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
Which agar is used for streptococcus pneumoniae?
Blood agar
Which agar is used for Neisseria meningitidis?
Chocolate agar
Why is gram positive not always diagnostic for disease?
Carriage rate for pneumococci - will show presence with no disease
Thus, +ve Gram not always diagnostic for disease
Gram positive diplococci and:
Epithelial cells ↑
PMNS ↑
Epithelial cells +++ and G+ve diplococci - carriage
PMNs + and G+ve diplococci - possible pneumococcal disease
Signs to look for in sample source
Naso-pharengeal aspirates, throat swabs, sputum…. -> large numbers of commensals
Need differential growth selection
Pathogen may be commensal e.g. S. pneumoniae
Look for other signs – inflammation e.g. neutrophils
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 is meningococcal sepsis
Septicaemia- more than meningitis
Shock/hypotension
Multiorgan failure
Rash
How does blood culturing work?
Resin at bottom of bottle changes colour as redox potential changes, chemical properties altered
Can be positive (for evil bacteria) or negative
What substance is Streptococcus pneumoniae susceptible to?
Optochin
and bile lyses it
Name a bacteria that produces oxidase
Neisseria meningitidis
on chocolate agar colonies turn blue
What technology is used to identify organisms in labs?
MALDI-TOF MS spectrometry
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
How do we choose the antibiotics to be tested?
Bacterial isolates are tested against panel of antibiotics.
• These are appropriate for the species (either known or predicted), the
specimen site and which antibiotics are on the hospital formulary and
recommended in guidelines.
• Some antibiotics may not be used for treatment but are proxies for
others.
• Further antibiotics may be tested if initial testing reveals resistance.
• Although all susceptibility results will be recorded, not all will be
reported to the clinician
Possible results of antibiotic testing
Resistant: high likelihood of treatment failure
Susceptible: high likelihood of treatment success
Intermediate/moderately sensitive: uncertain effect. It implies that an infection may be appropriately treated in body sites where the drugs are concentrated or when higher doses can be used
3 methods of antimicrobial susceptibility testing
Semi-quantitative and quantitative methods. These detect inhibition or lack of inhibition of growth of a microbe when exposed to an antimicrobial agent.
Detection of a phenotypic characteristics that predict resistance or susceptibility
Detection of a molecular characteristic that predicts resistance or susceptibility
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)
Quantitative methods- minimum inhibitory concentration (MIC)
The exact concentration of antibiotic needed to inhibit growth can be determined and is known as the MIC.
An MIC may be needed for several reasons. For some species/antibiotic combinations, disc diffusion techniques are not reliable e.g. Neisseria meningitidis and penicillin, cefotaxime, ciprofloxacin.
- In some infections the MIC determines choice and duration of treatment e.g. penicillin MIC and treatment of streptococcal endocarditis.
- MICs are used when investigating activity of new antimicrobials.
- Some laboratories use a breakpoint method for determining susceptibility for all clinical isolates, especially if they have automated systems
Continuous gradient disc diffusion method
works by establishing a concentration gradient of antibiotic on an agar plate. A commercially available form, known as the Etest, (Arvidson et al 1988), consists of a plastic strip that is coated on the underside with the antibiotic.
This is placed on the inoculated plate and the antibiotic diffuses out producing concentration gradient. The upper surface of the strip is marked with the antibiotic gradient concentrations.
The MIC is read at
the point that the growth (at the
edge of an elliptic zone of inhibition)
abuts the gradient scale.
Agar incorporation breakpoint method for AST
Each well contains agar incorporated amoxicillin with at a concentration of 8 mg/L. Isolates from
patient specimens have been inoculated onto the agar. A growth indicator is present – yellow =
growth and therefore resistant
Green = no growth and therefore sensitive
Macro and micro broth dilution method for
determining the MIC
Doubling dilutions of antibiotic are added to a liquid medium and this is inoculated with the test organism. The starting and finishing concentration
depends on the species and the antibiotic. The method can be used in standard test tubes (macrodilutuion) or a microtitre plate (microdiultion).
Name 2 public health control measures available for meningococcal disease?
Chemoprophylaxis- antibiotics
Vaccination
Virology investigations- Possible
test types
• Electron Microscopy • Virus isolation (cell culture) • Antigen detection • Antibody detection by serology • Nucleic acid amplification tests (NAATs e.g. PCR) • Sequencing for genotype and detection of antiviral resistance
Electron microscopy method
METHOD:
Specimens are dried on a grid
Can be stained with heavy metal e.g. uranyl acetate
Can be concentrated with application of antibody i.e. immuno-electron microscopy to concentrate the virus
Beams of electrons are used to produce images
Wavelength of electron beam is much shorter than light, resulting in much higher resolution than light microscopy
Viruses are identified by their morphology
Pros and cons of electron micrsoscopy
Cytopathic Effect (CPE)
Viruses require host cells to replicate and may cause a Cytopathic Effect (CPE) of cells when a patient sample containing a virus incubated with a cell layer
• Old method, now replaced by
molecular techniques, but still
needed for research or for rare
viruses
What antigen detection test techniques are being replaced by Nucleic acid detection methods due to
improved test performance
Viral antigens, usually proteins – either capsid structural proteins, secreted proteins
can be detected. Infected cells may display viral antigens on their surfaces.
Nasopharyngeal aspirates (NPA) – e.g. RSV, influenza
Blood (serum or plasma)
– Hepatitis B
– Dengue
Vesicle fluid
– Herpes simplex, varicella zoster
Faeces
– Rotavirus, adenovirus
Serology
Indirect detection of the pathogen
Diagnostic mode of choice for organisms which are refractory to culture
Serology can be used to:
– Detect an antibody response in symptomatic patients
– Determine if vaccination has been successful
– Directly look for antigen produced by pathogens
Serological tests are not limited to blood & serum
– can also be performed on other bodily fluids such as semen and saliva
3 formats of ELISA
Indirect
Direct (primarily antigen detection)
Sandwich
Pros of NAATS
May be automated
• Highly sensitive and specific, generates huge
numbers of amplicons
- Rapid
- Useful for detecting viruses to make a diagnosis
- At first time of infection e.g. measles, influenza
- During reactivation e.g. cytomegalovirus
• Useful for monitoring treatment response
- Quantitative e.g. HIV, HBV, HCV, CMV viral loads
Real time PCR
Different chemistries but all similar
• Real time as amplification AND detection
occur in REAL TIME i.e. simultaneously by
the release of fluorescence
• Avoids the use of gel electrophoresis or
line hybridisation
• Allows the use of multiplexing
Cons of NAATS
• May detect other viruses which are not
causing the infection
• Exquisitely sensitive and so may generate
large numbers of amplicons. This may
cause contamination.
• Need to have an idea of what viruses you
are looking for as will need primers and
probes that are specific for that target.
Multiplex PCR
more than one pair of primers is used in a PCR. It enables the amplification of multiple DNA targets in one tube e.g. detection of multiple viruses in one CSF specimen
e.g. HSV1, HSV2, VZV, enterovirus, mumps virus
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)
CASE:
3 year old Short history of fever, shaking, unwell Triage: high temp, flushed, no rash, unwell Seen by A&E SHO 10 mins later Temp 39.7C, HR 170, RR 55, drowsy ? Cause - referred to paediatrics
What things should be considered?
Well / unwell
- Age of child
- Hx
- Observations (HR, RR, cap refill)
Focus of infection (differential..)
- Localizing signs / symptoms (ENT!!)
- Rash, Immunisation status,Contact with illness (including travel, day care/nursery)
Meningococcal disease presentation in a young child
May look well in early septicaemia
Vital signs must be taken and repeated
Rash: blanching - single spot - purpura
Assess the underlying disease
By the time the rash appears in meningococcal disease, it is late
By the time the child is drowsy, VERY late
Shock is a medical emergency
CASE: See a child at 2am 20 months old Fever since 8pm - ‘burning up’ Mother very worried - not himself Mum describes some shaking episodes Miserable, Temp 40.5, HR 150 No focus found
What are the worrying features?
2am!!! Short history of illness High temp Looks unwell Worried mother No focus Tachycardic
Action: Full assessment of observations -- RR, cap refill, BP (interpret with caution) ‘Septic screen’ Admit patient
What is involved in a septic screen?
When to do a lumbar puncture?
When you want to rule out meningitis
Can be non-specific in children
Classic
– Neck stiffness, photophobia, headache
Infants
– Poor feeding, irritability, hypotonia, altered cry, opisthotonus, bulging fontanelle
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
Top 3 causes of bacterial meningitis
Neisseria meningitidis
Streptococcus pneumoniae
Group B streptococcus
Pneumonia treatment
A, B, C Oxygen Fluids IV antibiotics - Oral amoxycillin if possible - IV Augmentin - IV Cefuroxime
Epiglottitis presentation
Toxic, unwell Drooling Leaning forwards Soft stridor High fever
Epiglottitis treatment
Do not examine throat Do not cannulate Gentle wafting oxygen Controlled anaesthesia Intubation and ventilation IV antibiotics
CASE:
2 year old
Unwell 8 days
High fever (39C), Calpol no effect
Irritable
On examination: Conjunctivitis Cracked, sore lips Vague macular rash Cervical lymphadenopathy Swollen hands
Diagnosis?
Kawasaki disease: fever for 5 days plus 4/5 of
- Oropharyngeal changes
- Changes in peripheries (oedema first, peeling late)
- Bilateral non-purulent conjunctivitis
- Polymorphic rash
- Cervical lymphadenopathy
Young children can have ‘incomplete’ Kawasaki disease
Treatment of Kawasaki disease
High dose IVIG (2g/kg)
Aspirin
Echo to rule out coronary artery aneurysms (20% untreated cases)
- ECG to look for ischaemia while waiting for Echo if delay
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
Varicella complications
Secondary bacterial infection (Group A Strep can be very serious)
Pneumonitis (more common in adults)
Encephalitis
Severe, haemorrhagic varicella in immunocompromised
Reye’s syndrome
Most common organisms affecting children under 3 months
Group B Streptococcus
E Coli
other
Listeria (include ampicillin / amoxycillin)
What group of children have a very low threshold to do full septic screen
Febrile children under 3 months
highest risk for bacterial infection
