DISINFECTION 231
Of importance, also, is the finding that these
substances are sporicidal at elevated temperatures
(Berry et al. , 1937; Russell and Loosemore, 1964).
As a result of the findings of Berry et al. (1937),
one method of sterilizing certain injections by
heating them with 0.2% w/v chlorocresol is still an
official method in Britain (British Pharmacopoeia,
1980).
3) pH. The phenols are more active at an acid pH
than in alkaline solution, as phenates (phenox-
ides) are formed at high pH. Acid pH also results
in a more effective, although still slow, sporicidal
action (Sykes and Hooper, 1954).
4) Organic matter. The presence of organic matter
may decrease the antimicrobial activity of these
compounds; this was early recognized in the
design of the Chick–Martin test for evaluating
phenolic disinfectants (Chick and Martin, 1910;
Garrod, 1934, 1935). The results do, however,
depend on the actual phenol used and on the kind
of organic matter, and the interference is less than
with other disinfectants such as the quaternary
ammonium compounds (Cook, 1960).
5) Oxygen tension. Anaerobic bacteria are generally
more resistant than aerobes to phenols. Moreover,
facultative organisms, e.g., E. coli, are more resis-
tant when grown under anaerobic conditions.
6) Type of organism. As described above, these
compounds are bactericidal and sporostatic at
low concentrations, and sporostatic and not spo-
ricidal even at high concentrations. As a group,
however, they are also fungicidal to several
moulds, and use is made of this in the inclusion
of cresol and chlorocresol as preservatives in
creams which are liable to fungal contamination
(Wedderbern, 1964).
Morris and Darlow (1971) have pointed out
that phenolic compounds with high R-W coeffi-
cients are effective against some viruses, but that
they are generally too variable in their activity to
be suitable as general virucidal agents.
7) Chemical nature. Dihydric and trihydric phenols
(Figure 3) are generally less active than phe-
nols, and alkylation of monohydric phenols to
give cresols potentiates the antimicrobial activ-
ity. Also halogenation of the phenols increases
their activity (although to a lesser extent if the
halogenation is in the ortho- than in the parapo-
sition) and this is even more pronounced when
accompanied by the introduction of aliphatic or
aromatic groups into the nucleus, e.g., p -chloro-
m -cresol (chlorocresol). This increase in anti-
microbial activity is, however, paralleled by a
decrease in water solubility.To overcome this decrease in aqueous solubility, vari-
ous soaps have been used to render water-soluble (solubi-
lize) these substances. However, the effect of soap on thebiological efficacy of the phenols depends on two factors,
firstly the nature of the soap, and second the proportion of
soap to phenol. Solution of cresol with soap (Lysol, BP), for
example, contains 50% of cresol in a saponaceous solvent
and has a bactericidal activity which depends on the nature
and amount of soap used. The ratio of cresols to soap may
be critical, the optimal cresol–soap ratio being of the order
of 2:1. In lysol, the soap content is c. 22%, and the ratio is
thus 2.2:1. The soap solutions are able to solubilize insoluble
phenols in the micelles; the critical micelle concentrations
(cmcs) of different soaps vary, and this explains differences
in bactericidal action noted above.
Lysol and the so-called “black-fluids,” which consist of
the lower coal tar phenols, are formulated in sufficient soap
so that they are retained in solution when diluted with water.
In contrast, the white fluids consist of concentrated emul-
sions of high boiling phenols stabilized with protective col-
loids; they can be diluted with hard or soft waters, whereas
black fluids should be diluted with soft waters only.
Micelles have been considered as being reservoirs of phe-
nols, so that when the concentrated solution is diluted before
use, the phenols are released by dilution below the cmc to
give a highly active solution. Two types of system have been
investigated experimentally in attempts to assess the exact
role of the micelles; these are (a) constant phenol concentra-
tion, and (b) a constant phenol/soap ratio (Berry, 1951; Berry
and Briggs, 1956; Berry, Cook and Wills, 1956; Cook, 1960).
With a constant phenol system, there is a rapid increase in
bactericidal activity below the cmc of the soap which could
be the result of an increased uptake of phenol together with
an increased permeability of the bacterial surface (Mulley,
1964); however, above the cmc in this system, there is a
decrease in bactericidal activity, as the increasing numbers
of micelles being formed compete for the phenol with the
cell surface. In systems where there is a constant phenol/soap
ratio, there is likewise an increase in activity below the cmc,OH OH OHOHOH OHPyrogallol
Resorcinolm-Cresol ChlorocresolPhenolOH OHCH 3 CH 3ClFIGURE 3C004_002_r03.indd 231C004_002_r03.indd 231 11/18/2005 10:19:28 AM11/18/2005 10:19:28 AM