Wk6 - clinical microbiology 1 Flashcards
Structure of gram +ve bacteria wall
Cell membrane
Thick peptidoglycan cell wall
Structure of gram -ve bacteria wall
Cell membrane
Peptidoglycan cell wall
Outer membrane
Periplasm
Bacteriocidal
Achieve sterilisation of the infected site by directly killing the bacteria
Lysis of bacteria can lead to release of toxins and inflammatory material
Bacteriostatic
Suppresses growth but does not directly sterilise infected site
Requires additional factors to clear bacteria - immune mediated killing
Antibiotic spectrum
Spectrum refers to the range of bacterial species effectively treated by the antibiotic
Broad spectrum of antibiotics
Antibiotics that are active against a wide range of bacteria
Treat most causes of infection but also have a substantial effect on colonising bacteria
Action of Meropenem
Active against almost all gram +ve and gram -ve species.
Resistance is rare except
Broad spectrum antibiotic
Action of Benzyl-pencillin
Highly active against streptococci.
Most other disease causing bacteria are resistant
Narrow spectrum antibiotic
Narrow spectrum of antibiotics
Antibiotics that are active against a limited range of bacteria
Useful only where the cause of the infection is well defined
Have a much more limited effect on colonising bacteria
Different ways of using antibiotics (different therapies)
Guided therapy:
Depends on identifying cause of infection and selecting agent based on sensitivity testing
Narrow spectrum options - mostly
Empirical therapy:
Best (educated) guess therapy based on clinical/epidemiological acumen
Used when therapy cannot wait for culture
Broad spectrum - mostly
Prophylactic therapy:
Preventing infection before it begins
Antibiotic associated harm
Disruption of bacterial flora leads to:
Overgrowth with yeasts – thrush
Overgrowth of bowel – diarrhoea
Antibiotic use associated with:
development of C. difficile colitis
future colonisation and infection with resistant organisms
Beta-Lactam antibiotics - 4 main types
Penicillins
Cephalosporins
Carbapenems
Monobactams (Aztreonam)
Main examples of beta-lactam antibiotics
Benzylpenicillin Flucloxacillin Amoxicillin Ceftriaxone Meropenem Aztreonam
Mechanism of action of beta-lactams
All β-lactams share the same structural feature
β-lactam motif analogue of branching structure of peptidoglycan
Inhibits crosslinking of cell wall peptidoglycan
Causes lysis of bacteria - bacteriostatic
Beta-lactamases
Enzymes that lyse and inactivate beta-lactam drugs
Commonly secreted by Gram –ves and S. aureus
Confer high level resistance to antibiotic:
Total antibiotic failure is likely to result
How are beta lactams given?
Most β-lactams poorly absorbed from GI tract:
must be given IV
Some can be effective orally:
amoxicillin, flucloxacillin most commonly used
Vomiting limits dose
Half life of e.g.s of beta lactams
Benzylpenicillin ≈ 1 hour
Ceftriaxone ≈ 8 hours
Adverse effects of beta-lactams
Usually very well tolerated, eveen in high doses
GI toxicity, n&v, diarrhoea, cholestasis
Infection - candidiasis, c.diff
HYpersensitivity - Type 1 - anaphylaxis, Type 4 - mild to seevre dermatology, Iterstitial nephritis
Rare: seizure, haemolysis, leukoaenia
Type 1 hypersensitivty
Relatively common allergy (0.7 – 4% of penicillin courses)
Most patients develop an urticarial rash
Anaphylaxis is the most feared complication
Cross reaction between classes is variable
Cross reactivity
Patients allergic to a penicillin will usually be allergic to other penicillins
Cross reactivity with other antibiotic classes is much lower
Some patients with penicillin allergy may be safely managed with other β-lactams
Particularly important if patient presents with life-threatening infection (esp. meningitis)
Features of Benzylpenicillin
MOA of beta-lactams - Penicillins
Chemically similar to original penicillin
Administered by the intra-venous route
There is an oral agent (Penicillin V) but not often used
Remains the first choice antibiotic for serious streptococcal infection (i.e. erysipelas)
Narrow spectrum agent
MOA:
Attaches to penicillin-binding proteins on forming bacterial cell walls.
This inhibits the transpeptidase enzyme which cross-links the bacterial cell wall.
Failure to cross-link induces bacterial cell autolysis.
Amoxicillin provides some amount of gram-negative cover in addition to gram-positive drugs
Features of Amoxicillin
Semi-synthetic penicillin:
- Greatly increased activity against gram negative organisms (although resistance is now common)
- Much more orally bioavailable than natural penicillins
Widely used in the treatment of a wide range of infections
A lot of resistance to Amoxicillin now
Used against Streptococci (resp. tract infections)
FEatures of FLucloxacillin
Synthetic penicillin developed to be resistant to beta-lactamase produced by staphylococci
Antibiotic highly active against:
Staphylococcus aureus (not MRSA)
Streptococci
No activity at all against gram negative organisms
Can be given orally but nausea limits dose
Beta-lactamase inhibitors
Effectively inhibit some beta-lactamases
Co-administered with penicillin antibiotic
Greatly broadens spectrum of penicillins against Gram –ves and S. aureus
There are many beta-lactamases that are not inhibited leading to antibiotic failure
Co-amoxiclav - broad spectrum
Tazocin - very broad spectrum
Cephalosporins
- MOA of Cephalopsporins
Initially isolated from Sardinian sewage outflow
Found to have good activity against Gram +ves and Gram –ves
Less susceptible to beta-lactamases than penicillins (due to them having more side chains)
Multiple generations of cephalosporins
Gram negative spectrum increases with each generation
Some loss of Gram positive activity
Recent introduction of MRSA active cephalosporins
e.g. Ceftriaxone - broad spectrum
MOA:
Attaches to penicillin-binding-proteins on forming bacterial cell walls.
This inhibits transpeptidase enzyme which cross-links the bacterial cell wall.
Failure to cross-link induces bacterial cell autolysis.
Less susceptible to beta-lactamases than penicillins
- Provides both gram-positive and gram-negative cover
Carbapenems
Ultra-broad spectrum beta-lactam antibiotics developed during search for beta-lactamase inhibitors
Excellent spectrum of activity against Gram +ves and Gram –ves
No activity against MRSA
Resistant to beta-lactamases
New beta-lactamases are emerging which lyse carbapenems
e.g. Meropenem
Monobactams
Aztreonam only member of the monobactam class Beta-lactam antibiotic but no cross reactivity to penicillins so can be given to those with penicillin allergy (except anaphylaxis) Only given IV – no oral absorption
Only work against gram -ves
e.g. Aztreonam
Vancomycin - a Glycopeptide
MOA of Vancomycin
Discovered in 1950s but did not find widespread use until rise of MRSA
Inhibits cell wall formation in Gram +ves only (no Gram –ve action)
Not dependent on PBP binding so effective against resistant organisms
Resistance in clinical isolates is extremely rare
1st drug against MRSA
Not absorbed from GI tract so almost always given IV
Oral route only used for treatment of C. diff
Resistance occurs but is uncommon (esp. Staph)
Long half life so loading doses usually given
MOA:
Bactericidal, inhibiting cell-wall synthesis in gram positive bacteria
Toxicity of Vancomycin
Nephrotoxicity – more likely with higher doses Red-man syndrome if injected too rapidly Anaphylactoid reaction Very rare now infusion rates slow Ototoxicity - rare
Therapeutic drug monitoring undertaken
Narrow therapeutic range
Aim higher in severe illnesses
Main issue with vancomycin in clinical use is underdosing
Protein synthesis inhibitors - classes
50S ribosomal subunit: Macrolides Erythromycin Clarithromycin Azithromycin Clindamycin Chloramphenicol
30s Ribosomal subunit:
Aminoglycosides - Gentamicin
Tetracyclines - Doxycycline
Macrolides - features
MOA
Good spectrum of activity against Gram positives and respiratory Gram –ves
Active against “atypicals” (atypical pneummonia)
- Legionella
- Mycoplasma
- Chlamydia
Excellent oral absorption:
Oral even in severe infection
e.g. Clarithromycin - Strep, haemophilus, Atypicals - good for resp. tract infections
MOA:
Binds to 50s ribosomal subunit
Inhibits bacterial protein synthesis
Macrolides - adverse effects
Diarrhoea & Vomiting
QT prolongation
Hearing loss with long term use
Drug interactions with macrolides
Simvastatin - avoid co-prescription, temporarly stop simvastatin - increases risk of simvastatin toxicity
Atrovastatin
Warfarin - anticoagulant increases too much
Clindamycin - MOA
Similar in many respects to macrolides:
- Same mechanism of action
- Excellent oral absorption
- Principle action against Gram positives
Some key differences:
- No action against aerobic Gram negatives or “atypicals”
- Excellent activity against anaerobes
Clindamycin highly effective at stopping exotoxin production
Added to patients with Gram positive toxin mediated disease:
Toxic shock syndrome
Necrotising fasciitis
Streps, Staphs - sever gram +ve infection
Broad anaearobic cover - causes sever destruction of colonic flora –> causes high risk of C.difficile
How does clindamycin cause c.diff (and other antibiotics)
Antibiotics dramatically alter the colonic flora
C. difficile commonly colonises the human colon
Forms spores which can be difficult to eradiacate from hospitals
Has developed resistance to common antibiotic classes
4Cs:
- Clindamycin
- Co-amoxiclav
- Cephalosporins
- Ciprofloxacin
All antibiotics cause C. Diff (even those that treat it)
Keep antibiotics as narrow spectrum as possible
Chloramphenicol
Also inhibits the 50S ribosome Excellent broad spectrum of activity Unfortunately very toxic: - Bone marrow suppression - Aplastic anaemia - Optic neuritis
Not used systemically
Used topically for eye infections
Used for bacterial meningitis with beta-lactam allergy
Aminoglycosides (e.g. Gentamicin)
MOA of aminoglycosides
30S inhibitors
Recommended dose is 3 days.
Increased in importance over the last 5 years in UK:
Improved dosing regimens
Restriction of other broad spectrum antibiotics
Use of gentamicin in Glasgow more than doubled
Used in severe gram-positive infections (e.g. biliary tract infection, pyleonephritis, HA pneumonia)
Some severe gram-positive infecdtions (such as soft tissue infection and endocarditis)
MOA:
Binds to 30s ribosomal subunit, inhibiting protein synthesis, inducing a prolonged post-antibiotic bacteriostatic effect.
Additionally, bacteriocidal action on bacterial cell wall results in rapid killing early in dosing interval and is prominent at high dosese.
Also provides a synergistic effect when used alongside other antibiotics (such as flucloxacillin or vancomycin in gram-positive infections)
How does Gentamicin work - MOA
Binds to 30s ribosomal subunit, inhibiting protein synthesis, inducing a prolonged post-antibiotic bacteriostatic effect.
Additionally, bactericidal action on bacterial cell wall results in rapid killing early in dosing interval and is prominent at high doses.
Also provides a synergistic effect when used alongside other antibiotics (such as flucloxacillin or vancomycin in gram-positive infections).
Poorly understood action on the cell membrane:
Bactericidal action
Prominent at high concentrations
Results in rapid killing early in dosing interval
Aminoglycosides toxicity (Gentamicin)
Nephrotoxicity Ototoxicity: Hearing loss Loss of balance Oscillopsia Neuromuscular blockade: Usually only significant in myaesthenia gravis (rare)
Aminoglycosides once-daily dosing
Give high initial dose to take advantage of rapid killing
Leave long dosing interval (24-48hrs) to minimise toxicity
Measure trough level to ensure drug not accumulating
Give for 3 days only (to prevent toxicity developing)
Gentamicin - what its used fro
Gram negatives
E.coli
Pseudomonas
Tetracyclines
Similar spectrum of activity to macrolides Also active against “atypical” organisms Relatively non-toxic Avoid in children and pregnant women: - Bone abnormalities - Tooth discolouration
Doxycycline -w hats its used for
Staphs, streps, Atypicals
The 2 types of antibiotics that affect DNA repair and replication
Quinolones
- Ciprofloxacin
- Levofloxacin
Rifampicin
Quinolones
MOA of quinolones e.g. ciprofloxacin
Subtype of DNA repair and replication inhibitors
Broad spectrum, bactericidal antibiotics
Two quinolones in common clinical use:
Ciprofloxacin: good against Gram –ves, weaker against Gram +ves – commonly used in UTI/abdominal infection – urinary tract
Levofloxacin: sacrifices some Gram –ve activity for stronger Gram +ve action – respiratory tract
Excellent oral bioavailability: can use oral dosing even in severe infection
Active against many “atypical” pathogens, inc legionella
MOA:
Interferes with bacterial DNA replication and repair.
Broad-spectrum bactericidal antibiotics; provides both gram-positive and gram-negative cover.
Toxicity of quinolones
- Gastrointestinal toxicity
- QT prolongation
- Tendonitis
Other therapeutic problems:
- Resistance emerging on therapy/tendon damage
- C. diff infection (esp. in North America)
Ciprofloxacin - what its used for
Mainly gram negatives
Atypicals
Levofloxacin
Mainly gram positives - Strep and Staph (use in resp. tract infections)
Atypicals
Rifampicin - what its used for and how it works
Subtype of DNA repair and replication inhibitors
Principally used for two indications in the UK:
Tuberculosis (in combination therapy)
In addition to another antibiotic in serious Gram positive infection (esp. Staph. aureus)
Interactions are very important:
Rifampicin is a potent CYP450 enzyme inducer
Most drugs that undergo hepatic metabolism affected
Important to look up interactions when starting
4 principle TB drugs
Isoniazid
Rifampicin
Pyrazinamide
Ethambutol (only bacteriostatic)
Toxicity associated with all of them
Antibiotics - inhibitors of folate synthesis
MOA and examples
MOA:
Inhibition of folate metabolism pathway leads to impaired nucleotide synthesis and therefore impaired bacterial DNA replication
Examples:
Trimethoprim
Sulfamethoxaole
Trimethoprim
Orally administered antibiotic
Good range of action against Gram +ves and Gram –ves
However, resistance a major problem in clinical use
Today limited to use in uncomplicated UTI
Trimethoprim - toxicity
Elevation of serum creatinine
Does not reflect fall in GFR
Related to action on proximal tubules
Elevation of serum K+
Problematic in patients with chronic renal impairment
Rash and GI disturbance relatively uncommon
What is Co-trimoxazole
Combination of Trimethoprim and sulphamethoxazole
Significant additional toxicity: Bone marrow suppression Stevens Johnson Syndrome Relatively few advantages: Used in certain uncommon infections by specialists Pneumocystis jirovecii pneumonia
Metronidazole
Miscellaneous antibiotic
Enters by passive diffusion and produces free radicals
Effective against most anaerobic bacteria
Not actinomyces
Often added to therapy in intra-abdominal infections, esp abscess
Causes unpleasant reaction with alcohol
Peripheral neuropathy with long term use
How to treat lower UTI
Trimethoprim:
Currently first line agent for most cases
Avoid in 1st trimester of pregancy (as folate synthesis inhibitor)
Penetrates well into prostate so good choice for men
Nitrofuratoin:
Excellent, broad spectrum of activity
Concentrated in urine so no effect on other tissues
Failure to concentrate in urine in renal failure – avoid
Relatively non-toxic in short courses:
- Pulmonary fibrosis with long term use
Antibiotics in pregnancy
Thought to be safe: Most beta-lactams: Broad spectrum agents may be associated with NEC in premature infants Macrolides (e.g. clarithromycin) Anti-tuberculants
Not considered safe: Tetracyclines - Bone/tooth abnormalities Trimethoprim - Neural tube defects (1st Tri) Nitrofurantoin - Haemolytic anaemic (3rd Tri) Aminoglycosides - Ototoxicity (2nd/3rd Tri) Quinolones - Bone/joint abnormalities, tendon problems
Always need to consider risks vs benefits
Acquired vs inherent resistance
Inherently resistant antibiotics lack a pathway or target which a drug interacts with, or the drug is unable to gain access to the target.
Acquired resistance is where a drug which was previously sensitive has gained some genetic material encoding for resistance.
Examples of inherent resistance include vancomycin against Gram negative bacteria. These do not update the antibiotic meaning it has no opportunity to act on the cell wall. Also metronidazole against aerobic bacteria. It need anaerobically reduced to its active form meaning it is not activated by aerobes and therefore has no activity.
It is these mechanisms of acquired resistance which we’ll be focussing on today.
What are the 4 main ways antibiotics can develop resistance
- They can produce enzymes that inactivate or modify antimicrobials. Examples of these include the beta-lactamases which we’ll talk more about later.
- They change change the drug target so that the antibiotic no longer has any effect. Examples of target modification include methylation of the 23S ribosomal subunit resulting in resistance to erythromycin.
- Decreasing the permeability of the cell to the drug, meaning that the concentration required for the drug to be effective is not achieved, for example porins which permit drug to pass into a cell can be down regulated.
- The final mechanism is where bacteria are able to export the drug from inside the cell. Examples of this mechanism includes the production of multi-drug resistance efflux pumps, bacteria such as Pseudomonas can produce these allowing them to become resistant to may drugs through a single mechanism. The drug is usually exchanged for protons.
What are the 4 main ways that bacterial cells can become resistant to antibiotics
- Chromosomal mutation
- Acquisition of a mobile piece of DNA such as plasmid, integron or transposon
- DNA uptake can occur through transformation
- Pieces of DNA can be transferred between bacteria by viruses
Horizontal vs gene transfer
What are the 4 methods that drive evolution of antimicrobial resistance
Vertical gene transfer is where genetic information is transferred from parent cell to progeny via binary fission. Horizontal gene transfer is where genes are transferred other than through traditional reproduction. Importantly it is the primary reason for antibiotic resistance. If these mechanisms for genetic exchange didnt occur antibiotic resistance wouldnt be the significant problem that it is.
4 methods: Spontaneous mutation - vertical transmission Conjugation - horizontal Transduction Transformation - horizontal
Spontaneous mutation
Conjugation
Spontaneous mutation - once a mutation coding for antibiotic resistance has occurred in a cell, they will transfer this mutation to all their progency - vertical transmission of resistance.
Conjugation:
Requires cell to cell contact between two bacteria
Small pieces of DNA called plasmids are transferred.
Most important mechanism of horizontal gene transfer.
Features of plasmids
Plasmids are pieces of circular double stranded DNA
Genetic information that can be carried on plasmids can include resistance to antibiotics, heavy metals, UV light.
Can carry genes which encode pili and mediate adherence and can encode toxins.
As well as carrying genes of interest to the host cell they also carry genetic information to allow themselves to replicate and pass between cells.
Can either exist free within the cell or become integrated into the host chromosome
Plasmids are the most important mechainsm of ‘horizontal’ gene transfer.
They are found in Gram positives and Gram negatives and several different types of plasmids can exist within a single cell.
Plasmids are a very effective way of spreading resistance, they can multiply in high numbers have a high rate of cell to cell transfer and can be picked up by different species.
Transduction
Transformation
How do you test for MRSA
Transduction - This is where small pieces of DNA are transferred between bacteria by a virus
Bacteriophages are viruses which infect bacteria
Transformation - is the changing of a bacterial cell wall or other proteins within the bacteria, causing resistance to drugs which act against these properties.
Do an MRSA Screen:
MRSA (like MSSA) likes to live in moist areas of the body. MRSA screen consists of a swab of the nose and perineum (nose, axilla, groin). Usually takes 1-2 days for a result.
Extended spectrum beta-lactamases (ESBLs)
Usually plasmid encoded
These are enzymes which are able to hydrolyse the beta-lactam ring of not only penicillins but also cephalosporins
Options for treatment include ciprofloxacin, temocillin, gentamicin and meropenem.
Ways to prevent spread of resistance
Narrow spectrum where possible
Follow the empirical prescribing guidance
Short courses e.g. 3 days for UTI, 5-7 days for LRTI
Immunodeficiency - primary vs secondary
Primary:
inherited
Exposure in utero to environmental factors
Rare
Secondary:
Underlying disease state
Treatment for disease
Common
What has caused inc. population of immunocompromised patients
Improved survival at extremes of life Improved cancer treatment Developments in transplant techniques Developments in intensive care Management of chronic inflammatory conditions Steroids
WHat is the prinicpal cause of morbidity and mortality in the immunocompromised host
Infection
What is most important risk factor for infection
Neutropenia:
Cytotoxic chemotherpay therapeutic irradiation (TBI) –>
Dec. proliferation of haemopoietic progenitor cells –>
depletion of marrow reserves –> Neutropenia
DEfine as <0.5x10^9/L
How does cytotoxic drugs, irradiation and steroids affect enutrophil function/
DEc. chemotaxis, phagocytic activity, intracellular killing
Chronic granulomatous disease
Inherited immunodeficiency disorder 1/200,000 X linked most common Defect in gene coding for NADPH oxidase > deficient production of oxygen radicals > defective intracellular killing
Recurrent bacterial and fungal infections –> abscesses in lung, lymph nodes, skin
Inflammatory responses with widespread granuloma formation.
Cellular immunity is suppressed by
DiGeorge syndrome (primary deficiency, rare)
Malignant lymphoma
Cytotoxic chemotherapy
Extensive irradiation
Immunosuppressive drugs
- corticosteroids
- cyclosporin – immunosuppressant used to prevent organ rejection
- tacrolimus – more potent than cyclosporin
- alemtuzumab – anti-CD52 monoclonal
- rituximab – anti-CD20 monoclonal
- purine analogues eg fludarabine – causes profound lymphopaenia
Allogeneic stem cell transplantation especially if GVHD
Infections – HIV, mycobacterial infections, measles, EBV, CMV
Humoral immunity
Bruton agammaglobulinaemia (primary, rare)
Antibody production in lymphoproliferative disorders
- CLL, multiple myeloma
Usually preserved in acute leukaemia
Intensive radiotherapy and chemotherapy will
ultimately cause hypogammaglobulinaemia
Physical barriers of infection
Skin Conjunctivae Mucous membranes - gut, - respiratory tract - GU tract
How are types of bacteria split up?
Gram +ve - Streptococcus, Staphylococcus, Enterococcus
Gram -ve - E.coli, Neisseria (meningitis, gonorrhoea), Haemophilus, Pseudomonas
Anaerobes - Clostridium (gram +ve), Bacteriodes (gram -ve)
Mucosal barrier injury & colonisation resistance
High mitotic index - affected by chemotherapy & irradiation
Gi lymphoid tissue responds with inflammatory response –>
Mucositis –>
Pain, dysphagia, xerostoma, ulceration –>
Impairment of GI function
Alterations in permeability –>
Altered nutritional status
H2 antagonists, PPIs
Antibiotics
Diarrhoea –>
Altered microbiome
Severe nutritional deficiency =
<75% ideal body weight
OR
Rapid weight loss
+ Hypoalbuminaemia
4 ways in which bacteria can develop resistance
- Produce enzymes that inactivate/modify the antibiotics e.g. beta-lactamases
- Change the drug target so the antibiotic no longer has any affect
- Decreasing permeability of the cell to the drug e.g. porins which permit drugs into the cell can be down-regulated
- Bacteria can deevlop mechanisms to export the drug from inside the cell to the outside e.g. pseudomonas can develop this mechanism. The drug is usually exchanged for protons
3 ways in which bacteria acquire DNA
- Conjugation - requires cell to cell contact between 2 bacteria. Plasmids are transferred. Most important mechanism of horizontal gene transfer
- Transduction - this is where small pieces of DNA are transferred between bacteria by a virus. Bacteriophages are viruses which infect bacteria
- Transformation - when bacteria die, they release some naked DNA into the environment. Some bacteria are capable of up taking this naked NA and utilising it e.g. strep pneumoniae uses this method to become resistant to penicillin
Mechanism of action of azithromycin
Inhibits 50S ribosomal subunit
Which antimicrobial agents inhibits bacterial nucleic acid synthesis by preventing the production of tetrahydrofolic acid?
Trimethoprim
How does metronidazole work?
is a prodrug that undergoes reductive activation in anaerobic bacteria, leading to the formation free radicals and cytotoxic-intermediate products.
How does Gentamicin work?
disrupts peptide elongation by binding to the 16S ribosomal RNA.
How does Rifampicin work?
inhibits bacterial DNA-dependent RNA polymerase.
You are an FY1 on the Cardiology Ward reviewing a patient with sub-acute bacterial endocarditis, with Streptococcus mitis/oralis having been cultured from 3 sets of blood cultures taken off antibiotic therapy.
Which of the following sites is the MOST likely source of the organism?
Oropharynx
Which of the following viruses is the LEAST common cause of lower respiratory tract infections in infants (less than 5 years of age)?
Select one:
a. Influenza A
b. Rhinovirus
c. Adenovirus
d. Respiratory syncytial virus
Rhinovirus
A 27 year old man with HIV has a raised red lesion on his back and a clinical diagnosis of Kaposi’s sarcoma is made.
What virus is associated with Kaposi’s sarcoma?
human herpes virus 8
Super-antigen production by Staphylococcus aureus is an important virulence factor involved in the pathogenesis of which of the following diseases? There are TWO correct answers, choose one.
Select one:
a. Pneumonia
b. Scalded skin syndrome
c. Toxic shock syndrome
d. Gastroenteritis
Gastroenteritis & Toxic Shock Syndrome
Which virulence factor produced by Staphylococcus aureus is associated with severe rapidly progressing necrotising infection?
Select one:
a. Coagulase
b. Panton-Valentine leucocidin
c. Exfoliative toxins
d. Enterotoxin
Panton-Valentine leucocidin
A previously well 19 year old woman has cramping abdominal pain, bloody diarrhoea, and malaise for eight hours. She ate undercooked chicken the previous evening. Bacterial enterocolitis is suspected.
What is the most likely pathogen?
Campylobacter jejuni
ILO - understand the mechanism of action for each major class of antibiotic
Beta-lactams - are bacteriostatic as inhibit cross-linking of cell wall peptidoglycan, so therefore cause lysis of bacteria
Protein synthesis inhibitors - either inhibit 50S or 30S ribosomes, preventing production of necessary proteins. Gentamicin, for example, reversibly binds to 30S ribosome, causing bacteriostatic effect, however has also been seen to have bacteriocidal action as well.
ILO - understand side effects of commonly used antibiotics
Beta-lactams - usually well tolerated however can cause GI toxicity with N&V, diarrhoea and cholestasis. Can also cause infection (candida or c.diff) due to effect on normal gut flora. Hypersensitivtity an occur (either type I or IV)
Vancomycin - nephrotoxicity, Red-man syndrome (if injected too quickly - anaphylactic reaction)
Macrolides - D&V, QT prolongation, hearing loss with long term
Chloramphenicol - bone marrow suppresion, aplastic anaemia, optic neuritis
Aminoglycosides - nephrotoxicity, ototoxicity (hearing loss, loss of balance), neuromuscular bloackade
Quinolones - GI toxicity, QT prolongation, Tendonitis, C.diff infection
Rifampicin - potent CYP450 enzyme inducer so therefore most drugs which under hepatic metabolism are effected
Trimethoprim - elevation of serum creatinine, elevation of serum K+ is problematic which chronic renal impairment
ILO - have a basic understanding of an appropriate antibioitc in common conditions e.g. c.diff, UTIs, abdo infections, RTIs, TB
C diff can be treated using the 4 Cs - Clindamycin, Co-amoxiclav, Cephalosporins, Ciprfloxacin
UTIs:
Uncomplicated –> Trimethoprim
Complicated –> Ciprofloxacin
Severely unwell –> Amoxicillin and Gentamicin
Abdo infections - ciprofloxacilin
Respiratory tract infections - Levofloxacin
TB requires long duration (6/12) as difficult to reach granuloma due to inadequate blood supply - Rifampicin
What baseline investigations would you like to organise when suspecting endocarditis?
• Urinalysis to look for microscopic haematuria and proteinuria.
• Full blood count and biochemistry (urea and electrolyte, liver function tests).
• Any other relevant cultures – perhaps sputum and urine culture?
• Chest X - ray.
Blood cultures.
- Echocardiogram -
How should blood cultures be taken when dealing with a patient with possible bacterial endocarditis (BE)?
- 3 sets of blood cultures
- Taken from peripheral veins
- 10 mls of blood in each bottle
- Meticulous sterile technique
- Taken prior to antibiotics
(taking 3 sets of blood cultures with the first and the last cultures at least 1 hour apart)
What initial investigations are you going to do when thinking possible sepsis?
- Blood cultures – prior to the administration of any antimicrobial agent
- Urine culture
- Urea & Electrolytes
- Liver function tests
- Amylase
- CRP
- Full blood count & platelets
- Repeat lactate
- Chest X-ray
Infection control strategies
Water control
Rodent control
Isolation
Portal of exit fro common HAI
TB: Respiratory tract Salmonella: Faeces Norovirus: vomit Blood borne viruses - cuts and injuries Enterovirus: Conjuctival secretions
Modes of transmission of infection
Direct - Direct contact; Droplet spread
Indirect - Airborne, Vehicle borne (food, water fomites); Vectorborne (mechanical or biologic)
Portal of entry of any infection
Respiratory tract
Mucous membranes
Skin - non-intact
Mouth (faecal-oral)
Features of the chain of infection
Infectious agent Reservoirs Portal of exit Means of transmission Portal of entry Susceptible host
Transmission based precautions
Contact - isolation, cleaning, gloves, apron
Droplet - surgical mask and eye protection
Airborne - FFP3 masks (to stop you breathing it in) - TB, Pandemic flu, Aerosol Generating Procedures for resp pathogens, measles varciella zoster
Strategies for HAI
Isolation Screening Cohorting Standard and transmission based precautions Surveillence Antimicrobial stewardship
Aseptic technique
Reduce activity in the area
Keep exposure of a susceptible site to a minimum
Check sterile packs for evidence of damage or moisture
Ensure all fluids material in date.
Do not re-use single use items
Hand decontamination prior to the procedure
Protect uniform/clothing with a disposable apron.
Use sterile gloves.
Appropriate waste disposal.
How should blood cultures be taken when dealing with a patient with possible bacterial endocarditis (BE)?
3 sets of blood cultures
Taken from peripheral veins
10 mls of blood in each bottle
Taken prior to antibioitics
The volume of blood collected is the most significant factor in indentifying microorganisms from blood cultures.
Endocarditis leads to sustained bacteraemia and this is best established by taking 3 sets of blood cultures with the first and last cultures at least 1 hour apart.
The need for culture prior to antibiotic administration is self-evident. In BE, bacteraemia is almost constant, which has 2 implications: There is no rationale for delaying blood sampling to coincide with peaks of fever; and virtually all blood cultures are positive.
As a result a signle positive blood culture should be regarded cautiously for eastablishing the diagnosis of BE
What investigation is needed to aid your diagnosis of endocarditis (along with blood cultures)?
Echocardiogram is required.
IE is an infection of the endocardial surface of the heart valves. It can involve previously normal valves, a valve damaged as a result of prior endocarditis, rheumatic heart disease r a degenerative process, or a prosthetic valve.
Fibrin and platelets attach to the surface of damaged heart vlaves producing a sterile thrombotic endocarditis. Certain bacteria can adhere to lthese lesions on the valvular surface and initiate an inflammatory response that leads to further fibrin deposition, thus leading to maturation of the lesions. Such masses of fibrin and bacteria are termed ‘vegetations’. The heart and its appendages are easily viewed by an echocardiogram. A TTE is a non-invasive procedure that defines both the structure and the function of the heart. Unfortunately, TTE is not as sensitive as TOE. The heart is more clearly visualised by placing the transducer in the oesophagus. TOE is more invasive, and patients need to be sedated before the procedure.
TTE is more commonly doen for IE
Antibiotic Tx of Streptococcal IE
IV Vancomycin with IV Gentamicin
Classification of streptococci
Alpha-haemolytic streptococci:
- Streptococcus pneumoniae
- Streptococus viridans (most common Streptococcus species associated with BE
Beta-haemolytic streptococci:
- Group A streptococcus (Streptococcus pyogenes)
- Group B streptococcus (Streptococcus agalactiae)
- Group D streptococcus (Enterococcus)
Gamma (Non) - Haemolytic Streptococci
What is the name of the criteria used to define endocarditis?
Duke’s criteria
2 of the major criteria - Sustained bacteraemia with a typical organism and an echocardiogram that is consistent with endocarditis
Blood culture comes back (suspected BE) - shows has grown an alpha-haemolytic streptococcus belonging to the viridans group. All isolates are susceptible to penicillin, erythromycin, clindamycin, vancomycin and tetracycline. How will you treat this patient?
What do you need to monitor over the course of admission?
A commonly used regimen for treatment of streptococcal endocarditis of a native valve is 4 weeks of benzylpenicilllin combined with gentamicin for th first 2 weeks. Although streptococci are intrinsically resistant to gentamicin, benzylpenicillin disrupts the cell wall and allows gentamicin to penetrat inside the cell. Thus, they are synergestic when used in combination. The nature and duration of treatment depend upon the organism isolated and also the nature of the valve - native or prosthetic - and thus it is important to refer to the appropriate guideline.
Often 4 weeks IV antibiotics.
Should monitor renal function as gentamicin is a nephrotoxic drug. Regular trough levels of gentamicin need to be ascertained. A high trough level correlates with increased toxicity.
Single day dosing of gentamicin has become common in serious infection, but this has not been validated for Tx of endocarditis
Tx would you start with biliary sepsis/acute cholecystitis
Intravenous fluids - as dehydrated
Urinary catheter
Oxygen mask to maintain sats at >95%
IV antibiotics - e.g. Amoxicillin, Gentamicin and Metronadizole
(Take blood culture, lactate, urine output)
Likely to be referred to the surgical team (as is intraabdominal)
Pathogenesis of acute cholecystitis
Gallbladder infections usually result from gallstone formation and impaction in the cystic duct, with impaired biliary drainage leading to:
Infection, oedema, compressive effects on the local blood supply which may lead to gangrene of the gallbladder
Infection within the gallbladder may lead to:
Bloodstream infection, cholangitis, liver abscess formation
What is a presentation of Systemic Inflammatory Response Syndrome (SIRS)?
SIRS is 2 or more of: Temperature >30 or <36 Tachycardia >90 Tachypnoea RR >20/min WBC >12 x10^9/L
Young fit patients can look well at first - as they can compensate for severe illness in the early stages
Differential diagnosis of young male presenting with 2 day history of redness and pain in his left leg, following a minor injury whilst playing football. Presents with SIRS
Cellulitis Necrotising fasciitis Myositis Osteomyelitis DVT (unlikely in young man) Abscess (IVDU?)
First line investigations of suspected cellulitis or necrotising fasciitis
FBC (looking at WCC), U+Es, Lactate, CRP, Doppler ultrasound of leg, Blood cultures
ECG as is tachycardic
Initial Tx plan for cellulitis/necrotising fasciitis
The patient requires admission.
Given his SIRS status, IV antibioitcs are warranted. Students should follow local prescribing policy, but an initial regime of IV Flucloxacillin and Benzylpenicillin would be reasonable (Vancomycin could be an option if has penicillin allergy).
LMWH should be given.
Analgesia would be appropriate (e.g. co-codamol) - NSAIDs have been associated with increased risk of developing necrotising fasciitis from cellulitis, and should be withheld.
The area of erythema should be marked and closely monitored overnight.
Clues of necrotising fasciitis
Disproportionate pain NSAID use Mild preceding trauma SIRS/sepsis Rapid progress over several hours Dusky discolouration
What are the 4 classes of necrotising fasciitis
I - Synergistic infection with anaerobes (e.g. bacteriodes, peptostreptococcus) and anaerobes (strep, enterobacteriaciae). More common in elderly diabetic patients. This is why very broad antibiotic cover is required.
II - Infection with Group A Streptococci (S.pyogens or occassionally S.aureus), mediated by toxin production.
III - Vibrio vulnificus - after trauma in sea water)
IV - Fungal
Urgent Tx of necrotising fasciitis
The patient requires urgent surgical/plastics/ortho review for wide debridement of all the infected tissue.
Tissue from theatre should be sent to microbiology urgently for gram stain and culture.
IV antibiotics are important at this stage, but are unlikely to work whilst the necrotic tissue is present: this is avascular and full of organisms. Very broad cover is needed here - a common combination is IV Flucloxacillin (for Staph), IV Benzylpenicillin (for Strep), IV Gentamicin (for gram negatives), IV Metronidazole (for anaerobes) and PO/IV Clindamycin (for anaerobes/reduction in toxin production). If penicillin allergy/MRSA positive Vancomycin can be used.
He is hypotensive (BP 95/80) and requires IV fluids rapidly, and catheterisation for monitoring of his urine output
Will most likely have to go to ITU post-op
A man has a fractured neck of femur and is identified as MRSA colonised. He requires an operation to prevent avascular necrosis. What surgical prophylaxis would you give?
Surgical prophylaxis appropriate to cover MRSA should include a glycopeptide e.g. vancomycin or teicoplanin
Give an example of an antibiotic that has some (but by no means all) activity against MRSA
Gentamicin
Side effects of cefuroxime on an elderly patient
Increases the risk of CDI
How is c.diff diagnosed
A diarrhoeal stool sample needs to be sent to microbiology
Management of a patient in hospital with c.diff
The severity of the CDI should be assessed and appropriate treatment initiated i.e. a decision made as to whether to start metronidazole or vancomycin.
Any unnecessary antibiotics should be stopped (e.g. cefuroxime)
The laxatives and any PPIs should be stopped.
Standard infection control precautions need to be in place as well as isolation of the patient and appropriate hand hygiene with soap and water and not alcohol gels..
Oral metronadizole (?)
Despite your best efforts to optimise the antibiotic choices to treat an MRSA wound infection and treat C.difficile infection, the patient died from a bowel perforation as a consequence of toxic megacolon resulting from pseudomembranous colitis.
What would you put on the death certificate?
Does the Procurator Fiscal need to be informed? Does anyone else need to be informed?
The primary cause of death is large bowel perforation secondary to toxic megacolon secondary to Clostridium infection. An MRSA wound infection is also a contributing factor.
Yes the procurator fiscal should be informed as the patient has died as a direct consequence of a Hospital Acquired Infection (HAI)
It is good practice to inform the patients GP that the patient has dies. Should also inform the infection control team. The medical director should also be informed as there may be issues regarding the competence of the doctors who were looking after the patient which need urgently to be addressed.
What is tenesmus?
A constant desire to defacate
What key facts should you establish in a sexual history?
Date of last sex
Establish facts about practice rather than questions about orientation
What gender was the partner
What type of sex: insertive or receptive, oral or anal or vaginal
Was a condom used?
Have they ever had a sexual health check-up and/or HIV test?
Question style:
Non-judgemental approach/Open questions
Language they understand
Question examples:
“Tell me a bit about the last time you had any type of sex with anyone”
“Was that a male or female partner?”
“Did he use a condom all the way through?”
“Did you discuss HIV status at all?”
“Have you noticed anything else wrong with your health recently?” - Start a symptoms screen for HIV:
- acute seroconversion e.g. fever, rash, headache, myalgia, sore throat, swollen glands
- chronic HIV
What tests are generally taken at sexual health checks?
Blood test for HIV and syphilis
NAAT (nucleic acid amplification test) for gonorrhoea and chlamydia. Tests are taken from relevant anatomical sites (e.g. anal, pharayngeal swabs and urine)
Hepatitis B should eb tested for in MSM (blood test) as it can be sexually transmitted and vaccination offered
Spread of HSV
HSV can be shed on about 1 day in 50 with no symptoms at all: many people do not understand the risk of ‘asymptomatic shedding’ - therefore important to discolse to future partners
Symptoms of proctitis
Proctitis = inflammation of the anus lining
Pain - tenesmus Bleeding - bloody stools Discharge Constipation Other - rectal fullness, 'pencil-shaped' stools
Why does gonorrhoea have multiple drug resistance determinants?
What can be done to minimize the development of drug resistant strains of gonorrhoea?
Gonorrhoea is naturally transformable so can easily acquire plasmids and genetic material between resistant and sensitive organisms.
Rapid accurate diagnosis (microscopy, NAAT)
Avoidance of blind therapy with inappropriate drugs
Partner notification to limit the onward spread of resistant infection
Epidemiological monitoring of and policy reaction to resistance data.
A man presents with generalised maculopapular rash particularly affecting the hands and feet, alopecia, malaise and flu-like symptoms.
What is the likely diagnosis?
What else should you be worried about?
Why has this symptom (the rash) taken 3 months to develop?
This is a classic rash of secondary syphilis involving the palms. In this case the sexual history is a large clue.
A key differential is acute HIV infection although the rash is not typical this can be a co-infection alongside syphilis. HIV and syphilis antibody tests can take up to three months to be positive. However currently available HIV tests look for HIV p24 antigen and will detect the great majority of individuals who have been infected with HIV at 4 weeks after specific exposure
Primary syphilis can take up to 90 days and secondary syphilis up to 6 months to present as treponemes (the causative agent) are slowly growing with a long division time of several days. Infection can only become apparent once the organism is present in sufficient numbers to disturb the host.
A man with secondary syphilis (maculopapular rash) is started on IM benzathine penicillin. A few hours later he attends an out-of-hours service with chills, fever of 38.5 and malaise.
What is the biological cause of this reaction? Is it a drug allergy or due to something else?
This reaction is caused by the penicillin lysing cell walls of the organism which can only happen at the time of cell division. The cell fragments released lead to an escalating immune response triggering release of pro-inflammatory cytokines. In severe cases such as neurosyphilis this can be life-threatening.
What are the important questions to ask when taking a history from a patient with acute diarrhoea
Food history - classic culprits for gastroenteritis include undercooked poultry, reheated rice and frozen food that has been incompletely defrosted prior to cooking.
Ill contacts - many forms of gastroenteritis are passed from person to person. This is particularly important when thinking about viral gastroenteritis where contact with others (often children) with diarrhoea and vomiting is common.
Travel history - should include questions about pre-travel vaccinations and food and water precautions when traveling. Travel to more exotic locations increases the risk of parasitic causes of gastroenteritis and changes the spectrum of possible pathogens
Past medical history - important when thinking about a non-infectious aetiology for a patients diarrhoea. If patient reports several years of recurrent loose stool or ‘irritable bowel syndrome’ then exacerbation of a more chronic bowel disease should be considered. PmHx should also include an assessment of the patients medication. Many drugs can include diarrhoea as a side effect.
Risk factors for CDI - the main risk factors are age >65y, recent hospitalisation and a recent course of antibiotics. The presence of two of these three risk factors should prompt the consideration of CDI as a cause of the patients diarrhoea.
What are the important non-infectious causes of diarrhoea to consider?
IBD (UC & Crohn’s), bowel cancer, diverticular disease, chronic pancreatitis, HIV infection and ischaemic bowel.
Non GI infection can also present with diarrhoea as this can be a manifestation of sepsis syndrome (e.g. pneuomococal bacteraemia)
Investigations for someone presenting with acute diarrhoea
FBC, U&Es, Blood culture
Sample of diarrhoea for culture to try and identify the causative organism. It is best to obtain more than one stool sample as culture of bacterial organisms associated with gastroenteritis can be difficult
Management of gastroenteritis
She is dehydrated (can tell as has raised urea). Should be rehydrated with oral fluids (if she can tolerate them and ideally oral rehydration salts) or with IV fluids.
Antibiotics are not indicated - they have little clinical use in gastroenteritis as they only reduce the length of diarrhoea by a short period of time. An exception is when the patient has signs of sepsis (SIRS criteria) or there is concern about CDI
- Rehydration
- Antibiotics - if c.diff - quinolones (e.g. ciprofloxacin)
- Infection control - most causes of gastroenteritis are transmissible from person to person. Need to be careful about hand washing - if work as food handler cannot return to work until she has been symptom free for 48 hours
Which viruses cause a vesicular rash?
Herpes simplex virus - 1/2
Varicella zoster virus (e.g. chicken pox)
Enterovirus (e.g. Hand, foot and mouth disease (HFMD))
How to establish the cause of a vesicular rash
Polymerase chain reaction (PCR) should be requested on a vesicle swab
What are the routes of transmission of a virus causing a vesicular rash?
Exposure to on set of the rash is an average of 14-16 days (range 10-21 days). Varicella is contagious for 48 hours before the onset of the rash until every lesion has crusted over:
- Respiratory transmission - coughs, sneezes, droplets are inhaled or acquired via surfaces contaminated by infected droplets
- Contact with vesicle fluid
Immunocompromised patients include
Patients with severe primary immunodeficiency e.g. SCID, Wiskott-Aldrich sydrome
Patients being treated for malignant disease with immunosuppressive chemotherapy or radiotherapy, and for at least 6 months after terminating the Tx.
Patients who have received a solid organ transplatn and are currently on immunosuppressive Tx.
Patients who have received a solid organ transplant until at least 12 months after finishing all immunosuppressive Tx
Patients receiving systemic high-dose steroids until at least 3 months after Tx has stopped.
Varicella zoster immunoglobulin (VZIG) prophylaxis is recommended for individuals who fulfill all of the following 3 criteria
What therapy is available if the prophylaxis was unsuccessful?
Significant exposure to chickenpox or herpes zoster
A clinical condition that increases the risk of severe varicella; this includes immunosuppressed patients, neonates and pregnant women.
No antibodies to varicella zoster virus
If unsuccessful:
Treatment of varicella-zoster is aciclovir 10mg/kg IV every 8 hours in the immunocompromised.
If the patient is well enough for oral therapy valaciclovir 1g TDS daily continued for 2 days after crusting of lesions.
