Bacteria and their antibiotic sensitivities

What is the minimum inhibitory concentration?

The lowest concentration of a drug that will inhibit visible growth after overnight incubation.

What organisms are only visible with an electron microscope?

Viruses

Prions

What organisms are visible with a light microscope?

Protozoa

Bacteria

Fungi

What organisms are visible with the naked eye?

Ectoparasites

Helminths

3 roles of a microbiology lab

identify relevant pathogens

which antimicrobial agent is best

epidemiology and surveillance

What is an antimicrobial agent?

Substance with inhibitory properties against micro-organisms

What is an antibiotic?

Substance produced by micro-organisms with inhibitory effects against other micro-organisms

What is a bacteriostatic antibiotic?

Substance that inhibits the growth of a micro-organism

What is a bactericidal antibiotic?

Substance that kills a micro-organism

What is selective toxicity?

Maximum damage to the micro-organism with minimal/no damage to the host

What are the four main sites of action of an antibiotic?

Cell wall synthesis

Protein synthesis

Nucleic acid synthesis

Cell membrane function

What is innate antibiotic resistance?

Organism does not possess the correct target site or is impermeable to the agent

What is acquired antibiotic resistance?

Change in genetic makeup; either from plasmid or chromosome

What are the four mechanisms of antibiotic resistance?

Altered target site

Altered uptake

Drug inactivation by production of enzymes

Drug efflux

What is the minimum inhibitory concentration (MIC)?

lowest concentration of a drug that will inhibit visible growth after overnight incubation

the lower the MIC...

... the more susceptible the organism

What concentration of antimicrobial is usually required for a clinical response?

MIC x4

Why do we do susceptibility testing in the laboratory?

An attempt to predict the likely in vitro response of an organism to a range of antimicrobial agents

what is sensitivity testing

susceptibility testing - attempt to predict response of an organism to a range of antimicrobial agents

What are the two limitations of susceptibility testing?

Performed under optimal growth conditions i.e. does not reflect what actually happens in the body

Does not consider the pharmacokinetics of the agents

Breakpoints are determined by considering what three factors?

Distribution of susceptibilities of the pathogen

Pharmacological properties of the antibiotic

Clinical outcome data

What is a breakpoint?

The MIC at which an organism is deemed either susceptible or resistant to an antibiotic

Why must specimen transport to the lab be quick and safe?

Organisms are fragile

Organisms can die

Delay in transit may allow overgrowth of commensal organisms

What is acyclovir used for?

Herpes simplex virus

Cold sores

What is the mechanism of action of a beta lactam antibiotic?

Inhibit cell wall synthesis

Bactericidal

What is the mechanism of resistance of a beta lactam antibiotic?

Drug inactivation

Production of beta lactamases

Give three examples of a beta lactam antibiotic

Amoxicillin

Co-amoxiclav

Cefalexin

Where is amoxicillin mainly excreted - %?

In the urine - 60-90%

What is a common side effect of amoxicillin?

Rash

What are three clinical uses of amoxicillin in dentistry?

Acute abscesses

Periodontitis/gingivitis

Oral-antral fistula

What is co-amoxiclav?

Amoxicillin + clavulanate

what does clavulanate do

inhibits beta-lactamase

What is the main clinical use of cefalexin in dentistry?

Penicillin allergy (rash)

where is most of cefalexin absorbed - %?

from GI - 80-95%

what is structure of macrolides?

14 membered macrocyclic lactone ring with 2 sugars

What is the mechanism of action of a macrolide?

Inhibits protein synthesis

Bacteriostatic

2 examples of macrolides

erythromycin and clarithromycin

What are the two mechanisms of resistance of a macrolide?

Efflux pumps

Alteration of drug and/or target

Describe the metabolisation and excretion of macrolides

Metabolised by the liver

Excreted in bile and urine

Important side effects of macrolides

Prolonged QT interval

Careful with prescription to a patient on cardiac medication

Pyloric stenosis in infants

What are the two clinical uses of macrolides in dentistry?

Alternative to beta lactam allergies

Common medical indication for erythromycin

Whooping cough

Give an example of a Lincosamide

Clindamycin

What is the mechanism of action of a Lincosamide?

Inhibit protein synthesis

Bacteriostatic

What is the mechanism of resistance of a Lincosamide?

Alteration of target site

Cross resistance with macrolides

when would you use lincosamide

if allergic to erythromycin

what antibiotic is good alternative to erythromycin if allergic

lincosamide

Describe the excretion of lincosamides

Excreted in the bile and urine

What is a common side effect associated with lincosamide use?

C.difficile colitis - common in old people, avoid if at risk

structure of tetacycline

4 ringed antibiotic produced by streptomyces aureofaciens

2 examples of tetracyclines

doxycycline and tetracycline

What is the mechanism of action of tetracyclines?

Inhibit protein synthesis

Bacteriostatic

What are the mechanisms of resistance of tetracyclines?

Drug efflux

Altered target binding site

What is absorption of tetracyclines reduced by?

Milk and antacids

Describe the excretion of tetracyclines

Excreted in urine

Doxycycline excreted in faeces

What are side effects of tetracycline use?

Deposition in bones and teeth - grey marks

Avoid in growing children and pregnant or breast feeding women

Teeth staining

What are the clinical uses of tetracyclines in dentistry?

Adjunct to scaling and root planing in periodontitis

Aggressive periodontitis in conjunction with root surface debridement

What is the mechanism of action of nitroimidazoles?

Disruption of microbial nucleic acid synthesis

Give an example of a nitroimidazole

Metronidazole

Describe the resistance to metronidazole

Rare

Describe the metabolisation and excretion of nitroimidazoles

Metabolised by the liver

Excreted by the kidneys

Two common side effects of metronidazole

Metallic taste

Disulfiram-like reaction with alcohol

What are the two clinical uses of metronidazole in dentistry?

Acute abscesses

Periodontitis & Gingivitis

Describe the guidance for prescription of antibiotics for an acute abscess

First line - Amoxicillin

Second line - Metronidazole

Third line - Clarithromycin

Describe the guidance for prescription of antibiotics for acute necrotising ulcerative gingivitis

First line - Metronidazole

Second line - Amoxicillin

followed by periodontal therapy after acute phase

Describe the guidance for prescription of antibiotics for aggressive periodontitis

First line - Doxycycline

Second line - Metronidazole and amoxicillin

Describe the guidance for prescription of antibiotics for Oro-antral fistula

First line - Amoxicillin

Second line - Doxycycline

The anticoagulant effect of warfarin is enhanced by which three antibiotics?

Clarithromycin/erythromycin

Cephalosporins

Metronidazole

Antibiotic prophylaxis is no longer recommended for what patients?

Patients with structural heart abnormality or heart valvopathy

What antibiotic would you prescribe a 48 year old female, on no other medication and with no allergies, with an acute abscess?

Amoxicillin 500mg 3-7 days

What antibiotic would you prescribe a 35 year old male, with a history of anaphylaxis to penicillin, with an acute abscess?

Metronidazole 200mg 3-7 days

What antibiotic would you prescribe a 25 year old pregnant female, with aggressive periodontitis?

Metronidazole 200mg and amoxicillin 500mg 3-7 days

What antibiotic would you prescribe a 78 year old on warfarin with acute necrotising gingivitis?

Amoxicillin 500mg for 3 days