Biocide Susceptibility Research Articles

Acanthamoeba castellanii: Growth, encystment, excystment and biocide susceptibility

Stages in the encystment and excystment processes of Acanthamoeba castellanii have been studied. The kinetics of encystment involved measurements of the three phases (pre-encystment, cyst initiation and cyst wall synthesis). Excystment, starting from a mature cyst, involved pre-emergence, penetration outwards of the cyst wall and free trophozoite. The sensitivity to biocides of trophozoites in the exponential growth phase, pre-encystment trophozoites, mature cysts and pre-excystment cysts has been investigated. Some differences in relative sensitivity to a bisbiguanide (chlorhexidine) and a polymeric biguanide (polyhexamethylene biguanide) were observed, but mature cysts were always the most resistant cellular form.

Plasmids and bacterial resistance to biocides.

Plasmid-encoded bacterial resistance to antibiotics and to anions and cation (including important mercurial and silver compounds) has been widely studied. Plasmid-mediated resistance to organic cationic agents which are important biocides has been described for chlorhexidine and quaternary ammonium compounds (and also for the less important acridine, diamidines and ethidium bromide) in antibiotic-resistant Staphylococcus aureus and Staph. epidermidis strains. Plasmids may also encode reduced biocide susceptibility of Gram-negative bacteria, but intrinsic resistance is likely to be of greater significance. Antibiotic resistance and biocide resistance may be linked but this is not always found clinically.

Decreased biocide susceptibility of adherent Legionella pneumophila

In a study of the in vitro effectiveness of biocides against Legionella pneumophila, some aspects of the cooling tower environment were replicated in the laboratory, paying particular attention to water hardness and pH. Pieces of Douglas fir and polyvinyl chloride were colonized in a recirculating system and the comparative efficacy of two biocides (Bronopol and Kathon) against the sessile and planktonic populations was examined. While the biocides were relatively effective against the planktonic L. pneumophila population over a short period of time (minimum 9-12 h), substantially longer periods of time (maximum greater than 48 h) were required to reduce the number of cultivable bacteria to below detectable levels in the adherent population. The results indicate that failure to monitor the sessile population of L. pneumophila in laboratory studies of biocides may result in the use of incorrect dosages and/or contact times in field trials with apparently reduced in situ efficacy.