A strain of Pseudomonas aeruginosa survives exposure to hydrogen peroxide for as long as 60 min at a concentration of ~1.7% (9). But < 0.1% of the initial bacterial
concentration survived. Use concentrations of hydrogen peroxide still kill the bacteria strain within minutes. The reduced
susceptibility of P. aeruginosa to hydrogen peroxide is caused by its antioxidant defenses.
The presence of a biofilm (e.g., in potable-water pipes and cooling towers) significantly reduces the efficacy of hypochlorites.
For example, LeChevallier reported that a bacterial biofilm grown on various surfaces were 150 to 3000 times more resistant
to hypochlorous acid than planktonic cells were (10). Some types of bacteria such as Pseudomonads segregate extracellular mucopolysaccharides that enhance adhesion to solid surfaces. This is the hypothesized mechanism by
which certain strains of P. aeruginosa resist disinfection with chlorine in swimming pools.
The hypothetical explanation for the reduced susceptibility and tolerance of bacteria within biofilms has two components:
retarded penetration of chlorine into the biofilm and the consumption of free chlorine by organic materials before it can
fully penetrate the biofilm surface (11). A similar theory explains the reduced susceptibility of biofilms to hydrogen peroxide.
In an ostensible case of resistance to a disinfectant, the Centers for Disease Control (CDC) reported several peritoneal
infections in infants and false-positive blood cultures from patients in 1989. These infections were associated with an iodophor
antiseptic solution, namely povidone-iodine (PI), contaminated with Burkholderia cepacia (12). PI is a stable chemical complex of polyvinylpyrrolidone and iodine. A follow-up study of the PI solution confirmed the
contamination and recovered the bacterium from the contaminated iodophor after 29 weeks of sampling. Scanning electron microscopic
(SEM) examination of the contaminated PI solution showed bacterial cells embedded in extracellular material and strands of
glycocalyx between cells.
The extended survival of B. cepacia in the PI solution was attributed to the extracellular glycocalyx-like material that microorganisms form and deposit on various
surfaces (12). In this case, susceptibility and resistance to iodine were not reduced; bacteria were protected by extracellular
materials. No report of true microbial tolerance or resistance to PI has been confirmed.
Alkalis such as sodium hydroxide and potassium hydroxide kill bacteria through the action of hydroxide free radicals. In
solution, sodium hydroxide and potassium hydroxide disassociate into metal cations and hydroxide free radicals, which oxidize
lipids, proteins, and DNA.
Isopropyl alcohol (IPA) and ethyl alcohol (EA).
Reduced susceptibility to IPA and EA at use concentrations in sensitive microorganisms has not been substantiated. Increase
of the MIC—below use concentrations—has been documented for some gram-negative bacteria such as Escherichia coli and Salmonella species because of the oxidative-stress (SOS) response. The SOS response induces production of neutralizing enzymes to prevent cellular
damage and to repair DNA injuries (13). Most alcohol disinfection failures in the field result from the presence of biofilms,
poor cleaning, or inadequate contact times. IPA and EA are not effective in penetrating organic material and tissue. Hence,
biofilms have reduced susceptibility, and sometimes resistance, to alcohols. The most common reason for efficacy loss with
IPA and EA is evaporation that yields suboptimal contact times.
Reduced susceptibility and resistance to FA are most often found in gram-negative bacteria such as Pseudomonas spp. and Enterobacteriaceae. A strain of P. aeruginosa grows at an FA concentration of 0.075% (14). A strain of E. coli grows at an FA concentration of 0.02% (15). Use concentrations of FA are 1–8% for immersion and wetting disinfection, and
0.05–0.2% for preservation. Studies of E. coli (strain VU3695) indicate that reduced susceptibility to FA is caused by enzymes such as FA dehydrogenases that inactivate
it (15). Use concentrations of 1–8%, however, will readily kill these two bacterial strains.