The germicidal properties of some metals and metal compounds were
investigated in relation to their electro-chemical properties that may play a
role in the inactivation of E. coli bacteria. These properties included the
atomic and ionic radii, ionization energy, oxidation state, energy of formation
with hydro-sulfide groups, and the redox potential of the metals. Cultures of
E. coli bacteria with predetermined numbers of colony-forming units (CFU’s)
were brought in contact with the metals as well as metal compounds, using Eosin
methylene blue agar medium and sterilized, distilled water. The rate of
inactivation was determined by counting the CFU’s at predefined intervals of
time after inoculation. The experimental results showed that the rate of
inactivation increases with increasing ionization energy of the metals. While
the rate of inactivation increases with decreasing atomic radii for some of the
transition metals, there is no apparent relationship between ionic radius and
rate of inactivation for the metal compounds. In addition, non-transition group
III metals such as aluminum and indium showed higher rates of inactivation that
are comparable to the action of silver. This is probably due to the increase in
coagulation potential and the resulting adsorption of bacteria because of the
larger number of ionisable electrons carried by these atoms. In general, there
is a difference between the atoms and the ions in terms of their rate of
inactivation. This difference increases amongst the transition metals that have
lower oxidation potential, lower ionization potential as well as larger ionic
radius. The results also showed that for the metals, adsorption through
coagulation is an important factor that is responsible for inactivation of E.
coli. For the metal compounds, additional mechanisms such as direct reaction
through complex formation, physico-chemical distortion of the cell structure
through direct entry of the ions into the cell, may contribute towards greater
inactivation.
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