232 DISINFECTION
and a decrease in activity at the cmc. However, in this system
the activity again increases at higher soap concentrations;
thus, this increased bactericidal activity parallels a saturation
of the soap micelles with the phenol. The reasons for this
remain unclear, for although the soap itself could contrib-
ute to or be responsible for the increased activity, systems
in which the soap has a low activity give similar results. The
actual role of the micelles is thus difficult to assess. Of the
two systems described, it is apparent that the system with
the constant phenol/soap ratio is more important from the
viewpoint of practical disinfection. It is necessary to use the
lowest possible proportion of soap to phenol, thus giving a
comparable situation to bactericides in two-phase systems, in
which the bactericidal efficiency is related to the concentra-
tion in the aqueous phase (Bean and Heman-Ackah, 1965).
Phenol itself is an effective bactericide and anti-fungal
agent, which is used as a preservative in some injections and
creams; it is also the standard reference substance in some
methods of testing phenolic bactericides (see earlier). Cresol,
a mixture of o -, m - and p -cresol, in which the meta -isomer
predominates, and of other phenols obtained from coal tar,
is highly bactericidal and fungicidal. Apart from being the
active constituent of Lysol, it is also used as a preservative
in certain injections and creams. Chlorocresol is a power-
ful antimicrobial agent which, in conjunction with heat, is
employed for the sterilization of certain injections. It is also
used as a preservative in certain cosmetic creams and lotions,
and is included in Sodium Benzoate and Chlorocresol
Solution, which may be used for the storage of sterilized
surgical instruments, the sodium benzoate delaying rusting.
Chloroxylenol has low water solubility and is solubilized
by means of soap, the solution being known as Roxenol. Its
antimicrobial activity is markedly reduced in the presence of
organic matter. Thymol is a potent bactericide and fungicide
which, in the form of Glycerin of Thymol, is employed as an
oral antiseptic. Hexachlorophane [2,2-methylenebis(3,4,6-
trichloro-phenol)] is most frequently used as a soap contain-
ing 2% of the substance; the effectiveness of this soap as
a skin disinfectant may depend upon the accumulation of
hexachlorophane on the skin. Pentachlorophenyl dodecano-
ate is extensively used as a fungicide and insecticide in the
textile and packing industries. Unlike the parent molecule,
pentachlorophenol, it is nontoxic to humans.
None of the above compounds can be relied upon to kill
bacterial spores at ordinary temperatures.
Another group, related to the phenols, consists of the
esters (parabens) of p -hydroxybenzoic acid (3-hydrobenzoic
acid). Unlike benzoic acid, the dissociation of which increases
with increasing pH with a corresponding decrease in activity,
the parabens are active against bacteria and fungi over a fairly
wide pH range. Their bactericidal and fungicidal properties
increase within the homologous series, but this is paralleled
by a decrease in aqueous solubility. The parabens are not spo-
ricidal, and although on their discovery they were heralded
as being the ideal preservatives, they are now used (often
in combination of two or more) mainly as preservatives in
various pharmaceutical and cosmetic products (Russell,
Jenkins and Harrison, 1967; Parker, 1982).The phenols and parabens have an effect on the cytoplas-
mic membranes of bacteria and fungi (Hugo, 1976a,b).Biguanides and BisbiguanidesBiguanides have the general formulaR^1R^2 H.NH NHNH
NC CN(1) (5) R^2Three distinct antimicrobial actions of N^1 , N^5 -substituted
biguanides have to date been recognized (Weinberg, 1968):
germicidal, antiviral and antimalarial. Unfortunately, no one
compound is generally active in more than one of these three
categories. The requirements for each type of activity are
fairly unique, e.g., for maximum broad-spectrum germicidal
activity, both N^1 and N^5 should have a halogen-substituted
aralkyl substituent.
Bisbiguanides have the general formulaNH R^1.NHNH NHNH NHNHNH NHR C C (CH 2 ) 6 CCNHFor maximum broad spectrum germicidal activity R and R^1
should consist of a halogen-substituted aryl group. A number
of bisbiguanides are known to be germicidal, and the most
important member of this group is chlorhexidine (R and R^1
are both C 6 H 5 Cl).
Chlorhexidine was first described in 1954 (Davies et al. ,
1954), and it is bacteriostatic in low concentrations; higher
concentrations are bactericidal to several bacterial species,
including strains of the genus Proteus, of which Pr. mirabilis
is the most resistant, and Pr. rettgeri and Pr. morganii are
the most sensitive to chlorhexidine, with Pr. vulgaris occu-
pying an intermediate position. Some strains of Ps. aerugi-
nosa may be highly resistant. Chlorhexidine also possesses
antifungal activity. It is a membrane-active agent (Hugo,
1976a,b, 1982).
Chlorhexidine, the active constituent of “Hibitane,” is
used in surface disinfection, as an antimicrobial agent in
eye-drops, and, in the presence of sodium nitrite to prevent
corrosion, for the storage of surgical instruments.Chlorine-Releasing CompoundsIt was observed by Dakin (1915, 1916) that the commercial
hypochlorites then in use were not of constant composition
and contained free alkali and sometimes free chlorine. He
thus developed a solution (Dakin’s Solution or Chlorinated
Soda Solution, Srugical) which is still in use today. The sta-
bility of free available chlorine in solution is dependent on a
number of factors, in particular on the chlorine concentration,
pH of the solution, the presence of catalysts, temperature, the
presence of organic matter, and light (Dychdala, 1977).C004_002_r03.indd 232C004_002_r03.indd 232 11/18/2005 10:19:28 AM11/18/2005 10:19:28 AM