Microbiology Flashcards
What does it mean that staphylococcus sp. are aerobic and facultatively anaerobic?
They grow best aerobically (in air) but also grow anaerobically
How can staph aureus be distinguished from other staph spp?
It is the only coagulase +ve staph spp.
How can coagulase -ve staph saprophyticus be distinguished from other staph spp?
It is the only novobiocin resistant staph spp
Which antibiotic is first-line for…
- Staph aureus
- MRSA
Staph aureus: Flucloxacillin (or Doxycycline if allergic)
MRSA: Vancomycin
Which antibiotic may be used alongside another to treat staph aureus but never by itself?
Rifampicin
PVL toxin secreted by staph aureus/MRSA can cause formation of…
Abscesses and necrotic lesions
What is osteomyelitis?
Inflammation of bone and the medullary cavity
Describe haematogenous osteomyelitis and which bones are most often affected in children vs adults
- Caused by blood spread
- Usually monomicrobial (one organism)
- Most common in long bones in children
- Most common in vertebrae in adults (due to good vascular supply)
Describe contiguous osteomyelitis and the most common causes in the young vs elderly
- Caused by local infection
- Usually polymicrobial (more than one bacteria)
- Seen after injury or surgery in the young
- Seen due to pressure sores or vascular insufficiency in the elderly
- Seen in diabetic foot
Which pathogens most commonly cause osteomyelitis in…
- Newborns and children
- Adults
- Newborns and children: Staph aureus, Group B strep
- Adults: Staph aureus (frequency of gram -ve bacteria increased in the elderly)
What symptoms should cause clinical suspicion of infection?
- Fever
- Pain
- Inflammation
- Loss of function
Describe the clinical presentation of acute vs chronic osteomyelitis
Acute: abrupt onset of intense pain, erythema
Chronic: gradually worsening pain over months/years, loss of function
Which blood investigation may be useful in osteomyelitis?
CRP is useful for monitoring response to treatment (raised in infection)
What is a probe to bone (PTB) test?
A test used in the examination of potential osteomyelitis
A probe is inserted into a suspicious ulcer and if a solid ‘click’ is heard and a gritty surface is felt inside, that means bone has been hit
What is the gold standard method for taking a sample for osteomyelitis investigations?
Bone biopsy
List indirect methods for confirming diagnosis of infection (2)
X-ray (may be normal)
Histology (inflammation)
List direct methods for confirming diagnosis of infection (2)
Microbial growth from sample
PCR
Name 4 different types of osteomyelitis
- Open fracture
- Diabetic/venous insufficiency
- Haematogenous
- Vertebral
Haematogenous osteomyelitis is most common in… (5)
- Pre-pubescent children
- PWID
- Central lines/dialysis patients
- Elderly
- Sickle cell disease
The most common causative organism for haematogenous osteomyelitis is…
Staph aureus
What are the 2 steps for treatment of osteomyelitis?
- Source control (may require surgical debridement in some cases)
- Antibiotics (can fight infection now that source is controlled)
Patients should be given empirical treatment while awaiting a microbiological diagnosis. T/F?
False
If the patient is not acutely unwell, wait for microbiology then use specific antibiotic
Empirical treatment is only used if the patient is septic or has a soft tissue infection
Name the first-line IV and oral antibiotics used for treating... - Gram +ve - Gram -ve - Anaerobes ... bone infection
Gram +ve
- IV: Flucloxacillin (Vancomycin if penicillin allergic)
- Oral: Doxycycline
Gram -ve
- IV: Gentamicin
- Oral: Cotrimoxazole/ Doxycycline
Anaerobes
- Metronidazole
What is the usual duration of antibiotic treatment for osteomyelitis?
6 weeks
Define…
- Primary bacteraemia
- Complicated bacteraemia
- Primary bacteraemia: bacteraemia with no known cause
- Complicated bacteraemia: bacteraemia with systemic effects e.g., infective endocarditis
Which beta-haemolytic strep are most often seen in osteomyelitis?
Group A strep
Group B strep
Which alpha-haemolytic strep are most often seen in osteomyelitis and where are the more often found?
Strep pneumoniae - pathogen that most often causes pneumonia
Strep viridans - commensal of mouth, throat, vagina etc
Which non-haemolytic strep are most often seen in osteomyelitis and where are they more commonly found?
Enterococcus spp - commensals of bowel, more commonly cause UTI
Why are people with sickle cell disease at higher risk of osteomyelitis?
They are prone to bone infarcts which are high risk for infection (staph aureus most common)
What is the most common presentation of skeletal tuberculosis (Pott’s disease)?
Chronic back pain localised to the site of involvement over months or years
What are the 2 mechanisms by which a prosthetic joint can become infected?
- Direct inoculation at time of surgery
- Seeding of joint at a later time due to haematogenous spread
What are the most common organisms that cause... - Early prosthetic joint infections - Chronic prosthetic joint infections - Haematogenous joint infections ...?
- Early: Staph aureus, coagulase -ve staph
- Chronic: (lower virulence organisms) coagulase -ve staph, cutibacterium, corynebacterium, staph aureus
- Haematogenous: (high virulence, abrupt onset) staph aureus, gram -ve bacteria
What is the difference between planktonic and sessile bacteria?
Planktonic: active and replicate rapidly, cause most inflammatory symptoms and are susceptible to antibiotics
Sessile: dormant bacteria that form biofilms and are more resistant to antibiotics
What is a biofilm?
A community of sessile microbes that adhere to a surface (e.g., a prosthetic or body surface) and show greater resistance to antibiotics
Which tests should be carried out to diagnose prosthetic joint infection?
Culture (at least 5 tissue samples to find causative organism)
Blood culture
CRP
Radiography
Which antibiotics are given empirically to treat prosthetic joint infections while awaiting culture results?
Vancomycin (gram +ve cover) and Gentamicin (gram -ve bacilli cover)
What is septic arthritis? How is it introduced?
Infection of the joint space
By haematogenous spread (most common), direct invasion of penetrating wound or spread from infectious bone/soft tissue
What are the most common organisms to cause of septic arthritis? (4)
Staph aureus
Streptococci
N gonorrhoea (think STIs)
H influenza (think preschool children)
List diagnostic investigations for septic arthritis (3)
Blood culture (take in all cases as haematogenous spread is most common cause)
Joint fluid aspiration for microscopy, culture and sensitivity
Imaging (xray, MRI)
Describe the colour, WCC and PMN count of fluid aspirate in septic arthritis
Colour: opaque
WCC >100,000
PMN count >90%
What is the empirical treatment for septic arthritis?
What about for <5 y/o?
High dose IV flucloxacillin
Add Ceftriaxone (for H influenza cover)
What is the directed treatment plan for septic arthritis?
- At least 1 week IV antibiotics then switch to oral
- Washout to reduce bacterial load and debris
- Treatment usually 2-4 weeks but can progress to 6 weeks if required
Describe the histological appearance of clostridium tetani
Gram +ve strictly anaerobic rods
Which organism causes tetanus?
Clostridium tetani
Name the first-line antibiotic used to treat infection with each of the following organisms:
- Staph aureus
- Staph epidermis
- Strep pyogenes
- Gram -ves
- Anaerobes
- Staph aureus: Flucloxacillin
- Staph epidermis: Vancomycin
- Strep pyogenes: Doxycycline
- Gram -ves: Clindamycin
- Anaerobes: Metronidazole
Cotrimoxazol
What is tetanus?
A toxin mediated illness that results in spastic paralysis
How does clostridium tetani cause tetanus?
It releases a neurotoxin which binds to inhibitory neurones and prevents release of neurotransmitters
This causes spastic paralysis
How is tetanus diagnosed?
- Clinical spastic paralysis
- Culture (difficult to obtain, anaerobic gram +ve, drumstick shaped spore)
- Serum and urine toxin assays
What is the treatment for tetanus?
- Surgical debridement
- Antitoxin
- Supportive measures
- Antibiotics (penicillin and metronidazole)
- Booster vaccination
Why do people recovering from tetanus still need a booster tetanus injection?
Survivors are not immune to future infection
List some possible causes of myositis? (4)
- Viral (e.g., HIV, HTLV-1, influenza, CMV)
- Bacterial (e.g., staph aureus, strep viridans)
- Fungal
- Parasites
How does clostridium tetani enter a host?
Spores (often found in soil) enter through a wound
What is Gaucher’s disease?
A genetic lysosomal storage disorder that causes lipids to build up in organs such as the liver and spleen
What is SAPHO?
A rare inflammatory disorder affecting the bone, joints and skin, causing pain and swelling in those areas
(synovitis acne pustolosis hyperostosis osteitis)
What is CRMO?
A rare autoimmune disease that causes inflammation of the bones, most often in children
