Microbiology Flashcards
What is a sinus?
A sinus is an abnormal communication from deep tissue to the skin, and it indicates active infection underneath (they can’t have a treated or cured infection).
Define osteomyelitis
A progressive infectious process resulting in inflammatory destruction, bone necrosis (sequestrum) and new bone formation (involucrum).
What are the 3 types of osteomyelitis I should know about?
Haematogenous, Contiguous and Diabetic
Tell me about haematogenous osteomyelitis
Following bacteraemia, especially in children (because their bones are very vascular), metaphyseal area of long bones (because this is where most of the metabolic activity is).
Tell me about contiguous osteomyelitis
After trauma or surgery, or overlying soft tissue infection. May be associated with prosthesis/pins/plates. Direct spread of infection. Affects all ages, any bone
Tell me about diabetic osteomyelitis
A consequence of reduced vascularity, neuropathic skin changes, decreased local immunity and metabolic disturbance. Often associated with foot ulcers. Assume osteomyelitis if bone evident at the base of the ulcer. Very hard to treat. Often results in amputation
What might you see on an x-ray in a patient with osteomyelitis?
Periosteal thickening/ elevation
What is the overall most common pathogen to cause osteomyelitis?
Staphyloccocus aureus
What is the most common pathogen to cause osteomyelitis in newborn babies? (Note this is not the most common cause of osteomyelitis overall)
Group B streptococci (normal vaginal flora)
What is the most common pathogen to cause septic arthritis?
Staphylococcus aureus
Is reactive arthritis a joint infection?
Not a joint infection, it is a post-infectious phenomenon that involves the joints and makes them painful. It is a sterile type of arthritis and if you aspirate the joint there will be white cells but no organisms present because there was never an infection in the joint itself- it is an immune response to an (usually GI/GU) infection
What is the most common pathogen to cause prosthetic joint infections?
Coagulase negative staphylococci
What are the treatment options for prosthetic joint infection?
Conservative - DAIR - debridement, antibiotics, implant retention Radical i.e. remove prosthesis- either 1 stage or 2 stage Lifelong suppressive therapy if unfit for surgery Do nothing- if elderly, comorbidities and current symptoms do not impact QOL
What tend to be the antibiotic treatment durations for osteomyelitis (paediatric and adult), septic arthritis and prosthetic joint infections?
2-3 weeks for septic arthritis 4 weeks for paediatric osteomyelitis 6-8 + weeks for adult osteomyelitis but PJI may require months if prosthesis still in, or years if persistent and can’t cure
What antibiotics tend to be given to treat a bone/joint infection caused by staph aureus?
Flucloxacillin + rifampicin, fusidic acid or gentamicin
What does ‘facultative’ mean with regards to optimum atmospheric conditions for bacteria?
Organisms that can grow in the presence or absence of oxygen.
What is meant by the terms ‘psychrophile’, ‘thermophile’ and ‘mesophile’ with respect to optimum temperature of bacteria.
Psychrophile = organisms capable of growth in low temperature ranging from -200 to 10 degrees Thermophile= organisms that thrive at unusually high temperatures between 40 and 122 degrees e.g. campylobacter Mesophiles= organisms that grow happily at 37 degrees e.g. E-coli
How do staphylococcus and streptococcus organisms sit next to each other?
Staphylococcus organisms = grape like Streptococcus = in a chain
Summarise key points about the gram stain
Exploits differences in the bacterial cell wall in the staining process. Purple = gram positive. Pink= gram negative - pink has an ‘n’ in it so it is gram negative
Tell me about cross linking within peptidoglycan
The strands (comprised of N-acetylmuramic acid and N-acetylglucosamine) are cross linked by short runs of amino acids. Gram negative bacteria have only a single layer, gram positive bacteria contain up to 40 layers of peptidoglycan.
How does penicillin work as an antibiotic?
Note that the last two amino acid molecules in peptidoglycan are always alanines. Penicillin (which is a beta-lactam antibiotic) is an analogue of alanine. The enzymes which add the alanine to the chains have a greater affinity for penicillin, and so the organism is unable to make its cell wall and in an osmotic environment, the cell will lyse.
What is endocarditis?
Bacterial (or fugal) infection of a heart valve or area of endocardium. Clinical presentation traditionally classified as either acute or subacute. If you have a short history and present acutely it is more likely to be staph aureus, whereas if you have been unwell for weeks/months it might be more likely to be one of the viridans streptococci.
What do people with endocarditis generally die of?
They don’t tend to die of the bloodstream infection, they die of the cardiac complications such as heart failure (because of the damaged heart valves). In treatment, we aim to reduce the amount of damage that is accred (whether that be medical or surgical Tx).
What are the four categories of infective endocarditis?
Native valve infective endocarditis Prosthetic valve infective endocarditis IVDU-associated endocarditis Nosocomial infective endocarditis
Tell me about native valve endocarditis
Congenital heart disease (high to lower pressure gradients greatest risk) Rheumatic heart disease Mitral valve prolapse Degenerative valve lesions
What organisms are typically involved in native valve endocarditis?
Viridans streptococci
Tell me about prosthetic valve endocarditis and the commonest organism which causes it
Only 1-5% of cases. Coagulase negative staphylococci predominate (e.g. staph epidermis, staph saprophyticus).
Tell me about IVDU associated endocarditis and the organism most likely to cause it
Right sided infection more common. Tricuspid 50%, aortic 25%, mitral 20%. Staphylococcus aureus predominates, but other organisms including fungi (e.g. candida tropicalis) sometimes responsible.
What are the HÁČEK organisms in infective endocarditis?
Although, staph aureus, streptococcus species and enterococci together are responsible for >80% of cases of IE, there is a group of gram negative organisms (bacilli) which can cause IE they are often found in the upper airway and usually younger people with underlying cardiac problems tend to present with these organisms.
What infecting organisms may be ‘culture negtive’ in IE?
Q fever (coxiella burnetti) -zoonotic infection from goats Bartonella app Trypheryma whipplei
What are the clinical features of infective endocarditis?
Malaise (95%), pyrexia (90%), arthralgia (25%) Cardiac murmurs (90%), cardiac failure (5%) Oslers nodes (15%) Janeway lesions (5%), splinter haemorrhages (10%), Roth spots (5%). Oslers nodes are painful whereas janeway lesions are painless. Roth spots are flame-shaped haemorrhages on the retina. Splenomegaly (40%), cerebral emboli (20%), haematuria (70%) haematuria because they have a degree of glomerulonephritis
How does the clinical criteria work for IE? (Duke’s criteria, Durack et al.)
(2 major) or (1 major and 3 minor) or (5 minor)
What are the major criteria for IE?
Positive blood culture- typical organisms for IE from 2 separate blood cultures. Persistently positive blood cultures. Evidence of endocardial involvement- positive echocardiogram- vegetation’s, abscess, new partial dehiscence of prosthetic valve, new valvular regurgitation
What are the minor criteria for IE?
Predisposition (heart condition, IVDA) Fever Vascular phenomena (major arterial emboli, septic pulmonary infarcts, intracranial haemorrhage, Janeway lesions) Immunological phenomena: glomerulonephritis, Osler’s nodes, Roth spots, rheumatoid factor Microbiological evidence: positive blood culture but not meeting major criteria Echocardiogram: consistent with IE, but not meeting major criteria
bacterial causes of gastroenteritis
