Infection Flashcards
CASE:
1 day history of fever, headache, joint pains, lethargy
Pulse rate 130 (tachycardic), BP 80/50, Resp rate 30
Cap refill 3 seconds
Drowsy, neck stiffness
Non-blanching rash
Diagnosis?
Meningococcal meningitis
Neisseria meningitidis
Tumbler for blanching vs non blanching rash?
NON BLANCHING
Press tumbler against petechial or purpuric rash- if it does not blanch and remains visible through the glass
Non blanching rash in febrile/unwell patient constitutes a medical emergency
When are antibiotics started if infection is suspected?
start before lab results
Rapid tests for infection:
FBC- look at Hb and WBC (neutrophilia?) and platelets (low?)
CRP- C reactive protein (is it raised?)
Procalcitonin- also a marker of infection
When can you wait for further lab results?
Non-microbiological tests
Microbiological tests
but NOT in a life threatening situation eg meningitis
Bacterial microbiological investigation- what 3 key things do you look at?
Microscopy:
visualise the infectious agent with presumptive identification
enumerate white blood cells
Culture:
isolate the infectious agent
identify the infectious agent
sub-type the infectious agent
Sensitivity:
Antimicrobial susceptibility testing
Results may come in stages
What bacterial meningitis are neonates at risk of getting?
- Group B Streptococcus(Streptococcus agalactiae) COMMON
- Escherichia coli and other enterobacteriales LESS COMMON
- Listeria monocytogenes, Streptococcus pneumoniae and Haemophilus influenzae (non-capsular strains) RARE
What bacterial meningitis are children and adults at risk of getting?
Neisseria meningitidis (meningococcus),
H. influenzae (type b, rare in those older than 5)
S. pneumoniae (pneumococcus),
L. monocytogenes (immunocompromised)
What bacterial meningitis are elderly adults at risk of getting?
S. pneumoniae
L. monocytogenes
Mycobacterium tuberculosis (any age but commoner in adults)
(Fungal e.g. Cryptococcus neoformans (usually immunocompromised))
What is the mortality rate of N meningitidis, H influenzae, Strep. pneumoniae?
low to high
5% Neisseria meningitidis
8% Haemophilus influenzae type b
25% Streptococcus pneumoniae
Long term complications of meningitis?
33% survivors have permanent deficit:
e.g. limb loss, deafness, learning difficulties,
blindness, seizures, hydrocephalus.
‘Aseptic meningitis’ causes
Enteroviruses
- mainly young kids under 5
- less severe than bacterial
- 7-10 days illness
- little pathology
Mumps – no parotitis in 40-50%
Varicella rotavirus
Herpesvirus – HSV-2 - 0.5 - 3% (often recuurent – Mollaret’s meningitis)
HIV
Identifying the causative agent allows what to occur?
Targeted treatment
- Agent and duration, adjunctive actions
Prognostication
Prevention in others
- Antibiotic prophylaxis, vaccination, isolation
Epidemiological information
- Public heath and vaccination strategies, antimicrobial susceptibility data useful for stewardship
Generates research questions
How long do microscopy and anitgen detection tests take?
Initial results from microscopy and antigen detection tests can guide therapy on day 1
Further results will be available on day 2. Some tests take longer
Why would a CSF sample be cloudy?
Increased protein and white cells
eg in meninngitis
CSF parameters investigated for suspected meningitis
Protein
Glucose
WBC
Visualization of microbes- what can be seen using light microscopy vs electron microscopy?
BACTERIA
FUNGI
PROTOZOA, HELMINTHS
Can be seen using light microscopy x400 – x1000
VIRUSES
Electron microscopy x40,000
Which bacteria move more rapidly?
Motile bacteria with flagella move rapidly
Non-motile bacteria also move “on the spot”
i.e. Brownian motion
GRAM STAINING
Gram positive – blue/purple
Gram negative- red
Which gram bacteria is each one?
Which one is gram neg and gram pos?
Purple= gram positive Pink= gram negative
Morphology of:
Cocci
Rods/bacilli
Cocci
- These are spherical
Rods or bacilli
- These are cylindrical or “sausage” shaped
Some other bacteria have a spiral or helical appearance e.g. spirochaetes, or vibrios
Identify the bacteria
Gram positive cocci in chains e.g. Streptococci
Identify the bacteria
Gram positive cocci in clusters e.g. Staphylococci
Identify the bacteria
Gram positive rod (bacillus)
e.g. Bacillus anthracis
Identify the bacteria and the highlighted structures?
Gram negative cocci in pairs (diplococci)
e.g Neisseria meningitidis
and Neutrophils
Identify the bacteria
Gram negative rods (bacilli)
e.g Escherichia coli
Identify the bacteria
Vibrio
Gram negative
e.g Vibrio cholerae
Identify the bacteria and what colour stain it has
Spirochaete
e.g. leptospira
(silver stain)
Gram stain of a CSF from a 60 year old man
with sudden onset signs of meningitis
Identify the bacteria
Streptococcus pneumoniae (Pneumococcus) Gram positive diplococci
Gram stain of a CSF from a 3 year old child with meningitis
Identify the bacteria
Shows Gram negative rods which on culture grew
Haemophilus influenzae
Gram negative bacilli
short
What are these structures?
What is the detection rate for Listeria?
Only about 40% detection rate for Listeria ( vs >80% for other bacteria causing meningitis)
- sparse organisms; intracellular
image- gram positive rods
Identify the bacteria
Neisseria meningitidis
CSF Gram stain from a case of meningitis
showing bean shaped Gram-negative diplococci
and numerous polymorphs
What rapid detection tests are there?
Polymerase Chain reaction (PCR)
Latex agglutination tests
Polymerase Chain reaction (PCR)
detects bacteria specific DNA (or viral RNA or DNA)
very sensitive;
May still be positive after antibiotics, unlike culture
Latex agglutination tests
Less sensitive than PCR, but more rapid (though PCR is becoming more cost effective and quick)
Can detect:
Group B streptococcus (S. agalactiae)
Haemophilus influenzae type b
Streptococcus pneumoniae
Neisseria meningitidis types A,B,C,Y and W
E.coli K1
Latex agglutination result for:
N. meningitidis
H. influenzae
positive agglutination for Neisseria meningitidis
negative agglutination for range of other bacteria e.g.
- H. influenzae, group B strep, E coli etc
Serotyping of bacteria by latex agglutination
Bacteria can be “serotyped” either directly from a CSF sample, provided there are sufficient bacteria present, or from an isolated colony from a culture plate.
The bacteria are mixed with “Group specific” antibody reagents that discriminate the range of different “serotypes” that usually circulate in the community.
Agglutination indicates the Serotype
Identify the bacteria and structures
Non hemolytic colonies
non-hemolytic colonies are typical of normal upper respiratory (mouth) flora seen
after culture on blood agar of a sputum specimen from suspected bacterial pneumonia
Beta hemolytic colonies
The presence of beta-hemolytic colonies indicates the possibility of Streptococcus pyogenes
(Group A streptococcus) infection
bacterial flora cultured on a blood agar plate.
What substance is Streptococcus pneumoniae susceptible to?
Optochin
and bile lyses it
How does MALDI-TOF work?
• Analyte is co-crystalised with an
excess of a matrix
• Matrix is a UV absorbing weak
organic acid
• Matrix absorbs energy form the
laser pulse and is vaporised
carrying the analyte with it
• Because the matrix absorbs most
of the energy, the analyte
molecules are protected
• Ionisation of the analyte occurs
probably by protonation during
the desorption phase – usually as
a single-charged ion
• Analyte ions are now in a
gas phase
• They are accelerated via an
electrostatic field
• Ejected through a metal
flight tube subjected to a
vacuum until they reach a
detector
• Detector generates an
electrical signature which is
then displayed as a
spectrum
MALDI-TOF: What is time of flight dependant on?
– The mass (m) of the molecule
– The charge of the molecule (z)
– It is proportional to the square root of m/z
• MALDI usually produces single charge so usually m/z is equivalent to the mass of the molecule
Semi quantitative method for AST?
Disc Diffusion
most widely used method
The isolate is inoculated on semisolid agar
medium with antibiotic impregnated discs
and incubated for 18-20 hours
The diameter of the zone of inhibition of
growth around the disc is measured and
the bacterium is categorised as resistant,
intermediate or susceptible depending on
the zone size and pre-defined criteria.
No zone inhibition if resistant (eg the two in the image with no circles)
Pros and cons of electron micrsoscopy
What criteria is the traffic light system based on?
Colour Activity Respiratory Hydration Other
GREEN (low risk)
AMBER (intermediate risk)
Hydration: dry mucus membranes + poor feeding in infants
- CRT > 3 seconds
- reduced urine output
Other: fever for 5 or more days
- limb/joint swelling
- non-weight bearing/not using an extremity
- new lump > 2cm
RED (high risk)
What is involved in a septic screen?
Neurological contra-indications to lumbar puncture
Raised ICP/ risk of incipient herniation
--> Reduced (GCS <9) or fluctuating GCS (drop of ≥3) Relative bradycardia & hypertension Focal neurological signs Abnormal posture or posturing Unequal, dilated or poorly responsive pupils Papilloedema Abnormal ‘doll’s eye’ movements
–>
Seizures:
- Recent (within 30 minutes) or prolonged (over 30 minutes) convulsive seizures
- Focal or tonic seizures
From NICE Guidelines 2010: Bacterial meningitis and meningococcal septicaemia in children
Severely ill child, how to treat if:
Septicaemia
Meningitis
CASE:
4 year old child Never immunised Presents with 5 days of illness Fever, conjunctivitis, miserable Croupy cough Cervical lymphadenopathy Developed rash previous day
Diagnosis?
Measles
Presentation of varicella
