Microbiologically it was demonstrated that amino acids, e.g. cysteine (CySH), and othercompounds, e.g. sodium thioglycollate, containing thiol groups neutralized the activity of silvernitrate against Pseudomonas aeruginosa PAOl. Amino acids with disulphide bonds wereinactive, with the exception of l‐cystine dimethyl ester, as were all amino acids with nosulphur groups. Iodoacetamide reacted with CySH to produce a CyS–acetamide complex thatwas unable to quench the activity of Ag+. Chemical analyses using cyclic voltammetrydemonstrated that high coordination numbers (3·1) were obtained with thiol‐containingamino acids and low numbers (0·28–0·4) with other amino acids. Bothmicrobiologically and chemically, the results imply that interaction of Ag+ with thiolgroups plays an essential role in bacterial inactivation.

No abstract

A synchronous encystment method was used to study the order of development of resistance of Acanthamoeba castellanii to a range of biocides. The emerging resistance during encystation to short-term exposure to the minimum amoebicidal concentrations of each biocide tested was recorded during the first 36 h of the differentiation process. Hydrochloric acid and moist heat were tested as possible resistance markers. Development of the acid-insoluble, proteincontaining, ectocyst wall and the cellulose endocyst wall was followed by quantification of the acid- and alkali-insoluble residues of cell samples removed from synchronous encystment cultures up to 36 h. Resistance to chemical agents (polyhexamethylene biguanide, benzalkonium chloride, propamidine isethionate, pentamidine isethionate, dibromopropamidine isethionate, hydrogen peroxide) and to moist heat was seen to develop between 14 and 24 h after trophozoites were inoculated into the encystment media. Resistance to hydrochloric acid developed between 0 and 2 h and to chlorhexidine diacetate between 24 and 36 h. Levels of acid-insoluble residues began to increase after 8 h and alkali-insoluble residues (cellulose) were detected after 16 h and coincided with the emergence of resistance to all the agents tested except hydrochloric acid. The results suggest that resistance to the biocides tested probably results largely from the physical barrier of the cyst walls rather than as a consequence of a metabolically dormant cyst.

A new chloroxylenol preparation (DC) containing ethylenediamine tetraacetic acid (EDTA). has been shown to be an effective bactericidal agent against various Gram positive and Gram negative bacteria. It possessed significant and rapid activity against two strains of Pseudomonas aeruginosa as well as against chlorhexidine‐and chloroxylenol‐resistant derivatives of these strains. DC showed high activity at 20° and 30 °C, and (unlike chlorhexidine) when prepared in hard water or when used in the presence of organic matter. Repeated challenge testing at 20 °C of DC (but not of chloroxylenol) with Ps. aeuruginosa indicated that contamination of DC will not prove to be a problem in practice and that the activity of solutions will be retained during storage.

Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX) have been used to examine chlorhexidine diacetate (CHA)-sensitive and -resistant isolates of Pseudomonas stutzeri and to determine the effects of CHA on the cells. Significant differences were observed in the structure, size and elemental composition of CHA-sensitive and -resistant cells. Treatment with CHA produced considerably greater changes in CHA-sensitive cells, with widespread peeling of the outer membrane, a substantial loss of cytoplasmic electron-dense material and extensive lysis. Cells from the resistant isolates showed no blebbing of the outer membrane and no structural damage. X-ray mapping confirmed the difference in CHA uptake between CHA-sensitive and CHA-resistant cells. It is proposed that changes in the outer membrane form a major mechanism of resistance to CHA in P. stutzeri.