most sensitive parameter: levels rise & fall sooner than ESR (peaks after 2 days, returns to normal soon after treatment)
Whereas ESR may be normal or modestly elevated 1st few days, only rising after ≥1 week
**BUT. Elevated ESR &/ or CRP not specific to osteomyelitis. These must be interpreted with clinical symptoms & signs consistent with osteomyelitis.
Elevated White blood Cell count
May appear normal or only slightly elevated in neonates & children
Elevated Procalcitonin (biomarker for bacterial sepsis)
(IL-6 currently being investigated for use in diagnosis, as it correlates well with CRP)
State other points to consider for diagnosis.
*Negative (no growth) culture results do not mean absence of PJI :
Specific S. aureus phenotypes i.e. ‘Small-Colony Variants’ are slow-growers & may be missed in routine clinical cultures
Cultures specifically for fastidious, slow growing anaerobes, fungi & AFB should also be done especially when there are evident clinical symptoms & other PJI criteria are present
Administration of antibiotics affect culture results. So it’s recommended that if a patient is taking antibiotics, synovial fluid aspiration should be delayed for ~2wks. A longer antibiotic-free interval may be necessary for fastidious organisms
Also, fluid, bone & tissue samples if taken intraoperatively should also be sent for cultures & other tests.
Outline Microbial etiology by age.
Hematogenous (usually one microorganism)
- Infants (less than 1 year)
- Children (1-16 years)
- More than 16 years
- Infants (less than 1 year)
*Streptococci agalactiae
*Staphylococcus aureus
*E. coli - Children (1-16 years)
**Staphylococcus aureus
*Streptococci pyrogens
*H influenza - More than 16 years
*Staphylococcus epidermidis
*S, aureus
*Pseudomonas aeruginosa
*Serratia marcescens
*E. coli ** Contiguous (usually polymicrobial)
In children the incidence of Hemophilus influenzae & Streptococcus spp. reduced following implementation of childhood vaccinations
Kingella kingae (Gram negative coccobacillus- emerging pathogen in infants & children
In children born with hemoglobin disease e.g. sickle cell disease Salmonella species common cause of osteomyelitis
What is the leading cause of acute hematogenous osteomyelitis (AHO) in all age groups?
Accounts for how many % AHO infections in children?
S. aureus leading cause of acute hematogenous osteomyelitis (AHO) in all age groups.
Accounts for 70–90% AHO infections in children.
What are virulence factors?
What drives the success of S. aureus?
Extensive armor of cell-associated & extracellular factors contributing to virulence & pathogenicity.
Adhesins: facilitate attachment to extracellular matrix proteins (fibrinogen, fibronectin, collagen & bone sialoprotein). Crucial for early colonization of host tissues & implanted prosthetic devices.
Evasion from host immune defenses e.g. intracellular infection, protein A & capsular polysaccharides,
Toxins: facilitate tissue penetration & lysis of blood cells e.g. neutrophils, rbc’s
Survival in mammalian host cells e.g. osteoblasts (changing morphology to Small Colony Variants)
Why does S. aureus have a predilection for bone?
Correct ‘tools for the work’ i.e. adherence & virulence factors.
Bone formed by stringently regulated activities between osteoblasts & osteoclasts
Osteoblasts
Comprise collagen & non-collagenous proteins
Enveloped by bone extracellular matrix (BEM)
Osteoblast activity altered in pathological conditions i.e. infections, cancers & autoimmune diseases
What does s. aureus pathogenicity in osteomyelitis & joint infections depends on?
Its pathogenicity in osteomyelitis & joint infections depends on
*Adherence/ tissue tropism
*Evasion of host defense systems
*Ability to cause damage to host tissues
Adherence via ‘Microbial Surface Components Recognizing Matrix Molecules – ‘MSCRAMMS’
Anchored in S. aureus cell wall peptidoglycan
Several of its MSCRAMMS bind to BEM components
Describe the Infection process & virulence factors of S. aureus.
Infection process & virulence factors of S. aureus
Adherence/ tissue tropism via MSCRAMMs
Virulencefactorsthathelp in evasion of hostimmunedefenses
Toxins & other factorsthatinhibitneutrophil function & facilitate further infection.
Can change its metabolism & morphology to enable deep & chronic infection.
State examples of Microbial Surface Components Recognizing Matrix Molecules-MSCRAMMS.
- Collagen adhesin (Cna)
Binds to type I collagen (& cartilage)
Type I collagen highly expressed during osteoblast maturation i.e bone matrix deposition & mineralization
S. aureus Cna highly expressed during exponential growth; also assoc. with biofilm formation. - Bone sialoprotein binding protein (Bbp) binds to bone sialoprotein (& fibrinogen)
*High levels of sialoprotein in newly forming bones. Bbp commonly expressed in S. aureus strains causing hematogenous osteomyelitis in children.
NB. Cna & Bbp are major virulence factors esp. in strains causing PJIs.
- Fibronectin binding proteins A & B (FnBP A/B)
Indwelling implants become coated with plasma proteins e.g. fibronectin
Fibronectin acts as a bridge betweenS. aureus& osteoblast via α5β1 integrin. Facilitates internalization of S. aureus into phagocytic vesicles in the osteoblast. Bacteria safe from immune or antibiotic attack
S. aureus also induces expression RANKL in osteoblasts which in turn recruits osteoclasts & promotes their proliferation. Increasing bone resorption which further weakens the bone skeleton. - Clumping Factor A & Clumping Factor B (Clf A & Clf B)
*Also bind to fibrinogen & mediates platelet aggregation - Protein A
S. aureus surface polypeptide that binds to the Fcγ portion of immunoglobulins. Hindering opsonisation & phagocytosis
Interacts with von Willebrand factor via its Fcγ binding site which may also contribute to aggregation of S. aureus with platelets
Also binds to pre-osteoblastic cells via Tumour Necrosis Factor Receptor-1. Triggers premature signaling resulting in osteoblast apoptosis
- MHC II analog protein (Map)
Binds to osteopontin
Osteopontin involved in regulation of matrix mineralization during bone formation
Describe the evasion tactics used by S aureus
Coagulase (& von Willebrand factor)
In host: coagulation is innate defense mechanism against microbial pathogens. Traps & immobilizes them in a clot
Thrombin converts soluble fibrinogen into a fibrin network, seals off leaking blood vessels & generates barrier against bacterial spreading. Fibrin barrier serves as a scaffold for leukocyte adhesion & activation
S. aureus: expresses coagulase, it binds to prothrombin, forms ‘staphylothrombin’ (plasma clot around the bacterium) & coats with ‘fibrin’. A ‘pseudocapsule’ that facilitates escape from opsonophagocytic clearance
**Evasion of innate & adaptive immunity
Coagulase (& von Willebrand factor)
Facilitate establishment of microthrombi, thereby disrupting local bloodflow & promoting staphylococcal adhesion to vasculature
Endothelial cells in association with staphylococci in microthrombi more exposed to higher concentrations of S. aureus toxins
Promotes S. aureus colonization & invasion across blood vessels & into organ tissues or bones
Leading to local injury & inflammation in affected tissues &/ or bone
- Evasion tactics…..Biofilm & Small Colony Variants
Biofilm
Some S. aureus strains, secrete a exopolysaccharides (polysaccharide intercellular adhesin) external to their cell wall
Immediately after adhesion, it covers the bacteria, an integral step towards the development of a biofilm
Small Colony Variants
Inside osteoblasts and osteocytes S. aureus transition into Small Colony Variants (SCV). Slow-growing with low metabolic activity
Higher capacity for intracellular persistence & less liable to antibiotics
SCVs revert to virulent wild-type when exiting osteoblasts & infecting new cells
Describe Staphylococcus aureus activity process in Implanted prosthetic orthopedic devices.
S. aureus & S. epidermidis can also form biofilms (slime) on host tissue & ‘inert’ implanted prosthetic devices
Biofilm may be defined as “a structured community of bacterial cells enclosed in a self-produced polymeric matrix”
x100 –1000 less susceptible to antibiotics than planktonic (free-growing) bacteria
Evasion of host immune defenses (phagocytosis) & antibiotic treatment
Antibiotics suppress infection within a biofilm temporarily but relapses occur, unless there is removal of the device
Describe the Other S. aureus cytolytic factors: Phenol Soluble Modulins (PSMs).
Phenol Soluble Modulins (PSMs)
Small, amphipathic (hydrophilic and lipophilic properties) peptides with non-specific cytolytic activity
Facilitate escape of S. aureus from intracellular vesicle in the osteoblast & act on cell membrane causing osteoblast death
Activates plasminogen to plasmin, which degrades fibrin clots that keep staphylococcal infection localised. Releasing the bacteria & facilitating further bacterial penetration
S. aureus lysis of blood cells
S. aureus produces a variety of cytolytic toxins
Those that lyse red blood cells are termed _________& those that target white blood cells are __________.
Alpha-toxin first identified S. aureus haemolysin. A beta-barrel forming toxin, lytic to rbc’s & some leukocytes, but not neutrophils.
Bi-component leukotoxins that are structurally similar to alpha-toxin i.e. belong to beta-barrel pore-forming toxin family.
e.g. Panton-Valentine leucocidin, leukocidins LukDE & LukAB (LukGH) & gamma-toxin
S. aureus also produces lipases & nucleases, functions in pathogenesis poorly understood. Possibly, may decrease the antibacterial activity of neutrophils
S. aureus produces a variety of cytolytic toxins
Those that lyse red blood cells are termed haemolysins & those that target white blood cells are leukotoxins
S. aureus Membrane-damaging toxins.
State toxins that bind to specific receptors & form pores
Phenol Soluble Modulins thought to attach to the cytoplasmic membrane in non-specific manner, leading to membrane disintegration. Usually temporary or short-lived e.g. delta-toxin.
Alpha-toxin & the bi-component leukotoxins bind to specific receptors & form pores e.g. alpha-toxin, PVL, LukAB (LukGH) & LukDE.
