Antibacterial activity and mechanism of action of lipophilic antioxidants.

The emergence and spread of antibiotic resistance hampers effective treatment of bacterial
infections. This is particularly the case for infections involving a biofilm component, as
the activity of existing antibacterial drugs against these surface-attached communities is
limited. The work presented in this thesis sought to identify and characterise compounds
with antibacterial and antibiofilm activity against the important pathogen, Staphylococcus
aureus.
Antistaphylococcal activity was assessed for 16 antioxidants that are used in cosmetics,
traditional medicines or as food additives, and which have been reported previously
to have some antibacterial activity. Initial experiments with tert-butylhydroquinone
(TBHQ) showed that activity that had previously been ascribed to the antioxidant,
was a consequence of its conversion to tert-butylbenzoquinone (TBBQ) under culture
conditions. TBBQ displayed innate bactericidal activity against S. aureus that was
effected through perturbation of the bacterial membrane. The other antioxidants also
inhibited staphylococcal growth through perturbation of the cytoplasmic membrane, and
compounds that displayed selective action against bacterial membranes were identified.
Of the agents with bacterial specificity, TBBQ, celastrol and nordihydroguaiaretic acid
(NDGA) also eradicated staphylococcal biofilms; a rare property amongst antibacterial
agents. Although these antioxidants exhibited a similar membrane-damaging mode of
action, their mechanisms of antibiofilm activity differed. TBBQ eradicated preformed
biofilms through sterilisation of slow-growing and persister cell populations, whilst
celastrol and NDGA caused physical disruption of the biofilm. All three antioxidants
acted synergistically with gentamicin against biofilms, eradicating surface attached
populations at concentrations that did not cause irritation or visible damage to a human
skin equivalent.
The potent and selective antibacterial activity, and low resistance potential upon extended
subculture, suggest that these compounds could be used topically in combination with
gentamicin to treat infected wounds.