Medical Microbiology

Medical MicrobiologyAntimicrobial Susceptibility Testingisolation and identification treatment of systemic bacterial or fungal determination of antimicrobial susceptibility to various agentsmeasurement of the amount of antimicrobial present in serum or other body fluids.1Fig. 12.15a

2Fig. 12.15b

Fig. 12.15c

Medical MicrobiologyMethods for susceptibility testing should be:reproduciblerobustreliable

5Breakpoint concentrationsQualitativeSusceptible resistantintermediate Enzyme testingB-lactamase Synergism and antagonismMedical MicrobiologyDisc testing methodscheapflexiblevisiblefiddlyCDSNCCLSStokes7Fig. 12.18a

Fig. 12.18b

Fig. 12.18cMedical MicrobiologyMinimum inhibitory concentration (MIC)Dilution testing:agarbrothE testMinimum bactericidal concentration (MBC)Serum bactericidal titre11

Fig. 12.19 E-test

Fig. 12.20a MIC testFig. 12.20b MIC test

Antibacterial AgentsClassified according to:Bactericidal or bacteriostaticTarget site Cell wall synthesisProtein synthesisNucleic acid synthesisCell membrane functionChemical structure

15Bactericidal agents will kill the bacteria, while the action of bacteriostatic agents relies on inhibiting tha bacteria from growing, this allows the hosts defence mechanisms to kill the static population.Cell wall synthesis, protein synthesis, nucleic acid synthesis, cell membrane function

Antibacterial Agents (1)

Inhibitors of cell wall synthesisTargets peptidoglycan on the cell wall-lactamsPenicillins>10 different typesGlycopeptidesVancomycins

Antibacterial Agents (2)Inhibitors of protein synthesisAminoglycosidesInterfere with the binding of mRNAStreptomycin, gentamycinTetracyclinesInhibit RNA from entering acceptor sites on ribosomesChloroamphenicolPrevents protein bond synthesisMacrolidesErythromycin, lincomycin, clindamycin18

Antibacterial Agents (3)Inhibitors of nucleic acid synthesisSulphonamidesTrimethoprimQuinolonesPrevents coiling of bacterial chromosomeRifamycin (rifampicin)Blocks RNA synthesisAntibacterial Agents (4)Cell membrane functionPolymyxin BBinds to plasma membrane to disrupt structure & permeability propertiesResistance Not inhibited or killed by an antibacterial agent, at concentrations that can be achieved in the body.Some bacteria can be innately resistant due to cell wall structure or lack of target site.Genetics of ResistanceChromosomal mutationSingle mutationsSeries of mutationsTransmissible plasmidsR plasmidsJumping genesTransposons

R plasmids code for enzymes that destroy or modify drugs24Mechanisms of ResistanceTarget site may be alteredLower affinity for the drug

Altered uptakeDecreasing the permeability of the cell wallPumping the drug out of the cell

Enzyme productionActive against the drugBetalactamase25Extensive drug treatment eradicates susceptible bacteria Favours the increase of drug resistant strainsSuperinfection

Fig. 12.17