The regrowth observed in vitro for FA donors, used in preservation, has several causes. First, FA kills most bacteria in the culture medium. Second, the available FA is exhausted after reacting with the organic molecules in the medium and the bacteria. Third, bacteria that survived because they were protected by organic material (i.e., cellular debris) and received a sublethal dose may acclimatize to residual FA and grow back. Bacteria that have FA dehydrogenases will degrade FA and keep multiplying. As the FA is exhausted from the medium, other bacteria will grow (14, 16). FA donors are mainly used as preservatives at concentrations of 0.05–0.20% (free FA).

Phenol. Some bacteria can survive in low concentrations of phenol. Furthermore, some of them can use phenol as a source of carbon and thrive in low phenol concentrations (< 0.3%). For instance, a strain of Micrococcus pyogenes var. aureus is resistant to the bacteriostatic action of phenol in 0.2–0.3% concentrations (17). In addition, a strain of Brevundimonas putida metabolizes phenol and the isomers of cresol at low concentrations (18, 19). At use concentrations, none of these bacteria would survive. In fact, tolerance to phenol has only been demonstrated at concentrations < 0.5% (20). True resistance or tolerance to phenol at use concentrations has not been documented.

Triclosan (TLN). TLN is a derivative of halogenated phenolics intended to be used as an antiseptic. It has poor solubility in water, but is fat-soluble and easily crosses cell membranes. Once inside microorganisms, TLN poisons a specific enzyme that many bacteria and fungi need to live. TLN blocks the active site of the enzyme enoyl-acyl carrier-protein reductase (ENR), preventing microorganisms from synthesizing fatty acids they need for building cell membranes and other essential functions (21, 22).

Generally, TLN has little activity against P. aeruginosa, other gram-negative bacteria, and molds. Strains of Klebsiella pneumoniae, Enterobacter cloacae, Acinetobacter baumannii, Pseudomonas fluorescents, and E. coli–O157:H7 have grown on media containing TLN at a concentration of approximately 0.1% (24, 26). This concentration is near that of several consumer products. For example, some bars of soap contain 0.25–1.5% TLN by weight (27, 28).

Reduced susceptibility to TLN is seen in strains of Staphylococcus aureus. The typical TLN MIC for S. aureus is 0.016 μg/mL, but mutant strains had a MIC of 2 μg/mL: an increase of greater than fiftyfold. Payne revealed that a gene mutation is required for TLN resistance and that this gene must be overexpressed at levels three to five times higher than the level of expression in TLN-sensitive strains (23).

Although wild types of P. aeruginosa have the ENR enzyme, they are intrinsically resistant to TLN. Resistance comes from several factors, including efflux pumps and gene mutations (29, 30). Moreover, clinical and laboratory E. coli strains with a multidrug efflux pump have reduced susceptibility to TLN (24, 25). Efflux pumps are conveyor systems that move substances such as waste and harmful chemicals out of the cell.

Chlorhexidine (CHX). CHX compounds are low- to intermediate-level disinfectants and antiseptics. CHX interacts with the cell surface and promotes membrane damage, which in turn causes an irreversible loss of cytoplasmic components (31, 32). The killing action of CHX at relatively low concentrations (e.g., 2–2.5 μg/mL) is similar to the action of some antibiotics. At high concentrations (≥ 20 μg/mL), CHX causes coagulation of cytoplasm and precipitation of proteins and nucleic acids.