3.3. Disinfection susceptibility test by disk diffusion method
This test assayed bacteria to determine their susceptibilities to
sodium hypochlorite, either as 14.5% standard sodium hypochlorite
solution or 4.5% commercial bleach on the same agar plate. Bacteria
showed a broad range of susceptibility patterns producing zones of
inhibition between 7 mm and 65 mm in diameter against the two
disinfectants. We arbitrarily classified results to facilitate analysis
(there are no known standard metrics to define ‘resistance’), and 13
(8.8%) bacteria showed zones of inhibition 20 mm in diameter; 96
(64.9%) isolates showed zones of inhibition between 21 and 40 mm,
while 18 (12.2%) isolates produced zones of inhibition of 41 mm
(Table 2). In case of 4.5% commercial bleach, 98 (66.2%) isolates
showed zone of inhibition 20 mm, 29 (19.6%) isolates showed
between 21 and 40 mm, while no isolate showed any zone of
inhibition 41 mm (Table 2).
Comparing the means of size of zone of inhibition by two dis-
infectants indicated that (as expected) the standard sodium hypo-
chlorite was more effective against isolated bacteria (Table S2), but
interestingly 10 (6.8%) cultures (4 Bacillus spp., 2 Acidovorax spp., 1
Burkholderia sp., 1 Paenibacillus sp. and 2 unidentified bacteria)
were more sensitive to commercial bleach (Table S1); this may be
due to the presence of other antimicrobial agents, e.g., non-ionic
and cationic surfactants, or pH, of the commercial bleach solu-
tion. Twenty-one isolates were not tested as they did not form a
proper lawn on the agar plate as required for agar diffusion
method; at least three attempts to create a lawn were made for