Topic 4.4 - Antimicrobials and Prac notes

antimicrobials - def

kill or inhibit growth of microorganisms

methods of inhibiting microbial growth on a host - list (2)

bactericidal

bacteriostatic

bactericidal - def

kill susceptible microbes

bacteriostatic - def

inhibit growth

when removed, bacterial will resume growth

Properties of antimicrobial drugs - list (3)

selective toxicity

therapeutic dose

toxic dose

Properties of antimicrobial drugs - selective toxicity

ability of drug to kill or inhibit pathogen while damaging the host as little as possible

Properties of antimicrobial drugs - therapeutic dose

drug level required for clinical treatment

Properties of antimicrobial drugs - toxic dose

drug level at which drug becomes too toxic for patient

classifications of antimicrobials - list (2)

narrow spectrum

broad spectrum

effectiveness of antimicrobials - list (2)

minimal inhibitory concentration

minimal lethal concentration

classifications of antimicrobials - narrow spectrum (def)

effective against only a limited variety of bacterial pathogens

classifications of antimicrobials - broad spectrum (def)

effective against many different types of bacterial pathogens

effectiveness of antimicrobials - minimal inhibitory concentration (def)

lowest concentration of a drug that prevents growth of a bacterial pathogen

effectiveness of antimicrobials - minimal lethal concentration (def)

lowest concentration of a drug that kills the bacterial pathogen

main modes of antimicrobial action - summary

target key pathways essential to bacterial survival and replication

main modes of antimicrobial action - categories (4)

1. inhibitors of cell wall synthesis

2. protein synthesis inhibitors

3. metabolic antagonists

4. nucleic acid synthesis inhibition

penicillin - category

inhibits cell wall synthesis

penicillin - bactericidal or bacteriostatic

bactericidal

penicillin - mode of action (3)

1. beta-lactam blocks transpeptidase enzymes from catalysing transpeptidation → formation of cross-links in peptidoglycan

2. prevents the synthesis of a complete cell wall, leading to cell osmotic lysis

only acts on growing bacteria. that are synthesising new peptidoglycan

beta-lactams and transpeptidases - relationship

beta-lactams are compeititve inhibitors of transpeptidases → look like D-ala - D-ala

penicillin - structure

mostly 6-aminopenicillanic acid derivatives

differ in side chain attached to amino group

nosocomial infections - def

healthcare associated infections

infections acquireed in hospital and other health care settings

septicaemia - def

disease caused by microorganisms present and multiplying in blood circulation

pseudomonas aeruginosa - features (4)

Gram-negative rod

ubiquitous in water environments

prefers O2 for growth but thrives in low O2 or anaerobic environments

large, green-grey, wet-looking culture

staphylococcus epidermidis - features (3)

gram negative cocci in clusters

part of heathy skin microbiome

small, white, wet-looking culture

staphylococcus epidermidis as an oppertunistic pathogen

can be a threat if it invades the body

coats medical devices in a biofilm → protect form antibiotics and immune response via lots of adhesin production

cause of children in oncology ward being more susceptible to infections (4)

chemotherapy kills immune precursor cells

leukaemia causes low WBC count

close proximity to other infected patients

intravenous catheter acts as entry site for pathogens into blood stream

prac- gram-staining protocol (9)

1. crystal violet for 30

2. wash

3. jension’s iodine as mordant for 30

4. wash

5. acetone to decolourise for 2

6. wash

7. counterstain with carbol fuchsin for 10

8. rinse and blot

9. oil immersion lens light microscopy

prac - catalase test principle

Catalase decomposes H2O2 -> organisms that lack the cytochrome system (oxphos) usually also lack catalase

No bubbles = no catalase produced = no oxphos = not staphylococcus

prac - coagulase test

ability of staphylococcus to clot plasma via coagulase production

S. aureus = positive -> clotting

S. epidermidis = negative -> no clotting, uniform 'milky' look

prac - oxidase test purpose

differentiate between aerobic and facultatively anaerobic gram neg bacteria

prac - oxidase test results

Presence of cytochrome oxidase system (oxphos) reduces colourless reagent to form purple compound

Positive = pseudomonas

Negative = E. coli

prac - antimicrobial susceptibility test features (2)

Susceptible = zone of inhibition present

Resistant = zone of inhibition absent

prac - antimicrobial susceptibility test for P. aeruginosa

susceptible to TIC, TOB

resistant to AMP, CN, CAZ

prac - pulsed field gel electrophoresis principles (2)

Restriction enzymes cut circular DNA into several linear molecules

Changing electric field of gel allows for large DNA fragments to be studied → compared to unidirectional standard electrophoresis

prac - pulsed field gel electrophoresis results (2)

Two isolates from the same source = identical bands because restriction enzymes cut at the same location

Two isolates from different sources = different bands