Preservatives II Flashcards
Factors affecting the efficacy of preservatives
Concentration, temperature, pH, physical and chemical interactions, micro-organisms, formulation, contact time
Concentration
As the concentration increases, the death time decreases
There is an exponential relationship between potency and concentration
Practical meaning on concentration exponent
Phenol has a concentration exponent of 6. This means the activity will be reduced by the power of 6 upon dilution, so a 2-fold dilution will mean a decrease in activity of 2^6= 64, so phenols may be rapidly inactivated by dilution
The higher the concentration exponent is, the higher the loss of efficacy upon dilution is
Practical meaning of Q10
The value of Q10 for phenol is 4, this means that when the temperature increasesd by 10c, the activity will be increased by a factor of 4
pH and antimicrobial activity
Degree of ionisation- if the agent is an acid or base, its degree of ionisation will depend on the ppH
In some compounds, the active species is the non-ionised molecule so an increase in pH will reduce the activity by increasing the degree of dissociation of the molecules
In other cases the activity of the compound is due to the ionised molecule- an increase in pH will increase their antimicrobial activity
Interfering substances
Unbound preservatives contribute to the preservative system, interfering substances decrease preservative activity- decreasing amount available, neutralisation
Adsorption
Interactions with polymeric substances- alginates, starch mucilage, polyethylene glycols
Adsorption onto surface- parabens on magnesium trisilicate, chlorhexidine onto mineral earths (kaolin, calamine)
Adsorption to containers- rubber, plastic, glass
Incompatibility (neutralisation)- polyethylene glycol on parabens, cationic surfactants and ionic agents
Formulation: surface activity and preservative efficacy
Surface activity is an important factor in the antibacterial action of biocides, micelle formation is important to consider and depends on the surfactant and its concentration
The addition of low-concentration surface active compounds e.g. soap may potentiate the activity of an antimicrobial agent
Water activity
High sugar concentration (osmotic shock), low water activity usually means lower preservative efficacy
Bioavailability
Degree of binding of preservative with formulation components, vaporisation(chloroform), partitioning into oil/aqueous phase (depletion of the aqueous phase)
Microbial biofilms
The association of bacteria (and other micro-organisms) with solid surface leads to the formation of biofilms
A biofilm is a consortium of bacteria organised within an extensive polysaccharide exopolymer (glycocalyx)
Biofilms may consist of monocultures, several disperse species, mixed phenotypes of a given species
Why are bacteria in a biofilm much less sensitive to biocides?
Access of biocides to the underlying cells is prevented by the glycocalyx
Highly reactive biocides react chemically with the glycocalyx so that their antibacterial concentration is reduced
Slow growing micro-organisms are particularly recalcitrant to many biocides and antibiotics
Changes in the cell envelope composition of bacteria within the biofilm
Changes in micro-environment
Attached cells can increase production of enzymes that degrade antibacterial agents
Properties of the ideal preservative
Non toxic/ free of irritant effect, effective in preventing growth of micro-organisms, sufficiently soluble in water, adequate stability to heat and prolonged storage, chemically compatible with the other components of the formulation, not adversely affect container or its closure, have acceptable odour and colour, reasonable cost
Selection of the appropriate preservative system is comples
Understand and evaluate the preservative needs, problems of individual product, likely microbial contaminants, toxicity and application, regulation
