View/Open - ARAN - National University of Ireland, Galway

Chapter 4
4.4. Discussion
Suspension tests were performed in order to determine the minimum
inhibitory concentration (MIC) of the disinfectant agents. The MIC is used
to determine the concentration of disinfectants that inhibits visible
bacterial growth. The purpose of this was to establish the efficacy of the
disinfectants chosen against the strains studied in planktonic form. The
results of the suspensions tests described Table 4.2 indicate that
concentrations of 250mg/L (sodium hypochlorite), 0.5M (sodium
hydroxide) and 0.01% (benzalkonium chloride) were sufficient to inhibit
visible bacterial growth. If disinfectant products are examined solely using
a suspension test method such as the ASTM standard 1040 [131] it is
possible that while bacterial cells may be inhibited using low
concentration of disinfectant agents the agents may not be effective if
the bacterial cells have formed a biofilm.
Notwithstanding the susceptibility of the strain in planktonic form,
sodium hypochlorite and benzalkonium chloride (at a concentration
twofold higher than the MIC) were not effective at eliminating all biofilm
developed after 48 hours period and none of the 3 agents effective in
eliminating the 168 hour biofilm (SL1344 and SL483) after contact times
of 10, 45 and 90 minutes. Møretrø et al. also reported that the MIC of
disinfectant agents such as 0.01-0.005% benzalkonium chloride was
effective at eliminating bacterial growth in a suspension test however the
same concentration was not effective at eliminating all bacterial cells
from an established biofilm [134].
4.4.1. The log 10 reduction achieved by sodium hydroxide
Sodium hydroxide was the most effective disinfectant of all 3 examined.
When used to treat a 48 hour biofilm no viable cells were detected after 10
Page 141