Investigating control strategies to limit biofilm formation and/or quorum sensing by Aeromonas spp. isolates.
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Date
2013
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Abstract
Aeromonas spp. are important biofilm-forming fish pathogens causing great economic loss in
aquaculture. Bacterial cells within biofilms communicate with each other via the production
of quorum sensing (QS) signalling molecules called acyl-homoserine lactones (AHLs), which
influence biofilm development and production of virulence factors. QS together with efflux
pumps, extracellular polymeric substances (EPS) and eDNA are associated with resistance of
bacteria to antimicrobial agents. These mechanisms provide a target for different control
strategies. The objectives of this study were to: (i) determine effective antimicrobial agents
and exposure concentrations against aeromonad biofilms; (ii) ascertain whether Aeromonas
spp. produce QS molecules or display efflux pump phenotypes, and (iii) investigate the effect
of antimicrobial agents, lytic enzymes, efflux pump inhibitors and QS inhibitors on biofilm
formation by Aeromonas spp. isolates.signalling MICs of azithromycin, ciprofloxacin,
ceftazidime, and tetracycline ranged between 0.064-64 μg/ml. Gentamicin had the lowest
MICs which ranged between 0.0048-32 μg/ml.The highest MBIC at which antimicrobial
agents exhibited inhibition was 4096 μg/ml. Majority of the isolates displayed MIC levels
ranging from 2-32 μg/ml, and thus a ≥ 128-fold increase was observed for MBICs. Of the
sub-MIC, MIC and supra-MIC exposures tested, MIC exposure of biofilms was the most
effective. Gentamicin MIC exposures inhibited initial attachment of 100% (28/28) of isolates
tested, while azithromycin MIC exposure detached 82.1% (23/28) of isolates. Carbonyl
cyanide 3-chlorophenylhydrazone completely inhibited efflux of cefpodoximeby 14.8% of
isolates. However, 1-(1-naphthylmethyl)-piperazinewas more effective, decreasing adherence
of 98.1% (53/54) of isolates and increasing detachment of 100% (54/54) of isolates. DNase I
was more effective against the mature biofilm,where it increased biofilm detachment of
64.8% of isolates. Of the 48 Aeromonas spp. and six Plesiomonas spp. isolates used, only a
single isolate induced the production of violacein by the C. violaceum CV026 biosensor,
while all isolates induced the utilization of X-gal to produce a visible blue colour with the
A.tumefaciens A136 biosensor. Based on the reaction to the two biosensors, aeromonads
appeared to produce long-chain acylhomoserine lactones. By blocking QS, S-adenosyl
homoserinewas more effective in inhibiting both initial attachment (72.2% of isolates) and
pre-formed biofilms (detached 74.1% of isolates). The investigated strategies are promising
for Aeromonas spp. biofilm inhibition. Thesecould be explored aspotential therapeutic
measures in aquaculture systems to limit aeromonad pathogenicity and overcome
antimicrobial resistance.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.
Keywords
Aeromonas., Biofilms., Microbial ecology., Quorum sensing (Microbiology), Theses--Microbiology.